serialise_challenge.rb

rails g model user name:string
rails g model favorite name:string design_id:integer user_id:integer
rails g model design name:string

class User 
	has_many: favourites
end

class Favorite
	belongs_to: design
	belongs_to: user
end

class Design
	has_many: favourites
end


user_1 = User.create(name: "test_1")
user_2 = User.create(name: "test_2")
user_3 = User.create(name: "test_3")

design_1 = Design.create(name: "a great design")
design_2 = Design.create(name: "a really great design")
design_3 = Design.create(name: "a magnificent great design")

fav_1 = Favorite.create(name: "great 1", design: design_1, user: user_1 )
fav_2 = Favorite.create(name: "great_2", design: design_2, user: user_1 )
fav_3 = Favorite.create(name: "great_3", design: design_3, user: user_2 )

user_1.favorites << fav_1
user-2.favorites << fav_2

ok :))
Then

class UserSerializer < ActiveModel::Serializer
  attributes :name
end

class DesignSerializer < ActiveModel::Serializer
 attributes :id, :name 
end