add_collection_associations_to_factory_girl_factories.rb

# Goal: Allow addition of instances to a collection in a factory-built object
# when those instances require references to the parent.
# Typically occurs in Rails when one model has_many instances of another

# See more at:
# http://stackoverflow.com/questions/2937326/populating-an-association-with-children-in-factory-girl

class Factory
  def has_many(collection)
    # after_build is where you add instances to the factory-built collection. 
    # Typically you'll want to Factory.build() these instances.
    after_build { |instance|
      yield instance
    }

    # after_create will be called after after_build if the build strategy is Factory.create()
    after_create { |instance|
      instance.send(collection).each { |i| i.save! }
    }
  end
end

# Usage
# Foo has_many :bar

Factory.define :foo do |f|
  f.has_many :bar do |foo|
    foo.bar << Factory.build(:bar, :foo => foo)
  end
end