Question

Given the following models, using rails 2.1:

class Product < ActiveRecord::Base
  has_many :from_sources, :class_name => 'ProductSource', :foreign_key => 'to_product_id'
  has_many :to_sources, :class_name => 'ProductSource', :foreign_key => 'from_product_id'

  has_many :from_products, :through => :from_sources
  has_many :to_products, :through => :to_sources
end

class ProductSource < ActiveRecord::Base
  belongs_to :from_product
  belongs_to :to_product
end

class Supplier < ActiveRecord::Base
  has_many :products
end

I really can’t find a way to filter a supplier’s products that comes from another supplier.

Let me give and example with some data

• Supplier SA has products SAPA, SAPB
• Supplier SB has products SBPA, SBPB
• Supplier SC has products SCPA (redistributed from SBPB), SCPB (redistributed from SAPB)

I want to filter :products from supplier SC with only products that comes from SB.

Thanks!

Answer 1

not sure if i understand well your schema, but i think you will at least need your Product to belong_to a supplier, so you can filter the products table based on which supplier provides it:

Product.find(:conditions => {:supplier_id => @sb.id})

you can now :

• find all products WHERE supplier’s id is SC’s id,
• INNER JOIN them to their from_products,
• filter the rows WHERE the from_products’ supplier_id is SB’s id,
• and finally select DISTINCT id on the filtered rows so that you have only one occurence of SCPB.

so the actual SQL query should look like:

SELECT DISTINCT products.id, (etc...)
FROM products, product_sources
  INNER JOIN product_sources
  ON products.from_sources = product_sources.id
  LEFT JOIN products AS source
  ON product_sources.id = source.id
WHERE products.supplier_id = (SC's id) AND source.supplier_id = (SB's id)

Sorry, but im not really familiar to rails 2 query interface (rails 3 is so sweet),so i can’t tell more, but i think that is the right logic.