Ruby on Rails development from scratch (54)-ActiveRecord Foundation (Specify association Relationship)

Rails supports three of relationships between tables, one-to-one, One-to-many, Many-to-many, and you need to include a declaration in model to identify these associations: Has_one,has_many,belongs_to,has_and_belongs_to_many.

A one-to-one association may exist in a relationship such as an order and an invoice, and an order can have only one invoice, which we specify in rails:

Class Order < ActiveRecord::Base
Has_one:invoice ...
Class Invoice < activerecord::base
Belongs_to:order
...

The relationship between the order and the entry is One-to-many, and we declare this:

Class Order < ActiveRecord::Base
Has_many:line_items ...
Class LineItem < activerecord::base
Belongs_to:order
...

We may classify goods, a product may fall into several categories of goods, and a category of goods may have a variety of goods, goods and classifications of the relationship between the many to many, in rails, we declare:

Class Product < activerecord::base
has_and_belongs_to_many:categories
...
Class Category < activerecord::base
has_and_belongs_to_many:p roducts
...

The above definitions of associated relationships add methods to model to navigate through the associated objects, followed by a discussion of the three related relationships.