create_field.rb

def create_field(field, field_attributes)
	# Multiple values
	if field_attributes.is_a?Array or field_attributes.is_a?ActiveRecord::Associations::CollectionProxy
		multiple_values = [] 
		field_attributes.each do |atr|
			atr = atr.attributes if field_attributes.is_a?ActiveRecord::Associations::CollectionProxy
			atr.reject! {|k,v| k.eql?"id" }
			atr.merge!({ :user_id => self.address_book.user.id })
			multiple_values << field.singularize.camelize.constantize.new(atr)
		end
		self.contact.send "#{field}=", multiple_values 
	else
		# Single values
		field_attributes.reject! {|k,v| k.eql?"id" }
		field_attributes.merge!({ :user_id => self.address_book.user.id })
		self.contact.send "#{field}=", field.camelize.constantize.new(field_attributes)
	end
end

Uma outra abordagem: https://gist.github.com/gil27/ee734fdffbe2a2b61ac5 Assim tentamos respeitar o princípio de single responsability dos métodos.