rails中模型的关联关系 一对一、一对多、多对多及named_scope

1、1对1关系的实现  book与user

model层

 

book.rb

      belongs_to:user

 

 

user.rb

       has_one :book         #体现两者之间的1对1关系

 

db/migrate层

book.rb

  def self.up

    create_table :books do |t|

      t.integer :user_id     #存储 user的 id，

      t.string :title

      t.string :author

      t.timestamps

    end

  end

 

user.rb

  def self.up

    create_table :users do |t|

      t.string :name

      t.timestamps

    end

  end

 

例：

u= User.new

u.id= 2

u.save

 

b=Book.last

u.book= b

 

此时，b.user_id = 2

 

2、 单表继承   admin单表继承user

model层

admin.rb

classAdmin < User

  def im

    p "i am an admin"

  end

end

 

user.rb

classUser < ActiveRecord::Base

  def im

    p "i am an user"

  end 

end

 

db/migrate层

admin.rb是不需要的

  user.rb

    create_table :users do |t|

      t.string :name

      t.string :type                    #type为保留字，用于单表继承

      t.timestamps

    end

例：

 a = Admin.new

 a

  #<Admin id: nil, name: nil, type:"Admin", created_at: nil, updated_at: nil>

#a的type字段为"Admin"，此条记录仍保存在User表中

 

3、 1对多关系 book与department

 

model层

book.rb

        belongs_to:department

 

department.rb

       has_many:books

 

db/migrate层

book.rb

  def self.up

    create_table :books do |t|

      t.integer :department_id

      t.string :title

      t.string :author

      t.timestamps

    end

  end

 

department.rb

  def self.up

    create_table :departments do |t|

      t.string :name

      t.timestamps

    end

  end

end

 

例：

d =Department.new

d.name=”销售”

d.save

b = Book.new

b.title = “zh”

b.save

d.books <<b

 

此时，b.department_id= d.id

 

4、多对多的关系  book与classfy

 

model层

book.rb

  has_many :manifests

  has_many :classifies, :through =>:manifests, :source => :classify

 

classfy.rb

  has_many :manifests

  has_many :books, :through => :manifests,:source => :book

 

多对多的关系在第三个文件中体现   manifest.rb

  belongs_to :book

  belongs_to :classify

 

db/migrate层

book.rb

create_table:books do |t|

      t.string :title

      t.string :author

      t.timestamps

    end

 

classify.rb

    create_table :classifies do |t|

      t.string :kind

      t.timestamps

    end

 

manifest.rb            #此文件中需要存储有多对多关系对象的id

    create_table :manifests do |t|

      t.integer :book_id

      t.integer :classify_id

    end

 

c= Classify.new

 c.kind = "reading"

>>c.save

 

c= Classify.new

 c.kind = "science"

>>c.save

book表为

 

id           title

1             ch

2                 en

3                 jap

 

 b = Book.first

 c  =Classify.last

 c.books << b     #建立book与 Classify之间的关系

 

manifest表为：

id   book_id   Classify_id

1        1                  2

 

b= Book.last

b.classifies  << c

manifest表为：

id   book_id   Classify_id

1        1                  2

2         3                     2

 

 

5、多态    book、user、department

 

model层

book.rb

belongs_to:borrowed,:polymorphic => true

 

department.rb

has_many:books,:as => :borrowed, :dependent=> :destroy   # dependent的含义是如果book消失，则department所对应的对象消失

  def im

    p "i am an department"

  end

 

use.rb

has_many:books,:as => :borrowed, :dependent => :destroy

defim

    p "i am an user"

  end

db/migrate层

book.rb

  create_table :books do |t|

      t.integer :category_id

      t.integer :user_id

      t.string :title

      t.string :author

      t.string :borrowed_type

      t.integer :borrowed_id

      t.integer :department_id

      t.timestamps

    end

 

例：

u= User.first

 d =Department.first

u.im          #"i am an user"

d.im        #"i am an department"

 

 

u. books =  Book.last       #此时book表中Book.last  所对应对象的borrowed_type更新为user，borrowed_id更新为u. id

d.books = Book.first       #此时book表中Book.first 所对应对象的borrowed_type更新为department，borrowed_id更新为d. id

 

 

6、named_scope(用类名来调用)

 

model层

book.rb

  named_scope :name，:conditions => “SQL 语句”, order=>“SQL 语句”,  group=>”字段名”

 

  named_scope :unborrowed ,:conditions =>"borrowed_id is null",:order =>"title desc"

  named_scope :all_borrowed_type ,:group =>"borrowed_type"

 

例：

Book.unborrowed    #查出 borrowed_id字段为 null的对象，并按 title的降序排列