多态，has_many_and_belongs_to_many自参照关联

Question

I want to join two models with a polymorphic many to many association.

My table are Parents and Kids, and can become friends with each other. To achieve this, I want to create a Friend association table, such as Parents or Kids can become friends with other Parents or Kids

I read a few tutorial, covering the has_many, has_many through, and polymorphic associations, but nothing yet that could mix the two features together.

I tried the following:

Friend table

  t.integer :me
  t.string :me_type
  t.integer :him
  t.string :him_type

Kid model

class Kid < ActiveRecord::Base
  belongs_to :parent
  has_many :friends, as: :me, polymorphic: true
  belongs_to :friends, as: :you, polymorphic:true

Parent model

class Parent < ActiveRecord::Base
  has_many :friends, as: :me, polymorphic: true
  belongs_to :friends, as: :you, polymorphic:true

However, I'm stuck on how to define friend model. Any hint on how to define this relationship in rails ?

Answer 1

Try next associations,

Kid model

class Kid < ActiveRecord::Base
  belongs_to :parent
  has_many :my_friends,
    :as => :me,
    :class_name => "Friend"
  has_many :their_friends,
    :as => :you,
    :class_name => "Friend"
end

Parent model

class Parent < ActiveRecord::Base
  has_many :my_friends,
    :as => :me,
    :class_name => "Friend"
  has_many :their_friends,
    :as => :you,
    :class_name => "Friend"
end

Friend model

class Friend < ActiveRecord::Base
  belongs_to :me,
    :polymorphic => true
  belongs_to :you,
    :polymorphic => true
end

Answer 2

Another approach

class Friend < ActiveRecord::Base
  belongs_to :friendable, :polymorphic => true 
  belongs_to :parent
  belongs_to :kid
  belongs_to :...
end

Then, in each friend type (parent, child, cousin, etc.) model, add the relationship. For example, in your parent model

  # DB setup connection between parent and friends, 
  # Friend is polymorphic so id and type must match, 
  has_many :passive_friends, class_name:  "Friend",
                             foreign_key: "friend_id",
                             dependent:   :destroy
  # Controller setup for DB access through model
  has_many :friends, through: :passive_friends, 
                              source: :friendable,
                              source_type: 'Parent'

And in your kid model

# DB setup connection between kids and friends, 
# Friend is polymorphic so id and type must match, 
has_many :passive_friends, class_name:  "Friend",
                           foreign_key: "friend_id",
                           dependent:   :destroy
# Controller setup for DB access through model
has_many :friends, through: :passive_friends, 
                           source: :friendable,
                           source_type: 'Kid'

---

Then you can do things like

Mr. Smith has [<%= parent.friends.count.to_s %>] friends.

And it will include all friends of all types in the total.

(You may not want the dependent destroy param, but you should delete the friend record when the relationship is deleted.)