Rails Polymorphic has_many

Using Ruby on Rails, how can I achieve a polymorphic has_many relationship where the owner is always of a known but the items in the association will be of some polymorphic (but homogenous) type, specified by a column in the owner? For example, suppose the Producer class has_many products but producer instances might actually have many Bicycles, or Popsicles, or Shoelaces. I can easily have each product class (Bicycle, Popsicle, etc.) have a belongs_to relationship to a Producer but given a producer instance how can I get the collection of products if they are of varying types (per producer instance)?

Rails polymorphic associations allow producers to belong to many products, but I need the relationship to be the other way around. 开发者_C百科For example:

class Bicycle < ActiveRecord::Base
  belongs_to :producer
end

class Popsicle < ActiveRecord::Base
  belongs_to :producer
end

class Producer < ActiveRecord::Base
  has_many :products, :polymorphic_column => :type # last part is made-up...
end

So my Producer table already has a "type" column which corresponds to some product class (e.g. Bicycle, Popsicle, etc.) but how can I get Rails to let me do something like:

>> bike_producer.products
#=> [Bicycle@123, Bicycle@456, ...]
>> popsicle_producer.products
#=> [Popsicle@321, Popsicle@654, ...]

Sorry if this is obvious or a common repeat; I'm having surprising difficulty achieving it easily.

You have to use STI on the producers, not on the products. This way you have different behavior for each type of producer, but in a single producers table.

(almost) No polymorphism at all!

class Product < ActiveRecord::Base
  # does not have a 'type' column, so there is no STI here,
  # it is like an abstract superclass.
  belongs_to :producer
end

class Bicycle < Product
end

class Popsicle < Product
end

class Producer < ActiveRecord::Base
  # it has a 'type' column so we have STI here!!
end

class BicycleProducer < Producer
  has_many :products, :class_name => "Bicycle", :inverse_of => :producer
end

class PopsicleProducer < Producer
  has_many :products, :class_name => "Popsicle", :inverse_of => :producer
end

please take it on format

class Bicycle < ActiveRecord::Base 
  belongs_to :bicycle_obj,:polymorphic => true 
end 

class Popsicle < ActiveRecord::Base
  belongs_to :popsicle_obj , :polymorphic => true 
end 

class Producer < ActiveRecord::Base 
  has_many :bicycles , :as=>:bicycle_obj 
  has_many :popsicle , :as=>:popsicle_obj 
end 

Use this code. If you have any problem with it, please leave a comment.

Here is the workaround I'm currently using. It doesn't provide any of the convenience methods (collection operations) that you get from real ActiveRecord::Associations, but it does provide a way to get the list of products for a given producer:

class Bicycle < ActiveRecord::Base
  belongs_to :producer
end

class Popsicle < ActiveRecord::Base
  belongs_to :producer
end

class Producer < ActiveRecord::Base
  PRODUCT_TYPE_MAPPING = {
    'bicycle' => Bicycle,
    'popsicle' => Popsicle
  }.freeze
  def products
    klass = PRODUCT_TYPE_MAPPING[self.type]
    klass ? klass.find_all_by_producer_id(self.id) : []
  end
end

Another downside is that I must maintain the mapping of type strings to type classes but that could be automated. However, this solution will suffice for my purposes.

I find that polymorphic associations is under documented in Rails. There is a single table inheritance schema, which is what gets the most documentation, but if you are not using single table inheritance, then there is some missing information.

The belongs_to association can be enabled using the :polymorphic => true option. However, unless you are using single table inheritance, the has_many association does not work, because it would need to know the set of tables that could have a foreign key.

(From what I found), I think the clean solution is to have a table and model for the base class, and have the foreign key in the base table.

create_table "products", :force => true do |table|
    table.integer  "derived_product_id"
    table.string   "derived_product_type"
    table.integer  "producer_id"
  end

  class Product < ActiveRecord::Base
    belongs_to :producer
  end

  class Producer < ActiveRecord::Base
    has_many :products
  end

Then, for a Production object, producer, you should get the products with producer.products.derived_products.

I have not yet played with has_many through to condense the association to producer.derived_products, so I cannot comment on getting that to work.

class Note < ActiveRecord::Base

 belongs_to :note_obj, :polymorphic => true
 belongs_to :user


end


class Contact < ActiveRecord::Base

 belongs_to :contact_obj, :polymorphic => true
 belongs_to :phone_type 

end



class CarrierHq < ActiveRecord::Base


 has_many :contacts, :as => :contact_obj
 has_many :notes, :as => :note_obj


end