单程有很多次

如果您以“普通的”RubyonRails的方式执行此操作,那么您描述的数据库将类似于这样。如果您的数据库结构不是这样的,我建议您阅读更多关于如何在RubyonRails中实现关联的信息,因为这是正确的方法(您应该使用 t.references :category 在您的迁移中,因为它是为了使您的引用更容易不被弄乱而设计的)。

+----------------+  +----------------+ +----------------+
| categories     |  | subcategories  | | products       |
+----------------+  +----------------+ +----------------+
| id             |  | id             | | id             |
| ...            |  | category_id    | | subcategory_id |
|                |  | ...            | | ...            |
+----------------+  +----------------+ +----------------+

将此作为数据库结构, has_many :products, :through => subcategories 作品 Category 模型。

class Category < ActiveRecord::Base
  has_many :subcategories
  has_many :products, :through => :subcategories
end

class Subcategory < ActiveRecord::Base
  belongs_to :category
  has_many :products
end

class Product < ActiveRecord::Base
  belongs_to :subcategory
  has_one :category, :through => :subcategory  # don't need this, but it does work
end

>> c = Category.create
=> #<Category id: 1, ...>
>> c.subcategories.create
=> #<Subcategory id: 1, category_id: 1, ...>
>> p = s.products.create
=> #<Product id: 1, subcategory_id: 1, ...>
>> c.products
=> [#<Product id: 1, subcategory_id: 1, ...>]
>> p.category         # if you have the has_one assocation
=> #<Category id: 1, ...>