For a Shirt ...
- which user designed it?
- how many times has it been sold?
- which users have purchased it?
For a Purchase ...
- which user made the purchase?
- which shirt was purchased?
For a User ...
- which shirts did it design?
- how many shirts has it purchased?
- which shirts has it purchased?
- how many times have shirts that it's designed been sold?
- which users have bought shirts that it's designed
For a Shirt ...
- belongs_to :designer, class_name: 'User' # 3, 6
- has_many :sales, class_name: 'Purchase' # 4, 9
- has_many :purchasers, through: :salea # 5, 10
For a Purchase ...
- belongs_to :purchaser, class_name: 'User' # 1
- belongs_to :shirt # 2, 4
For a User ...
- has_many :purchases, foreign_key: :purchaser_id # 7
- has_many :designed_shirts, class_name: 'Shirt', foreign_key: :designer_id # 3, 6
- has_many :purchased_shirts, through: :purchases, source: :shirt # 5, 8
- has_many :sales, through: :designed_shirts #9
- has_many :clients, through: :sales, source: :purchasers # 10
