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提交 d5ee17ed 编写于 作者: M Mislav Marohnić
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improve Active Record README 

 - revise introductory text
 - improve examples
 - don't claim that Oracle, SQL Server, or DB2 are supported
 - remove lengthy "simple example"
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activerecord/README.rdoc
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= Active Record -- Object-relation mapping put on rails
= Active Record -- Object-relational mapping put on rails
Active Record connects business objects and database tables to create a persistable
domain model where logic and data are presented in one wrapping. It's an implementation
of the object-relational mapping (ORM) pattern[http://www.martinfowler.com/eaaCatalog/activeRecord.html]
by the same name as described by Martin Fowler:
Active Record connects classes to relational database tables to establish an
almost zero-configuration persistence layer for applications. The library
provides a base class that, when subclassed, sets up a mapping between the new
class and an existing table in the database. In context of an application,
these classes are commonly referred to as *models*. Models can also be
connected to other models; this is done by defining *associations*.
"An object that wraps a row in a database table or view, encapsulates
the database access, and adds domain logic on that data."
Active Record relies heavily on naming in that it uses class and association
names to establish mappings between respective database tables and foreign key
columns. Although these mappings can be defined explicitly, it's recommended
to follow naming conventions, especially when getting started with the
library.
Active Record's main contribution to the pattern is to relieve the original of two stunting problems:
lack of associations and inheritance. By adding a simple domain language-like set of macros to describe
the former and integrating the Single Table Inheritance pattern for the latter, Active Record narrows the
gap of functionality between the data mapper and active record approach.
A short rundown of the major features:
A short rundown of some of the major features:
* Automated mapping between classes and tables, attributes and columns.
class Product < ActiveRecord::Base; end
class Product < ActiveRecord::Base
end
...is automatically mapped to the table named "products", such as:
The Product class is automatically mapped to the table named "products",
which might look like this:
CREATE TABLE products (
id int(11) NOT NULL auto_increment,
name varchar(255),
PRIMARY KEY (id)
);
...which again gives Product#name and Product#name=(new_name)
This would also define the following accessors: `Product#name` and
`Product#name=(new_name)`
{Learn more}[link:classes/ActiveRecord/Base.html]
* Associations between objects controlled by simple meta-programming macros.
* Associations between objects defined by simple class methods.
class Firm < ActiveRecord::Base
has_many :clients
has_one :account
belongs_to :conglomorate
belongs_to :conglomerate
end
{Learn more}[link:classes/ActiveRecord/Associations/ClassMethods.html]
* Aggregations of value objects controlled by simple meta-programming macros.
* Aggregations of value objects.
class Account < ActiveRecord::Base
composed_of :balance, :class_name => "Money",
......@@ -65,23 +68,19 @@ A short rundown of the major features:
end
{Learn more}[link:classes/ActiveRecord/Validations.html]
* Callbacks as methods or queues on the entire lifecycle (instantiation, saving, destroying, validating, etc).
class Person < ActiveRecord::Base
def before_destroy # is called just before Person#destroy
CreditCard.find(credit_card_id).destroy
end
end
class Account < ActiveRecord::Base
after_find :eager_load, 'self.class.announce(#{id})'
* Callbacks available for the entire lifecycle (instantiation, saving, destroying, validating, etc.)
class Person < ActiveRecord::Base
before_destroy :invalidate_payment_plan
# the `invalidate_payment_plan` method gets called just before Person#destroy
end
{Learn more}[link:classes/ActiveRecord/Callbacks.html]
* Observers for the entire lifecycle
* Observers that react to changes in a model
class CommentObserver < ActiveRecord::Observer
def after_create(comment) # is called just after Comment#save
......@@ -122,40 +121,24 @@ A short rundown of the major features:
{Learn more}[link:classes/ActiveRecord/Reflection/ClassMethods.html]
* Direct manipulation (instead of service invocation)
* Database abstraction through simple adapters
So instead of (Hibernate[http://www.hibernate.org/] example):
# connect to SQLite3
ActiveRecord::Base.establish_connection(:adapter => "sqlite3", :database => "dbfile.sqlite3")
long pkId = 1234;
DomesticCat pk = (DomesticCat) sess.load( Cat.class, new Long(pkId) );
// something interesting involving a cat...
sess.save(cat);
sess.flush(); // force the SQL INSERT
# connect to MySQL with authentication
ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "localhost",
:username => "me",
:password => "secret",
:database => "activerecord"
)
Active Record lets you:
pkId = 1234
cat = Cat.find(pkId)
# something even more interesting involving the same cat...
cat.save
{Learn more}[link:classes/ActiveRecord/Base.html]
* Database abstraction through simple adapters (~100 lines) with a shared connector
ActiveRecord::Base.establish_connection(:adapter => "sqlite", :database => "dbfile")
ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "localhost",
:username => "me",
:password => "secret",
:database => "activerecord"
)
{Learn more}[link:classes/ActiveRecord/Base.html#M000081] and read about the built-in support for
MySQL[link:classes/ActiveRecord/ConnectionAdapters/MysqlAdapter.html], PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], SQLite[link:classes/ActiveRecord/ConnectionAdapters/SQLiteAdapter.html], Oracle[link:classes/ActiveRecord/ConnectionAdapters/OracleAdapter.html], SQLServer[link:classes/ActiveRecord/ConnectionAdapters/SQLServerAdapter.html], and DB2[link:classes/ActiveRecord/ConnectionAdapters/DB2Adapter.html].
{Learn more}[link:classes/ActiveRecord/Base.html] and read about the built-in support for
MySQL[link:classes/ActiveRecord/ConnectionAdapters/MysqlAdapter.html],
PostgreSQL[link:classes/ActiveRecord/ConnectionAdapters/PostgreSQLAdapter.html], and
SQLite3[link:classes/ActiveRecord/ConnectionAdapters/SQLite3Adapter.html].
* Logging support for Log4r[http://log4r.sourceforge.net] and Logger[http://www.ruby-doc.org/stdlib/libdoc/logger/rdoc]
......@@ -169,11 +152,11 @@ A short rundown of the major features:
class AddSystemSettings < ActiveRecord::Migration
def self.up
create_table :system_settings do |t|
t.string :name
t.string :label
t.text :value
t.string :type
t.integer :position
t.string :name
t.string :label
t.text :value
t.string :type
t.integer :position
end
SystemSetting.create :name => "notice", :label => "Use notice?", :value => 1
......@@ -186,110 +169,15 @@ A short rundown of the major features:
{Learn more}[link:classes/ActiveRecord/Migration.html]
== Simple example (1/2): Defining tables and classes (using MySQL)
Data definitions are specified only in the database. Active Record queries the database for
the column names (that then serves to determine which attributes are valid) on regular
object instantiation through the new constructor and relies on the column names in the rows
with the finders.
# CREATE TABLE companies (
# id int(11) unsigned NOT NULL auto_increment,
# client_of int(11),
# name varchar(255),
# type varchar(100),
# PRIMARY KEY (id)
# )
Active Record automatically links the "Company" object to the "companies" table
class Company < ActiveRecord::Base
has_many :people, :class_name => "Person"
end
class Firm < Company
has_many :clients
def people_with_all_clients
clients.inject([]) { |people, client| people + client.people }
end
end
The foreign_key is only necessary because we didn't use "firm_id" in the data definition
class Client < Company
belongs_to :firm, :foreign_key => "client_of"
end
# CREATE TABLE people (
# id int(11) unsigned NOT NULL auto_increment,
# name text,
# company_id text,
# PRIMARY KEY (id)
# )
Active Record will also automatically link the "Person" object to the "people" table
class Person < ActiveRecord::Base
belongs_to :company
end
== Simple example (2/2): Using the domain
Picking a database connection for all the Active Records
ActiveRecord::Base.establish_connection(
:adapter => "mysql",
:host => "localhost",
:username => "me",
:password => "secret",
:database => "activerecord"
)
Create some fixtures
firm = Firm.new("name" => "Next Angle")
# SQL: INSERT INTO companies (name, type) VALUES("Next Angle", "Firm")
firm.save
client = Client.new("name" => "37signals", "client_of" => firm.id)
# SQL: INSERT INTO companies (name, client_of, type) VALUES("37signals", 1, "Firm")
client.save
Lots of different finders
# SQL: SELECT * FROM companies WHERE id = 1
next_angle = Company.find(1)
# SQL: SELECT * FROM companies WHERE id = 1 AND type = 'Firm'
next_angle = Firm.find(1)
# SQL: SELECT * FROM companies WHERE id = 1 AND name = 'Next Angle'
next_angle = Company.find(:first, :conditions => "name = 'Next Angle'")
next_angle = Firm.find_by_sql("SELECT * FROM companies WHERE id = 1").first
The supertype, Company, will return subtype instances
Firm === next_angle
All the dynamic methods added by the has_many macro
next_angle.clients.empty? # true
next_angle.clients.size # total number of clients
all_clients = next_angle.clients
Constrained finds makes access security easier when ID comes from a web-app
# SQL: SELECT * FROM companies WHERE client_of = 1 AND type = 'Client' AND id = 2
thirty_seven_signals = next_angle.clients.find(2)
Bi-directional associations thanks to the "belongs_to" macro
thirty_seven_signals.firm.nil? # true
== Philosophy
Active Record is an implementation of the object-relational mapping (ORM)
pattern[http://www.martinfowler.com/eaaCatalog/activeRecord.html] by the same
name described by Martin Fowler:
== Philosophy
"An object that wraps a row in a database table or view,
encapsulates the database access, and adds domain logic on that data."
Active Record attempts to provide a coherent wrapper as a solution for the inconvenience that is
object-relational mapping. The prime directive for this mapping has been to minimize
......
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