新建Model

建立一个新的Model

rails g model category

请注意产生的xxxxx_create_categories.rb文件，这个文件记录了categories表的定义，我们可以给表增加几个列

class CreateCategories < ActiveRecord::Migration
  def change
    create_table :categories do |t|
      t.string :name
      t.integer :position
      t.timestamps
     end

    add_column :events, :category_id, :integer
    add_index :events, :category_id
  end
end

我们同时给events加上一个外键，让这两张表之间可以关联起来。
新建数据表：

rake db:migrate

Model关联

一对多关联one-to-many

activerecord-relationships-has_many_diagram.png

在上图中一个Event可以有多个Attendee，我们新建attendee Model

rails g model attendee name:string event_id:integer

生成数据表attendees

rake db:migrate

分别编辑event.rb和attendee.rb建立关联关系

class Event < ActiveRecord::Base
  has_many :attendees,  :class_name=>"Attendee", :foreign_key=>'event_id'
end

class Attendee < ActiveRecord::Base
  belongs_to :event, :class_name=>'Event', :foreign_key=>'event_id'
end

范例一、建立Attendee对象并关联到Event上：

e = Event.first
a = Attendee.new(:name => 'ihower', :event => e)
# a = Attendee.new(:name => 'ihower', :event_id => e.id)
a.save
e.attendees   #这是个数组
Attendee.first.event

范例二、从Event对象中建立一个Attendee

e = Event.first
a = e.attendees.build(:name => 'ihower')
a.save

范例三、从Event对象中建立一个Attendee，并直接存到数据库

e = Event.first
a = e.attendees.create(:name => 'ihower')

范例四、先建立Attendee再放入Event中

e = Event.first
a =  Attendee.create(:name => 'ihower')
e.attendees << a

范例五、删除

e = Event.first
e.attendees.destroy_all  #一个个删除e的attendees，并触发attendee的destroy回调
e.attendees.delete_all #一次性删除e的attendees，不会触发attendee的destroy回调

一对一关联 one-to-one

activerecord-relationships-has_one_diagram.png

假设每一个event都有一个location，增加一个Location Model，其中event_id就是外键

rails g model location name:string event_id:integer

然后执行rake db:migrate产生数据表.
分别编辑event.rb和location.rb

class Event < ActiveRecord::Base
    has_one :location, :class_name=>'Location', :foreign_key=>'event_id'
end

class Location < ActiveRecord::Base
    belongs_to :event, :class_name=>'Event', :foreign_key=>'event_id'
end

范例一、建立Location对象并关联到Event

e = Event.first
l = Location.new(:name=>'Hsinchu', :event=>e)
#l = Location.new(:name=>'Hsinchu', :event_id=>e.id)
l.save
e.location
l.event

范例二、从Event对象建立一个Location

e = Event.first
l = e.build_location(:name=>'Hsinchu')
l.save

范例三、直接从Event对象中建立一个Location

e = Event.first
l = e.create_location(:name=>'Hsinchu')

多对多关联many-to-many

activerecord-relationships-many_to_many_1_diagram.png

一个Event有多个Group，一个Group有多个Event。我们可以用一个额外的Model来建立关联。
我们可以建立如下的model

rails g model group name:string
rails g model event_groupship event_id:integer group_id:integer

执行rake db:migrate产生这连个数据表。
分别编辑event.rb、group.rb和event_groupship.rb文件

class Event < ActiveRecord::Base
    has_many :event_groupships
    has_many :groups, :through => :event_groupships
end

class EventGroupship < ActiveRecord::Base
    belongs_to :event
    belongs_to :group
end

class Group < ActiveRecord::Base
    has_many :event_groupships
    has_many :events, :through => :event_groupships
end

范例、建立双向关联

g = Group.create(:name=>'ruby tw')
e1 = Event.first
e2 = Event.create(:name=>'ruby tuesday')
EventGroupship.create(:event=>e1, :group=>g)
EventGroupship.create( :event => e2, :group => g )
g.events
e1.groups
e2.groups

不通过第三方Model建立多对多关联

rails g model group name:string
rails g migration create_table_event_groupship

编辑xxxx_ create_table_event_groupship.rb文件

class CreateTrainTeacher < ActiveRecord::Migration  
def up
    create_table :event_groupships do |t|
      t.integer :event_id
      t.integer :group_id
    end
  end
  def down
    drop_table :event_groupships
  end
end

执行rake db:migrate生成数据库表
编辑event.rb和group.rb文件

class Event < ActiveRecord::Base
  has_and_belongs_to_many :group, :class_name=>'Group', :foreign_key=>'event_id',
  :association_foreign_key=>'group_id', :join_table=>'event_grouphips'
end

class Group < ActiveRecord::Base
 has_and_belongs_to_many :event, :class_name=>'Event', :foreign_key=>'group_id',
 :association_foreign_key=>'event_id', :join_table=>'event_grouphips'
end

关联参数

order指定顺序

class Event < ActiveRecord::Base
  has_many :attendees, ->{ order("id DESC") }
end

串联where条件

class Event < ActiveRecord::Base
  has_many :attendees, ->{ where(["created_at > ?", Time.now - 7.day]).order("id DESC") }
end

删除依赖数据

class Event < ActiveRecord::Base
  has_one :location, :dependent => :destroy
  has_many :attendees, :dependent => :destroy
end