CODEWITHSUNDEEP
ruby-on-railsdatabaseactiverecordrails-activerecord
mferg
mferg

Reputation: 49

Active Record Associations- One Model can belong to different models at the same time

What is the correct way to create active record relationships for a scenario where-

One instance of a Model A can belong to multiple of instances of two other models, Model B and Model C at the same time.

Model B can only have one Model A.

Model C can have many Model A's.

An example is a job board where an employer is hiring for one position and candidate is applying and has many previous positions. I want both the candidate and employer positions to be from the same model.

Position Mode (Model A) can belong to both employers and candidates at the same time

Employer Model (Model B) has_one position

Candidate Model (Model C) has_many positions

Upvotes: 2

Views: 2197

Answers (1)

Jen
Jen

Reputation: 458

Here's one way of accomplishing what you described in the question:

class Position < ActiveRecord::Base
  belongs_to :candidate
  belongs_to :employer
end

class Employer < ActiveRecord::Base
  has_many :positions
end

class Candidate < ActiveRecord::Base
  has_many :positions
ens

Note that a position can only belong to one employer and one candidate at a time; this is kind of nonsensical if you are creating a job board - the relationship between position and employer is probably correct, but candidates might have many applications, and an application would belong_to a position, while a position has_many applications. Or possibly also "favorites" so candidates could keep track of a list of positions to apply to.

The "theoretical" example that you started with:

  • One instance of a Model A can belong to multiple of instances of two other models, Model B and Model C at the same time.
  • Model B can only have one Model A.
  • Model C can have many Model A's.

Code:

class ModelA < ActiveRecord::Base
  has_many :model_bs
  has_many :model_c_relationships # join table!
  has_many :model_cs, through: :model_c_relationships
end

class ModelB < ActiveRecord::Base
  belongs_to :model_a
end

class ModelCRelationship < ActiveRecord::Base
  belongs_to :model_a
  belongs_to :model_c
end

class ModelC < ActiveRecord::Base
  has_many :model_c_relationships
  has_many :model_as, through: :model_c_relationships
end

If there is a situation where one type of object can belong to one other object, but that other object can be one of several types, then it might need a polymorphic association. For example, StackOverflow allows you to comment on both questions and answers. A comment can only belong to one comment-able parent object, therefore the code might look like this:

class Comment < ActiveRecord::Base
  belongs_to :commentable, polymorphic: true
end

class Question < ActiveRecord::Base
  has_many :comments, as: :commentable
  has_many :answers
end

class Answer < ActiveRecord::Base
  belongs_to :question
  has_many :comments, as: :commentable
end

Upvotes: 2

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