Using a scope from a related model in another scope

Question

I have two related models such as this:

class PartCategory < ActiveRecord::Base 
  has_many :part_types 
  scope :engine, where(:name => 'Engine') 
end 

class PartType < ActiveRecord::Base 
  belongs_to :part_category 
end 

I would like to add a scope to the PartType model such as:

scope :engine_parts, lambda { joins(:part_category).engine } 

But when I try that, I get the following error:

NoMethodError: undefined method `default_scoped?' for ActiveRecord::Base:Class

I don't have a lot of experience with the scope thing, so I am probably missing something fundamental here. Can someone please tell me what it is.

Answer 1

Try this:

scope :engine_parts, lambda { joins(:part_category).merge(PartCategory.engine) } 

Basically, the result of joins(:part_category) is the join of two models, so you can't call .engine on it directly, you need to compose scopes in this manner.

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