How do you mock params when testing a Rails model's setter?

def new_task_attributes=(task_attributes)
    task_attributes.each do |attributes|
      tasks.build(attributes)
    end
  end

def test_adding_task_to_project
    p = Project.new
    params = {"new_tasks_attributes" => [{ "name" => "paint fence"}]}
    p.new_tasks_attributes=(params)
    p.save
    assert p.tasks.length == 1
  end

def test_adding_task_to_project
  p = Project.new
  new_tasks_attributes = [{ "name" => "paint fence"}]
  p.new_tasks_attributes = (new_tasks_attributes)
  p.save
  assert p.tasks.length == 1
end