[Rails] Associations between models

has_many/has_one vs belongs_to inverse_of dependent Related Articles

has_many/has_one vs belongs_to

class Book < ActiveRecord::Base
   belongs_to :subject
end

class Subject < ActiveRecord::Base
   has_many :books
end

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Also, there is has_one just like has_many

inverse_of

Active Record provides the :inverse_of option so you can explicitly declare bi-directional associations:

class Author < ApplicationRecord
  has_many :books, inverse_of: 'writer'
end

class Book < ApplicationRecord
  belongs_to :writer, class_name: 'Author', foreign_key: 'author_id'
end

a = Author.first
b = a.books.first
a.first_name == b.writer.first_name
=> true
a.first_name = 'David'
a.first_name == b.writer.first_name
=> true

dependent

If you set the :dependent option to:

:destroy, when the object is destroyed, destroy will be called on its associated objects.

:delete, when the object is destroyed, all its associated objects will be deleted directly from the database without calling their destroy method.

:destroy_async: when the object is destroyed, an ActiveRecord::DestroyAssociationAsyncJob job is enqueued which will call destroy on its associated objects. Active Job must be set up for this to work.

https://guides.rubyonrails.org/association_basics.html