http://guides.rubyonrails.org/association_basics.html
- belongs_to : TABLE_NAME
- has_one : TABLE_NAME [:through => :TABLE_NAME]
- has_many : TABLE_NAME [:through => :TABLE_NAME]
- has_and_belongs_to_many : TABLE_NAME [:join_table => :TABLE_NAME]
http://guides.rubyonrails.org/association_basics.html