2005年12月21日
#
1.Ruby Object in c
2.juke box extension
3.memory allocation
4.ruby type system
5.create an exception
6embed a ruby interpreter
7.bridge to other language
8.c api
1.
p269
Sometimes, though, life is more complicated. Perhaps you want to define a global variable
whose valuemust be calculated when it is accessed. You do this by defining hooked
and virtual variables. A hooked variable is a real variable that is initialized by a named
function when the corresponding Ruby variable is accessed. Virtual variables are similar
but are never stored: their value purely comes from evaluating the hook function.
See the API section that begins on page 294 for details.
If you create a Ruby object from C and store it in a C global variable without exporting
it to Ruby, you must at least tell the garbage collector about it, lest ye be reaped
inadvertently.
static VALUE obj;
// ...
obj = rb_ary_new();
rb_global_variable(obj);
You may have noticed that our sample database tables all define an integer
column called id as their primary key. This is an Active Record convention.
“But wait!” you cry. “Shouldn’t the primary key of my orders table be the
order number or some other meaningful column? Why use an artificial
primary key such as id?”
The reason is largely a practical one—the format of external data may
change over time. For example, you might think that the ISBN of a book
would make a good primary key in a table of books. After all, ISBNs are
Report erratum Prepared exclusively for Don Francis
PRIMARY KEYS AND IDS 198
unique. But as this particular book is being written, the publishing industry
in the US is gearing up for a major change as additional digits are
added to all ISBNs.
If we’d used the ISBN as the primary key in a table of books, we’d have to
go through and update each row to reflect this change. But then we’d have
another problem. There’ll be other tables in the database that reference
rows in the books table via the primary key. We can’t change the key in the
books table unless we first go through and update all of these references.
And that will involve dropping foreign key constraints, updating tables,
updating the books table, and finally reestablishing the constraints. All in
all, something of a pain.
If we use our own internal value as a primary key, things work out a lot
better. No third party can come along and arbitrarily tell us to change
things—we control our own keyspace. And if something such as the ISBN
does need to change, it can change without affecting any of the existing
relationships in the database. In effect, we’ve decoupled the knitting
together of rows from the external representation of data in those rows.
Now there’s nothing to say that we can’t expose the id value to our end
users. In the orders table, we could externally call it an order id and print
it on all the paperwork. But be careful doing this—at any time some regulator
may come along and mandate that order ids must follow an externally
imposed format, and you’d be back where you started.
If you’re creating a new schema for a Rails application, you’ll probably
want to go with the flow and give all of your tables an id column as their
primary key. If you need to work with an existing schema, Active Record
gives you a simple way of overriding the default name of the primary key
for a table.
class BadBook < ActiveRecord::Base
set_primary_key "isbn"
end
Normally, Active Record takes care of creating new primary key values
for records that you create and add to the database—they’ll be ascending
integers (possibly with some gaps in the sequence). However, if you override
the primary key column’s name, you also take on the responsibility
of setting the primary key to a unique value before you save a new row.
Perhaps surprisingly, you still set an attribute called id to do this. As far as
As we’ll see later, join tables are not included in this advice—they should not have an id column.
Active Record is concerned, the primary key attribute is always set using
an attribute called id. The set_primary_key declaration sets the name of the
column to use in the table. In the following code, we use an attribute
called id even though the primary key in the database is isbn.
book = BadBook.new
book.id = "0-12345-6789"
book.title = "My Great American Novel"
book.save
# ...
book = BadBook.find("0-12345-6789")
puts book.title # => "My Great American Novel"
p book.attributes #=> {"isbn" =>"0-12345-6789",
"title"=>"My Great American Novel"}
Just to make things more confusing, the attributes of the model object
have the column names isbn and title—id doesn’t appear. When you need
to set the primary key, use id. At all other times, use the actual column
name.
If a model object has an attribute named balance, you can access the
attribute’s value using the indexing operator, passing it either a string or
a symbol. Here we’ll use symbols.
account[:balance] #=> return current value
account[:balance] = 0.0 #=> set value of balance
However, this is
deprecated in normal code, as it considerably reduces
your options should you want to change the underlying implementation
of the attribute in the future. Instead, you should access values or model
attributes using Ruby
accessor methods.
account.balance #=> return current value
account.balance = 0.0 #=> set value of balance
The value returned using these two techniques will be cast by Active
Record to an appropriate Ruby type if possible (so, for example, if the
database column is a timestamp, a Time object will be returned). If you
want to get the raw value of an attribute, append _before_type_cast to the
method form of its name, as shown in the following code.
COLUMNS AND ATTRIBUTES 195
David Says. . .
Overriding Model Attributes
Here’s an example of the benefits of using accessors to get at the
attributes of models. Our account model will raise an exception immediately
when someone tries to set a balance below a minimum value.
class Account < ActiveRecord::Base
def balance=(value)
raise BalanceTooLow if value < MINIMUM_LEVEL
self[:balance] = value
end
end
account.balance_before_type_cast #=> "123.4", a string
account.release_date_before_type_cast #=> "20050301"
Finally, inside the code of the model itself, you can use the read_attribute( )
and write_attribute( ) private methods. These take the attribute name as a
string parameter.
Boolean AttributesSome databases support a boolean column type, others don’t. This makes
it hard for Active Record to abstract booleans. For example, if the underlying
database has no boolean type, some developers use a char(1) column
containing “t” or “f” to represent true or false. Others use integer columns,
where 0 is false and 1 is true. Even if the database supports boolean types
directly (such as MySQL and its bool column type), they might just be
stored as 0 or 1 internally.
The problem is that in Ruby the number 0 and the string “f” are both
interpreted as true values in conditions.4 This means that if you use the
value of the column directly, your code will interpret the column as true
when you intended it to be false.
# DON'T DO THIS
user = Users.find_by_name("Dave")
if user.superuser
grant_privileges
end
4Ruby has a simple definition of truth. Any value that is not nil or the constant false is
true.
To query a column in a condition, you must append a question mark to
the column’s name.
# INSTEAD, DO THIS
user = Users.find_by_name("Dave")
if user.superuser?
grant_privileges
end
This form of attribute accessor looks at the column’s value. It is interpreted
as false only if it is the number zero; one of the strings "0", "f", "false",
or "" (the empty string); a nil; or the constant false. Otherwise it is interpreted
as true.
If you work with legacy schemas or have databases in languages other than
English, the definition of truth in the previous paragraph may not hold.
In these cases, you can override the built-in definition of the predicate
methods. For example, in Dutch, the field might contain J or N (for Ja or
Nee). In this case, you could write
class User < ActiveRecord::Base
def superuser?
self.superuser == 'J'
end
# . . .
end
Storing Structured Data
It is sometimes convenient to store attributes containing arbitrary Ruby
objects directly into database tables. One way that Active Record supports
this is by serializing the Ruby object into a string (in YAML format) and
storing that string in the database column corresponding to the attribute.
In the schema, this column must be defined as type text.
Because Active Record will normally map a character or text column to a
plain Ruby string, you need to tell Active Record to use serialization if you
want to take advantage of this functionality. For example, we might want
to record the last five purchases made by our customers. We’ll create a
table containing a text column to hold this information.
File 6 create table purchases (
id int not null auto_increment,
name varchar(100) not null,
last_five text,
primary key (id)
);
In the Active Record class that wraps this table, we’ll use the serialize( )
declaration to tell Active Record to marshal objects into and out of this
column.
File 8 class Purchase < ActiveRecord::Base
serialize :last_five
# ...
end
When we create new Purchase objects, we can assign any Ruby object to
the last_five column. In this case, we set it to an array of strings.
File 8 purchase = Purchase.new
purchase.name = "Dave Thomas"
purchase.last_five = [ 'shoes', 'shirt', 'socks', 'ski mask', 'shorts' ]
purchase.save
When we later read it in, the attribute is set back to an array.
File 8 purchase = Purchase.find_by_name("Dave Thomas")
pp purchase.last_five
pp purchase.last_five[3]
This code outputs
["shoes", "shirt", "socks", "ski mask", "shorts"]
"ski mask"
Although powerful and convenient, this approach is problematic if you
ever need to be able to use the information in the serialized columns outside
a Ruby application. Unless that application understands the YAML
format, the column contents will be opaque to it. In particular, it will be
difficult to use the structure inside these columns in SQL queries. You
might instead want to consider using object aggregation, described in Section
15.2, Aggregation, on page 247, to achieve a similar effect.
Active Record is the object-relational mapping (ORM) layer supplied with
Rails. In this chapter, we’ll look at the basics of Active Record—connecting
to databases, mapping tables, and manipulating data. We’ll dig deeper
into the more advanced stuff in the next chapter.
Active Record closely follows the standard ORM model: tables map to
classes, rows to objects, and columns to object attributes. It differs from
most other ORM libraries in the way it is configured. By using a sensible
set of defaults, Active Record minimizes the amount of configuration that
developers perform. To illustrate this, here’s a program that uses Active
Record to wrap a table of orders in a MySQL database. After finding the
order with a particular id, it modifies the purchaser’s name and saves the
result back in the database, updating the original row.
require "rubygems"
require_gem "activerecord"
ActiveRecord::Base.establish_connection(:adapter => "mysql",
:host => "localhost", :database => "railsdb")
class Order < ActiveRecord::Base
end
order = Order.find(123)
order.name = "Dave Thomas"
order.save
That’s all there is to it—in this case no configuration information (apart
from the database connection stuff) is required. Somehow Active Record
figured out what we needed and got it right. Let’s have a look at how this
works.
14.1 Tables and Classes
When you create a subclass of ActiveRecord::Base, you’re creating something
that wraps a database table. By default, Active Record assumes that
the name of the table is the plural form of the name of the class. If the class
name contains multiple capitalized words, the table name is assumed to
have underscores between these words. Some irregular plurals are handled.
Class Name
Order
TaxAgency
Diagnosis
Batch
Table Name
tax_agencies
orders
batches
diagnoses
LineItem
Person
Datum
Quantity
Class Name
line_items
people
quantities
data
Table Name
These rules reflect DHH’s philosophy that class names should be singular
while the names of tables should be plural. If you don’t like this behavior,
you can disable it by setting a global flag in your configuration (the file
environment.rb in the config directory).
ActiveRecord::Base.pluralize_table_names = false
The algorithm used to derive the plural form of a table name is fairly simplistic.
It works in the majority of common cases, but if you have a class
named Sheep, it’ll valiantly try to find a table named sheeps. The assumption
that the table name and class names are related might also break
down if you’re operating with a legacy schema,2 where the table names
might otherwise force you to use strange or undesirable class names in
your code. For this reason, Active Record allows you to override the default
generation of a table name using the set_table_name directive.
14.2 Columns and Attributes
Active Record objects correspond to rows in a database table. The objects
have attributes corresponding to the columns in the table. You probably
noticed that our definition of class Order didn’t mention any of the columns
in the orders table. That’s because Active Record determines them dynamically
at runtime. Active Record reflects on the schema inside the database
to configure the classes that wrap tables.3
Our orders table might have been created with the following SQL.
File 6 create table orders (
id int not null auto_increment,
name varchar(100) not null,
email varchar(255) not null,
address text not null,
pay_type char(10) not null,
shipped_at datetime null,
primary key (id)
);
Logging in Rails
Rails has logging built right into the framework. Or, to be more accurate,
Rails exposes a Logger object to all the code in a Rails application.
Logger is a simple logging framework that ships with recent versions of
Ruby. (You can get more information by typing ri Logger at a command
prompt or by looking in the standard library documentation in Programming
Ruby [TH01]). For our purposes, it’s enough to know that we can
generate log messages at the warning, info, error, and fatal levels. We can
then decide (probably in an environment file) which levels of logging to
write to the log files.
logger.warn("I don't think that's a good idea")
logger.info("Dave's trying to do something bad")
logger.error("Now he's gone and broken it")
logger.fatal("I give up")
In a Rails application, these messages are written to a file in the log directory.
The file used depends on the environment in which your application
is running. A development application will log to log/development.log, an
application under test to test.log, and a production app to production.log.
13.7 Debugging Hints
Bugs happen. Even in Rails applications. This section has some hints on
tracking them down.
First and foremost, write tests! Rails makes it easy to write both unit
tests and functional tests (as we saw in Chapter 12, Task T: Testing, on
page 132). Use them, and you’ll find that your bug rate drops way down.
You’ll also decrease the likelihood of bugs suddenly appearing in code that
you wrote a month ago. Tests are cheap insurance.
Tests tell you whether something works or not, and they help you isolate
the code that has a problem. Sometimes, though, the cause isn’t immediately
apparent.
If the problem is in a model, you might be able to track it down by running
the offending class outside the context of a web application. The
scripts/console script lets you bring up part of a Rails application in an irb
session, letting you experiment with methods. Here’s a session where we
use the console to update the price of a product.
depot> ruby script/console
Loading development environment.
irb(main):001:0> pr = Product.find(:first)
=> #<Product:0x248acd0 @attributes={"image_url"=>"/images/sk..."
irb(main):002:0> pr.price
=> 29.95
irb(main):003:0> pr.price = 34.95
=> 34.95
irb(main):004:0> pr.save
=> true
Logging and tracing are a great way of understanding the dynamics of
complex applications. You’ll find a wealth of information in the development
log file. When something unexpected happens, this should probably
be the first place you look. It’s also worth inspecting the web server log for
anomalies. If you use WEBrick in development, this will be scrolling by on
the console you use to issue the script/server command.
You can add your own messages to the log with Logger object described in
the previous section. Sometimes the log files are so busy that it’s hard to
find the message you added. In those cases, and if you’re using WEBrick,
writing to STDERR will cause your message to appear on the WEBrick console,
intermixed with the normal WEBrick tracing..
If a page comes up displaying the wrong information, you might want to
dump out the objects being passed in from the controller. The debug( )
helper method is good for this. It formats objects nicely and makes sure
that their contents are valid HTML.
<h3>Your Order</h3>
<%= debug(@order) %>
<div id="ordersummary">
. . .
</div>
Finally, for those problems that just don’t seem to want to get fixed, you
can roll out the big guns and point a debugger at your running application.
This is normally available only for applications in the development
environment.
To use breakpoints:
1. Insert a call to the method breakpoint( ) at the point in your code where
you want your application to first stop. You can pass this method a
string if you’d like—this becomes an identifying message later.
2. On a convenient console, navigate to your application’s base directory
and enter the command
depot> ruby script/breakpointer
No connection to breakpoint service at
druby://localhost:42531 (DRb::DRbConnError)
Tries to connect will be made every 2 seconds...
Don’t worry about the No connection message—it just means that
your breakpoint hasn’t hit yet.
3. Using a browser, prod your application to make it hit the breakpoint( )
method. When it does, the console where breakpointer is running will
burst into life—you’ll be in an irb session, talking to your running
web application. You can inspect variables, set values, add other
breakpoints, and generally have a good time. When you quit irb, your
application will continue running.
By default, breakpoint support uses a local network connection to talk
between your application and the breakpointer client. You might be able to
use the -s option when you run breakpointer to connect to an application on
another machine.
Active Support is a set of libraries that are shared by all Rails components.
Much of what’s in there is intended for Rails internal use. However, Active
Support also extends some of Ruby’s built-in classes in interesting and
useful ways. In this section we’ll quickly list the most popular of these
extensions.
Extensions to Numbers
Class Fixnum gains the two instance methods even? and odd?.
All numeric objects gain a set of scaling methods.
puts 20.bytes #=> 20
puts 20.kilobytes #=> 20480
puts 20.megabytes #=> 20971520
puts 20.gigabytes #=> 21474836480
puts 20.terabytes #=> 21990232555520
There are also time-based scaling methods. These convert their receiver
into the equivalent number of seconds. The months( ) and years( ) methods
are approximations—months are assumed to be 30 days long, years 365
days long.
puts 20.minutes #=> 1200
puts 20.hours #=> 72000
puts 20.days #=> 1728000
puts 20.weeks #=> 12096000
puts 20.fortnights #=> 24192000
puts 20.months #=> 51840000
puts 20.years #=> 630720000
You can also calculate times relative to Time.now using the methods ago( )
and from_now( ) (or their aliases until( ) and since( ), respectively).
puts Time.now #=> Tue May 10 17:03:43 CDT 2005
puts 20.minutes.ago #=> Tue May 10 16:43:43 CDT 2005
puts 20.hours.from_now #=> Wed May 11 13:03:43 CDT 2005
puts 20.weeks.from_now #=> Tue Sep 27 17:03:43 CDT 2005
puts 20.months.ago #=> Thu Sep 18 17:03:43 CDT 2003
How cool is that?
Time Extensions
The Time class gains a number of useful methods, helping you calculate
relative times.
now = Time.now
puts now #=> Tue May 10 17:15:59 CDT 2005
puts now.ago(3600) #=> Tue May 10 16:15:59 CDT 2005
puts now.at_beginning_of_day #=> Tue May 10 00:00:00 CDT 2005
puts now.at_beginning_of_month #=> Sun May 01 00:00:00 CDT 2005
puts now.at_beginning_of_week #=> Mon May 09 00:00:00 CDT 2005
puts now.at_beginning_of_year #=> Sat Jan 01 00:00:00 CST 2005
puts now.at_midnight #=> Tue May 10 00:00:00 CDT 2005
puts now.change(:hour => 13) #=> Tue May 10 13:00:00 CDT 2005
puts now.last_month #=> Sun Apr 10 17:15:59 CDT 2005
puts now.last_year #=> Mon May 10 17:15:59 CDT 2004
puts now.midnight #=> Tue May 10 00:00:00 CDT 2005
puts now.monday #=> Mon May 09 00:00:00 CDT 2005
puts now.months_ago(2) #=> Thu Mar 10 17:15:59 CST 2005
puts now.months_since(2) #=> Sun Jul 10 17:15:59 CDT 2005
puts now.next_week #=> Mon May 16 00:00:00 CDT 2005
puts now.next_year #=> Wed May 10 17:15:59 CDT 2006
puts now.seconds_since_midnight #=> 62159.215938
puts now.since(7200) #=> Tue May 10 19:15:59 CDT 2005
puts now.tomorrow #=> Wed May 11 17:15:59 CDT 2005
puts now.years_ago(2) #=> Sat May 10 17:15:59 CDT 2003
puts now.years_since(2) #=> Thu May 10 17:15:59 CDT 2007
puts now.yesterday #=> Mon May 09 17:15:59 CDT 2005
Active Support also includes a TimeZone class. TimeZone objects encapsulate
the names and offset of a time zone. The class contains a list of the
world’s time zones. See the Active Support RDoc for details.
String Extensions
Active Support adds methods to all strings to support the way the Rails
core converts names from singular to plural, lowercase to mixed case, and
so on. Of these, two might be useful in the average application.
puts "cat".pluralize #=> cats
puts "cats".pluralize #=> cats
puts "erratum".pluralize #=> errata
puts "cats".singularize #=> cat
puts "errata".singularize #=> erratum
The rules here are the default conventions used by Rails. You can override
all of these conventions using the appropriate declarations in your Rails
classes.
We often name variables and classes using short phrases. In Ruby, the
convention is to have variable names where the letters are all lowercase,
and words are separated by
underscores. Classes and modules are named
differently: there are no underscores, and each word in the phrase (including
the first) is capitalized. (We’ll call this
mixed-case, for fairly obvious
reasons). These conventions lead to variable names such as order_status
and class names such as LineItem.
Rails takes this convention and extends it in two ways. First, it assumes
that database table names, like variable names, have lowercase letters and
underscores between the words. Rails also assumes that table names are
always plural. This leads to table names such as orders and third_parties.
On another axis, Rails assumes that files are named in lowercase with
underscores.
Rails uses this knowledge of naming conventions to convert names automatically.
For example, your application might contain a model class that
handles line items. You’d define the class using the Ruby naming convention,
calling it LineItem. From this name, Rails would automatically deduce
the following.
That the corresponding database table will be called line_items. That’s
the class name, converted to lowercase, with underscores between
the words and pluralized.
• Rails would also know to look for the class definition in a file called
line_item.rb (in the app/models directory).
Rails controllers have additional naming conventions. If our application
has a store controller, then the following happens.
• Rails assumes the class is called StoreController and that it’s in a file
named store_controller.rb in the app/controllers directory.
• It also assumes there’s a helper module named StoreHelper in the file
store_helper.rb located in the app/helpers directory.
• It will look for view templates for this controller in the app/views/store
directory.
• It will by default take the output of these views and wrap them in the
layout template contained in store.rhtml or store.rxml in the directory
app/views/layouts.
There’s one extra twist. In normal Ruby code you have to use the require
keyword to include Ruby source files before you reference the classes and
modules in those files. Because Rails knows the relationship between
filenames and class names, require is not necessary in a Rails application.
Instead, the first time you reference a class or module that isn’t known,
Rails uses the naming conventions to convert the class name to a filename
and tries to load that file behind the scenes. The net effect is that you can
Model Naming
URL
http://.../store/listFile app/views/store/list.rhtml (or .rxml)
View Naming
Helper module StoreHelper
File app/helpers/store_helper.rb
URL
http://.../store/listClass StoreController
File app/controllers/store_controller.rb
Controller Naming
Method list()
Layout app/views/layouts/store.rhtml
Figure 13.3: Naming Convention Summary
typically reference (say) the name of a model class, and that model will be
automatically loaded into your application.
As you’ll see, this scheme breaks down when your classes are stored in
sessions. In this case you’ll need to explicitly declare them. Even so, you
don’t use require. Instead, your controller would include a line such as
class StoreController < ApplicationController
model :line_item
# ...
Notice how the naming conventions are still used consistently here. The
symbol :line_item is lowercase with an underscore. It will cause the file
line_item.rb to be loaded, and that file will contain class LineItem.
Grouping Controllers into Modules
So far, all our controllers have lived in the app/controllers directory. It is
sometimes convenient to add more structure to this arrangement. For
example, our store might end up with a number of controllers performing
related but disjoint administration functions. Rather than pollute the top-
level namespace with each of these, we might choose to group them into a
single admin namespace.
Rails does this using a simple convention. If an incoming request has a
controller named (say) admin/book, Rails will look for the controller called
book_controller in the directory app/controllers/admin. That is, the final part
of the controller name will always resolve to a file called name_controller.rb,
and any leading path information will be used to navigate through subdirectories,
starting in the app/controllers directory.
Imagine that our application has two such groups of controllers (say,
admin/xxx and content/xxx) and that both groups defined a book controller.
There’d be a file called book_controller.rb in both the admin and content subdirectories
of app/controllers. Both of these controller files would define a
class named BookController. If Rails took no further steps, these two classes
would clash.
To deal with this, Rails assumes that controllers in subdirectories of the
directory app/controllers are in Ruby modules named after the subdirectory.
Thus, the book controller in the admin subdirectory would be declared as
class Admin::BookController < ApplicationController
# ...
end
The book controller in the content subdirectory would be in the Content
module.
class Content::BookController < ApplicationController
# ...
end
The two controllers are therefore kept separate inside your application.
The templates for these controllers appear in subdirectories of app/views.
Thus, the view template corresponding to the request
http://my.app/admin/book/edit/1234will be in the file
app/views/admin/book/edit.rhtml
You’ll be pleased to know that the controller generator understands the
concept of controllers in modules and lets you create them with commands
such as
myapp> ruby script/generate controller Admin::Book action1 action2 ...
This pattern of controller naming has ramifications when we start generating
URLs to link actions together. We’ll talk about this starting on
page 287.
The runtime configuration of your application is performed by two files.
One, config/environment.rb, is environment independent—it is used regardless
of the setting of RAILS_ENV.
The second file does depend on the environment:
Rails looks for a file named for the current environment in the
directory config/environments and loads it during the processing of environment.
rb.
The standard three environments (development.rb, production.rb,
and test.rb) are included by default. You can add your own file if you’ve
defined new environment types.
Environment files typically do three things.
• They set up the Ruby load path. This is how your application can
find things such as models and views when it’s running.
• They create resources used by your application (such as the logger).
• They set various configuration options, both for Rails and for your
application.
The first two of these are normally application-wide and so are done in
environment.rb. The configuration options often vary depending on the environment
and so are likely to be set in the environment-specific files in the
environments directory.
The Load Path
The standard environment automatically includes the following directories
(relative to your application’s base directory) into your application’s load
path.
1. test/mocks/environment. As these are first in the load path, classes
defined here override the real versions, enabling you to replace live
functionality with stub code during testing. This is described starting
on page 161.
2. All directories whose names start with an underscore or a lowercase
letter under app/models and components.子目录
3.The directories app, app/models, app/controllers, app/helpers, app/apis,
components, config, lib, vendor, and vendor/rails/*.
Each of these directories is added to the load path only if it exists.
Application-wide Resources
environment.rb creates an instance of a Logger that will log messages to
log/environment.log. It sets this to be the logger used by Active Record,
Action Controller, and Action Mailer (unless your environment-specific
configuration files had already set their own logger into any of these components).
environment.rb also tells Action Controller and Mailer to use app/views as
the starting point when looking for templates. Again, this can be overridden
in the environment-specific configurations.
Configuration Parameters
You configure Rails by setting various options in the Rails modules. Typically
you’ll make these settings either at the end of environment.rb (if you
want the setting to apply in all environments) or in one of the environmentspecific
files in the environments directory.
We provide a listing of all these configuration parameters in Appendix B
Services which are used in different applications can be defined only once by creating Global Service Definition files or services specific to an application can be restricted and available only to that application.
When used in a web application services are available to web events, which allow events to stay small and reuse existing logic in the Services Framework. Also, services can be defined as 'exportable' which means they are allowed to be accessed by outside parties.
Currently there is a SOAP EventHandler which allows services to be made available via SOAP. Other forms of remote invocation may be added to the framework in the future
The primary goal of the entity engine is to eliminate the need for entity specific persistence code in as many areas of a transactional application as possible. Granted that this sort of abstraction is a different issue for reporting and similar systems, but for transactional systems such as are used on a day to day basis in all businesses, an entity engine can save a great deal of development effort and dramatically reduce persistence related bugs in the system. These types of applications include everything from ecommerce to accounting to inventory and warehouse management to human resources and so on. These tools can be useful for reporting and analytical systems,
but really aren't meant to allow for the wide variety of custom queries that often take place in such tools.???
In order to achieve having as little entity specific code as possible, all value objects are generic, using a map to store the fields values of the entity instance by name. The get and set methods for the fields take a String with the fieldName in it which is used to verify that the field is part of the entity, and then either get or set a value as desired.
The danger of this flexibility is curtailed using a contract between the entity engine and the application; this is contained in a special XML file.???
Instead of writing entity specific code, entity definitions are read from an XML file and used by the entity engine to enforce a set of rules between the application and the data source, be it a database or some other source. These XML entity definitions specify the names of all of the entities and their fields along with which database tables and columns they correspond to. They are also used to specify a type for each field which is then looked up in the field types file for the given data source to find the Java and SQL data types. Relations between entities are also defined in this XML file. A relation is defined by specifying the related table, the type of relation (one or many) and the keymaps for the relation. A title can also be given to the relation which becomes part of its name to distinguish it from other relations to that specific related entity.
Using the Entity Engine as an abstraction layer, entity specific code can be easily created and modified. Code that uses the Entity Engine APIs to interact with entities can be deployed in various ways so that entity persistence can be done differently without changing the code that interacts with those entities on a higher level. An example of the usefulness of this abstraction is that, by changing a configuration file, an application written using the Entity Engine can switch from hitting a database directly through JDBC to using an EJB server and Entity Beans for persistence. The same code could also be used for custom data sources like legacy systems over HTTP or messaging services through a bit of custom coding within the same framework.
Kid is a simple XML based template language that uses embedded Python to do cool stuff. The syntax was inspired by a number of existing template languages, namely XSLT, TAL, and PHP.
This document describes the template language and will be most useful as reference to those developing Kid templates. For information about using templates from Python, the command line, or in web environments, see the User's Guide.
<?python
title = "A Kid Test Document"
fruits = ["apple", "orange", "kiwi", "M&M"]
from platform import system
?>
<html xmlns:py="http://purl.org/kid/ns#">
<head>
<title py:content="title">This is replaced.</title>
</head>
<body>
<p>These are some of my favorite fruits:</p>
<ul>
<li py:for="fruit in fruits">
I like ${fruit}s
</li>
</ul>
<p py:if="system() == 'Linux'">
Good for you!
</p>
</body>
</html>
Yields something like this:
<?xml version="1.0" encoding="utf-8"?>
<html>
<head>
<title>A Kid Test Document</title>
</head>
<body>
<p>These are some of my favorite fruits:</p>
<ul>
<li>I like apples</li>
<li>I like oranges</li>
<li>I like kiwis</li>
<li>I like M&Ms</li>
</ul>
<p>
Good for you!
</p>
</body>
</html>
All attributes described in this document must belong to the following namespace:
http://purl.org/kid/ns#
The namespace prefix py is used throughout this document to indicate that an item belongs to the Kid/Python namespace.
The <?python?> processing instruction (PI) contains Python code and MAY occur anywhere that is legal for processing instructions to occur in an XML document.
The rules for executing code found in a <?python?> PI is as follows:
- <?python?> PIs located outside of the document element (e.g. root element) contain Document Level code. This code SHOULD be executed in a global, shared scope for the document. The code SHOULD be executed once when the template is loaded and shared between multiple invocations of the template.
- <?python?> PIs located within the document element contain Local Level code. This code is executed each time the document is processed with a local scope specific to the invocation and the shared document level global scope.
Document Level and Local Level code work exactly like Module Level and Function Level code in normal Python modules. For example, the following Kid template:
<?python
x = 0
y = 0
?>
<html xmlns:py="http://purl.org/kid/ns#">
<?python
x = 1
if x == 1:
x = 10
?>
<p py:content="x"/>
<?python
global y
y = 30
?>
<p py:content="y"/>
</html>
May be considered equivalent to the following Python module:
x = 0
y = 0
def expand(handler):
handler.startDocument()
handler.startElement('html')
x = 1
if x == 1:
x = 10
handler.element('p', content=x) # output p element with x as value
global y
y = 30
handler.element('p', content=y) # output p element with value of y
handler.endElement('html')
handler.endDocument()
<?python?> PIs may contain any legal Python language construct including functions, classes, lamda forms, etc.
<?python
class Adder:
def __init__(self, x, y):
self.x = x
self.y = y
def doit(self):
return self.x + self.y
foo = Adder(x=400, y=20)
x = foo.doit()
?>
Single line <?python?> PIs are okay too:
<?python x = 10 ?>
There are multiple methods of generating content output from a template: py:content, py:replace, py:attrs, and ${} substitution. Each of these syntaxes have the same rules for what types of objects may result from the Python expression they contain.
- str, unicode
- The string is inserted as XML CDATA. That is, it is non-parsable character data that does not contain markup. The following characters are encoded as XML entities when serialized: '<', '&'. Attribute values containing content also encode the quote character: '"'.
- ElementTree.Element
-
When an ElementTree.Element is referenced from a content producing construct, the item is inserted into the document literally, i.e. it is not encoded as text, but becomes part of the output structure.
The XML() and document() functions can be used to turn a string into structured content and to retrieve an XML document from a URL, respectively.
Note that attribute values MUST NOT reference structured content. This applies to py:attrs and using ${} substitution in attribute values.
- sequence
- If a sequence type (list, tuple, or other iterable) is referenced, the rules are applied to each of the items in the sequence. For example, you could reference a list containing an Element and a string.
- Other
- If the result of evaluating the expression is any other type, an attempt is made to coerce the value to unicode as if by calling unicode(expr) and processing continues as if the object were a string or unicode object initially.
Attributes not belonging to the Kid namespace and text content MAY embed Python expressions by enclosing the expression within a dollar sign followed by curly braces: ${expr}. The result of evaluating the expression(s) is substituted with the rest of the attribute value or text content following rules defined for Content Producing Constructs.
<?python
verb = 'ran'
noun = 'store'
?>
<a title="I ${verb} to the ${noun}">...
... would result in:
<a title="I ran to the store">...
If an attribute value consists purely of substitution expressions and all expressions evaluate to None, the attribute is removed. This can be avoided by using expr or '' to force a zero length string to be returned instead of None. For example:
<?python
# set something to None
x = None
?>
<a title="${x}">...
... would result in:
<a>...
However, this:
<?python x = None?>
<a title="${x or ''}">...
... results in:
<a title="">...
For simple expressions consisting entirely variable names and object access operators (.), the curly braces may be omitted:
<a href="http://example.com/$page" title="$title">
Dots are allowed too: $object.another.attribute
</a>
However, it is good practice to use the curly brace form as it sets the substitution off from the other text a bit more providing a stronger visual clue as to what's going on.
$$ is an escape. $${bla} will output ${bla}.
All templates have a few default imports for convenience.
Expression substitution, py:content, and py:replace encode strings as text. That is, text is encoded according to the rules of the XML specification, which includes, among other things, replacing the literal characters < and & with their encoded counterparts (< &). If you have XML stored as a string and want it to be output as XML and not encoded text, you need to pass the string to the XML function.
For example, let's say there is a function, hello, that returns XML data that should be embedded in template output (let's say it returns <hello>world</hello>). Consider the following:
<p>${hello()}</p>
The result would be:
<p><hello>world<hello></p>
Calling the XML function would have given us the result we intended:
<p>${XML(hello())}</p>
<p><hello>world</hello></p>
The document function loads an XML document from a file or URL allowing it to be embedded in template output:
<div py:content="document('header.html')"></div>
The document function resolves paths relative to the current template file (if the template location is available).
<element py:for="target_list in expression_list" />
Works exactly like the Python for statement.
The py:for attribute may appear on any element to signify that the element should be processed multiple times, once for each value in the sequence specified:
<?python
bottles = range(1, 101)
bottles.reverse()
?>
<p py:for="num in bottles">
<span py:content="num">X</span> bottles of beer on the wall,
<span py:content="num">X</span> bottles of beer on the wall,
take one down, pass it around, <span py:content="num - 1">X - 1</span>
bottles of beer on the wall.
</p>
The py:for attribute is the first attribute to be processed if present. All other py: attributes are processed for each iteration of the loop.
<element py:if="expr" />
The py:if attribute may appear on any element to signify that the element and its decendant items should be output only if the boolean expression specified evaluates to true in Python:
<p py:if="5 * 5 == 25">
Python seems to be handling multiplication okay.
</p>
The py:if attribute is processed after the py:for attribute and is evaluated for each iteration. If the result of evaluating expr as a boolean expression is false in Python, no further py: attributes are processed for the current iteration or, if not in a py:for, at all.
Note
Evaluated as a boolean expression in Python, None, False, [], (), {}, 0, and '' are all considered to be false.
<element py:content="expr" />
This attribute MAY appear on any element to signify that the decendant items of the element are to be replaced with the result of evaluating expr.
<p py:content="time.strftime('%C %c')">The Time</p>
Results in:
<p>Tues, Jun 26, 2004 02:03:53 AM</p>
py:content is a Content Producing Construct and can output both character and structured data.
<element py:replace='expr' />
py:replace is shorthand for specifying a py:content and a py:strip="True" on the same element:
<?python
x = 10
?>
<p><span py:replace="x">...</span></p>
... results in:
<p>10</p>
... and is equivelant to specifying:
<?python #
x = 10
?>
<p><span py:strip="" py:content="x">...</span></p>
The py:replace attribute is processed after the py:for and py:if attributes. py:strip and py:content attributes are not processed and are discarded.
py:replace is a Content Producing Construct and can output both character and structured data.
<element py:attrs="expr" />
The py:attrs attribute may appear on any element to specify a set of attributes that should be set on the element when it is processed. The expression specified MUST evaluate to one of the following types of values:
- dict
- A dictionary with keys specifying attribute names and values specifying attribute values. These are added to the attributes of the current element by calling element.attrib.update(mapping), where element is an ElementTree Element object and mapping is the dictionary returned from the expression. Outer curly braces are not necessary to write down.
- list
- A list of tuples of the form (name, value) is also acceptable. Each item of the list is added to the current set of attributes by iterating over the list and calling element.set(name, value).
- keyword arguments
- The attributes can also be specified as comma separated keyword arguments of the form name=value.
The following lines:
<elem py:attrs="{'a':1, 'ns:b':2}" />
<elem py:attrs="'a':1, 'ns:b':2" />
<elem py:attrs="(('a',1), ('ns:b',2))" />
<elem py:attrs="a=1, ns:b=2" />
will all produce the same output:
<elem a="1" ns:b="2" />
Note that attributes whose values are None will be removed. If a blank attribute is desired, an empty string should be used.
If the expression specified is an empty dictionary or an empty list, the attributes are not modified in any way.
py:attrs is a Content Producing Construct, but can output only character data.
<element py:def="template_name(arg_list)" />
The py:def attribute may appear on any element to create a "Named Template Function". Markup contained within an py:def element is not output during normal template expansion but can be referenced from other Content Producing Constructs to insert the markup at the point referenced.
Like normal Python functions, Named Template Functions have an optional argument list that may use all of the jazzy features of Python argument lists like variable and keyword arguments.
Named Template Functions are invoked exactly like normal Python functions. They are generally invoked from Content Producing Constructs like py:content or ${} substitution.
<ul py:def="display_list(seq)">
<li py:for="item in seq" py:content="item" />
</ul>
<table py:def="display_dict(mapping)">
<tr>
<th>Key</th>
<th>Value</th>
</tr>
<tr py:for="key, value in mapping.items()">
<td py:content="key" />
<td py:content="value" />
</tr>
</table>
Here we've defined two Named Template Functions: display_list and display_dict. The first function takes a sequence and the second a mapping. We can invoke these functions from the same template by invoking them from a content producing construct:
<body>
${display_list(['apple', 'orange', 'kiwi'])}
<div py:replace="display_dict({'x' : 'y', 'p' : 'q'})">
Key/Value Table replaces this text
</div>
</body>
<element py:match="expr" />
The py:match attribute may appear on any element to create a "Match Template". Markup contained within a Match Template element is not output during normal template expansion. Instead, these constructs set up filters for expansion output that are capable of transforming content as it is generated.
Match Templates are generally used to insert content dynamically based on patterns in template expansion or to provide "custom tag" functionality similar to that found in JSP taglibs or XSLT.
A Match Template has two parts: the match expression part (expr) and the body part (the element and it's descendants).
Match Templates are processed as follows:
- Each element that is output from a template goes through the Match Template Filter.
- The Match Template Filter visits each of the Match Templates defined in the current template and the templates the current template extends in the order that they are defined and evaluates the associated match expression.
- If the match expression returns true as a boolean expression, the match template's body is expanded and replaces the original element and all of its descendants.
In both the match expression and in the match template's body, the item name is bound to the Element that is being output. However, there are some limitations to what can be accessed at each phase:
- During match expression evaluation, only the item Element and none of its descendants are available. This means that match expressions are limited to testing matches based on the immediate Element's tag and attributes .
- During match template expansion (that is, when the match expression is true), the element's descendants are available and may be referenced from Content Producing Constructs to output bits and pieces of the matched items structure.
The following simple example shows how to create a custom tag <greeting> that outputs one of two provided values based on the time of day the template is expanded:
<?xml version="1.0" encoding="utf-8"?>
<?python
from time import localtime
def timeofday():
"""Get time of day ('am' or 'pm')"""
return localtime().tm_hour < 12 and 'am' or 'pm'
?>
<html xmlns:py="http://purl.org/kid/ns#">
<!-- define the greeting match template -->
<span py:match="item.tag == 'greeting'"
py:replace="item.get(timeofday())">
</span>
<head>
<title>Time of day demo</title>
</head>
<body>
<p>
Good <greeting am="Morning!" pm="Afternoon" />
</p>
</body>
</html>
An important thing to note is that the py:match expression and the match template body have access to the <greeting> element via the variable item. The item.get(timeofday()) bit retrieves the value of the am attribute or the pm attribute based on what is returned from the timeofday function.
At 9:00 AM, output from this template would look like this:
<html>
<head>
<title>Time of day demo</title>
</head>
<body>
<p>
Good Morning!
</p>
</body>
</html>
The obvious question at this point is how to reuse Match Templates? The example above demonstrates the use of a Match Template from the same main template but it is often desirable to have "libraries" of Match Templates that could be used by multiple individual templates. The answer is to have the main template extend a common template containing the Match Templates needed. We can rewrite the above example as two separate templates: main.kid and common.kid.
The common template would look like this:
<?xml version="1.0" encoding="utf-8"?>
<?python
from time import localtime
def timeofday():
"""Get time of day ('am' or 'pm')"""
return localtime().tm_hour < 12 and 'am' or 'pm'
?>
<html xmlns:py="http://purl.org/kid/ns#">
<!-- define the greeting match template -->
<span py:match="item.tag == 'greeting'"
py:replace="item.get(timeofday())">
</span>
</html>
And the main template would look like this:
<?xml version="1.0" encoding="utf-8"?>
<html py:extends="'common.kid'">
<head>
<title>Time of day demo</title>
</head>
<body>
<p>
Good <greeting am="Morning!" pm="Afternoon" />
</p>
</body>
</html>
When a template extends another template (or set of templates), all of the Match Templates and Named Template Functions of the extended templates are available as if they were defined locally.
Warning
Match templates are an experimental feature. Syntax and semantics may change significantly or be removed entirely in future release. Actually, this statement applies to many aspects of Kid but this one is especially unstable.
<root py:extends="template1, template2, ...">
The py:extends attribute may appear on the root element to specify that the template should inherit the Named Template Functions and Match Templates defined in another template (or set of templates). If a py:extends attribute is specified, it MUST be on the root element of the document.
The py:extends may contain a list of Python expressions separated by commas that reference templates. The rules for what types of values may be specified are:
- string
-
The name of a template file, relative to the current template file.
Example:
<html py:extends="'common.kid'" />
- module or Template class
-
The py:extends variable references a module or a Template class. If a module is referenced, an attempt is made to find a class named Template belonging to the that module.
Example:
<?python
import common
?>
<html py:extends="common" ...
Multiple templates may be referenced by separating each by a comma. The following example references templates common and forms, imported using the import hooks and a template filename named other.kid:
<?python
import common, forms
?>
<html py:extends="common, forms, 'other.kid'" ...
For example, there is a template named common.kid that defines a template function, display_errors, and a match template that converts <b> elements to <strong> elements with uppercase content:
<html xmlns:py="http://purl.org/kid/ns#">
<ul py:def="display_errors(errors)">
<li py:for="error in errors" py:content="error" />
</ul>
<strong py:match="item.tag == 'b'"
py:content="item.text.upper()" />
</html>
The functions and match templates may be imported into another template by referencing them with py:extends:
<html py:extends="'common.kid'"
xmlns:py="http://purl.org/kid/ns#">
<head>
<title>Errors</title>
</head>
<body>
<p>The following <b>errors</b> were found:</p>
${ display_errors(["Field is required", "Must be phone number.."]) }
</body>
</html>
The <b>errors</b> item is transformed to <strong>ERRORS</strong> and the error list is displayed. Both the match template and the named template function are available in the derived template as if they were defined locally.
The order that py: attributes are processed is as follows:
- py:def
- py:match
- py:for
- py:if
- py:replace
- py:strip
- py:attrs
- py:content
Attribute substitution occurs after all other
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