A Guide to Active Record Associations

"4.3.2 Options for has_many"

is a connection between two Active Record models. Associations are implemented using macro-style calls, so that you can declaratively add features to your models. For example, by declaring that one model belongs_to another, you instruct Rails to maintain Primary Key–Foreign Key information between instances of the two models, and you also get a number of utility methods added to your model. Rails supports six types of associations:

Full article: http://guides.rubyonrails.org/association_basics.html#has...