belongs_to, has_many Default setting of: foreign

--belong_to: foreign_key option

By default in Rails, the column name that holds the foreign key of this model is assumed to be *** association name with _id ***.

--has_many: foreign_key option

By default in Rails, the column name that holds the foreign key of the (related) counterparty model is assumed to be *** this model name with _id added ***.

Reference: Rails Guides Active Record Association

belongs_to, has_many Default setting of: foreign_key option for association