rails4 has_one association scope with where query - ruby-on-rails

Question

I am developing a Product model that has an has_one association to itself including a raw material record. And the model returns data through REST API. I am using Active Model Serializer instead of JBuilder.
Product model has a 'code' field that contain a product code in string:
'001-000-01-01' (This is a product.)
'001-000-00-01' (This is a material.)
Only difference between two codes is the third number from right. '1' is product. '0' is material. I want to include a "raw_material" record when retrieving a product record. So, I try to set has_one association with scope that has a "where" clause (later I can compose a query to get a material from a product code). Now I simply pass "product" object in lambda and use it in where.
First I write in def raw_material, this works. However, I don't know how to pass an object to def and use it in where clause. Therefore I come up with the scope pattern in has_one, however it returns an error even though it generates exactly the same SELECT as the def pattern. I get "NoMethodError" instead.
class Product < ActiveRecord::Base
has_many :supplies, ->{order('row_order ASC') }, primary_key: :code, foreign_key: :product_code
#This Works!
#has_one :raw_material, class_name: 'Product', primary_key: :code, foreign_key: :code
#This Works!
#has_one :raw_material
#Does not work. Why?
has_one :raw_material, ->(product) { where('code = ?', product.code).take }, class_name: 'Product'
accepts_nested_attributes_for :supplies, allow_destroy: true
#def raw_material
# Product.where('code = ?', '001-000-01-01').take
#end
end
The "def" pattern works:
Started GET "/products/1.json" for ::1 at 2016-10-05 21:48:15 +0900
Processing by ProductsController#show as JSON
Parameters: {"id"=>"1"}
Product Load (0.3ms) SELECT "products".* FROM "products" WHERE "products"."id" = ? LIMIT 1 [["id", 1]]
[active_model_serializers] Supply Load (0.1ms) SELECT "supplies".* FROM "supplies" WHERE "supplies"."product_code" = ? ORDER BY row_order ASC [["product_code", "031-052-00-01"]]
[active_model_serializers] Product Load (0.1ms) SELECT "products".* FROM "products" WHERE (code = '001-000-01-01') LIMIT 1
[active_model_serializers] Rendered ProductSerializer with ActiveModelSerializers::Adapter::Attributes (6.71ms)
Completed 200 OK in 24ms (Views: 9.2ms | ActiveRecord: 1.4ms)
However, the scope pattern does not work:
Started GET "/products/1.json" for ::1 at 2016-10-06 08:19:13 +0900
Processing by ProductsController#show as JSON
Parameters: {"id"=>"1"}
Product Load (0.1ms) SELECT "products".* FROM "products" WHERE "products"."id" = ? LIMIT 1 [["id", 1]]
[active_model_serializers] Supply Load (0.1ms) SELECT "supplies".* FROM "supplies" WHERE "supplies"."product_code" = ? ORDER BY row_order ASC [["product_code", "031-052-00-01"]]
[active_model_serializers] Product Load (0.1ms) SELECT "products".* FROM "products" WHERE (code = '031-052-00-01') LIMIT 1
[active_model_serializers] Rendered ProductSerializer with ActiveModelSerializers::Adapter::Attributes (10.6ms)
Completed 500 Internal Server Error in 27ms (ActiveRecord: 1.2ms)
NoMethodError (undefined method `except' for #<Product:0x007fe97b090418>):
app/controllers/products_controller.rb:17:in `block (2 levels) in show'
app/controllers/products_controller.rb:14:in `show'
Rendered vendor/bundle/ruby/2.2.0/gems/actionpack-4.2.5/lib/action_dispatch/middleware/templates/rescues/_source.erb (8.8ms)
Product Controller simply defines show like this:
# GET /products/1
# GET /products/1.json
def show
respond_to do |format|
format.html
format.json do
render json: #product, include: ['raw_material']
end
end
end
product_serializer.rb is:
class ProductSerializer < ActiveModel::Serializer
attributes(*Product.attribute_names.map(&:to_sym))
has_one :raw_material
end
Any assistance is greatly appreciated.
Thank you
UPDATE:
I solved this issue myself. Please check my answer below. Thank all who wrote solutions.

Answer 1

You can customise the has_one :raw_material as follows.
class ProductSerializer < ActiveModel::Serializer
attributes(*Product.attribute_names.map(&:to_sym))
has_one :raw_material do
Product.where(code: object.code)
end
end

Answer 2

I could solve this myself. Since I don't have to write this in a line, the def(method) pattern is the best bet. How to override "has_many" could help my issue.
Overriding a has_many association getter
"self" can be used for has_one to get an object inside def.
has_one :raw_material
def raw_material
Product.where('code like ?', (self.code)[0,9] + '_' + (self.code)[10,3]).take
end
This works and can generate any dataset you like in a Model.

Answer 3

# Does not work. Why?
has_one :raw_material ->(product) { where('code = ?', product.code).take }...
because you are trying to define a scope here... not an association. if you want a scope, use a scope eg:
scope :raw_material ->(product) { where('code = ?', product.code).first }
if you want an association, then use an association eg
has_one :raw_material, class_name: 'Product', primary_key: :code, foreign_key: :code
don't try to mix the two.