Association Examples in Rails

association_models.rb

1.belongs_to association:
---------------------

  - Student and College:
  - a student belongs_to college
  - college_id is present in the college
  - Example:
  **************
    class Student < ApplicationRecord
      belongs_to :college
    end

    migration file
    **************
    add_column :students, :college_id, :integer

2.has_many association:
---------------------

  - student and college
  - college has many students
  - Example:
  **************
    class College < ApplicationRecord
      has_many :students
    end

3. has_one association:
-----------------------

  - student and profile
  - each student has a profile.
  - Example
  **************
    class Student < ApplicationRecord
      has_one :profile
    end

    class Profile < ApplicationRecord
      belongs_to :student
    end
  - student_id in profiles table.

4. has_many through association:
-------------------------------

- very important association
- many students applied for many companies
- many companies receives application from many students
- Example
**********
  class student < ApplicationRecord
    has_many :applications
    has_many :companies, through: applications
  end

  class Company < ApplicationRecord
    has_many :applications
    has_many :students, through: applications
  end

  class Application < ApplicationRecord
    belongs_to :student
    belongs_to :company
  end
  
- here the applications table contain the student_id and company_id

5. has_one :through association
-------------------------------

- here student has a profile and profile_history
- here one to one association
- Example:
************

  class Student < ApplicationRecord
    has_one :profile_history
    has_one :profile, through: profile_history
  end

  class Profile < ApplicationRecord
    has_one :profile_history
    has_one :student, through: profile_history
  end

  class ProfileHistory < ApplicationRecord
    belongs_to :profile
    belongs_to :student
  end


6. has_and_belongs_to_many
--------------------------

- rarely used
- Example:
***********
  class Student < ApplicationRecord
    has_and_belongs_to_many :companies
  end

  class Company < ApplicationRecord
    has_and_belongs_to_many :students
  end
- Create a table Application or adding a reference keys student_id, company_id to Application table.