HashStruct is similar to Struct from Ruby standard library, the difference is that HashStruct.generate creates a class which takes a hash with attributes rather than just positional arguments as Struct.new does.

The point is that this is behaviour which all the model have, so you can use HashStruct to test thinks which expect something what quacks like a model class.

User  = HashStruct.generate(:first_name, :last_name)
@user = User.new(first_name: "Jakub", last_name: "Stastny")

module TestMixin
  def say_hi() puts("Hi!") end
end

HashStruct.extend(TestMixin)
User = HashStruct.generate(:first_name, :last_name)
User.say_hi
# => "Hi!"

HashStruct.send(:include, TestMixin)
User = HashStruct.generate(:first_name, :last_name)
User.new.say_hi
# => "Hi!"

You may want to create your custom class inhterited from HashStruct in case you want to extend it.