Rspec unit test for updating a property of a model
It looks like you don't need the whole user-task-task item setup for the test you provided. It's actually a TaskItemUserReference spec, not Task spec.

RSpec.describe TaskItemUserReference, type: :model do

  describe "#toggle_property" do
    it "updates property" do
      task_item_user_reference = TaskItemUserReference.new(completed: true)

      expect do 
        task_item_user_reference.toggle_property(:completed)
      end.to change {task_item_user_reference.completed}.from(true).to(false)
    end
  end
end