codeclimate/codeclimate-rubocop

This cop looks for has_(one|many) and belongs_to associations where
ActiveRecord can't automatically determine the inverse association
because of a scope or the options used. This can result in unnecessary
queries in some circumstances. `:inverse_of` must be manually specified
for associations to work in both ways, or set to `false` to opt-out.

### Example:
    # good
    class Blog < ApplicationRecord
      has_many :posts
    end

    class Post < ApplicationRecord
      belongs_to :blog
    end

### Example:
    # bad
    class Blog < ApplicationRecord
      has_many :posts, -> { order(published_at: :desc) }
    end

    class Post < ApplicationRecord
      belongs_to :blog
    end

    # good
    class Blog < ApplicationRecord
      has_many(:posts,
        -> { order(published_at: :desc) },
        inverse_of: :blog
      )
    end

    class Post < ApplicationRecord
      belongs_to :blog
    end

    # good
    class Blog < ApplicationRecord
      with_options inverse_of: :blog do
        has_many :posts, -> { order(published_at: :desc) }
      end
    end

    class Post < ApplicationRecord
      belongs_to :blog
    end

### Example:
    # bad
    class Picture < ApplicationRecord
      belongs_to :imageable, polymorphic: true
    end

    class Employee < ApplicationRecord
      has_many :pictures, as: :imageable
    end

    class Product < ApplicationRecord
      has_many :pictures, as: :imageable
    end

    # good
    class Picture < ApplicationRecord
      belongs_to :imageable, polymorphic: true
    end

    class Employee < ApplicationRecord
      has_many :pictures, as: :imageable, inverse_of: :imageable
    end

    class Product < ApplicationRecord
      has_many :pictures, as: :imageable, inverse_of: :imageable
    end

### Example:
    # bad
    # However, RuboCop can not detect this pattern...
    class Physician < ApplicationRecord
      has_many :appointments
      has_many :patients, through: :appointments
    end

    class Appointment < ApplicationRecord
      belongs_to :physician
      belongs_to :patient
    end

    class Patient < ApplicationRecord
      has_many :appointments
      has_many :physicians, through: :appointments
    end

    # good
    class Physician < ApplicationRecord
      has_many :appointments
      has_many :patients, through: :appointments
    end

    class Appointment < ApplicationRecord
      belongs_to :physician, inverse_of: :appointments
      belongs_to :patient, inverse_of: :appointments
    end

    class Patient < ApplicationRecord
      has_many :appointments
      has_many :physicians, through: :appointments
    end

@see http://guides.rubyonrails.org/association_basics.html#bi-directional-associations
@see http://api.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#module-ActiveRecord::Associations::ClassMethods-label-Setting+Inverses