With the goal of getting coverage for a line that, if the object cannot be saved
render :new, status: :unprocessable_entity
writing a test, for a class where Post belongs_to :user, such as

    assert_no_difference "Post.count" do
      post posts_path,  params: { post: { user_id: nil }  }
    end
    assert_response :unprocessable_entity

will in practice never reach the response stage as ActiveRecord::RecordInvalid: will be invoked beforehand.

A block test assert_response :unprocessable_entity do generates
NoMethodError: undefined method 'response_code' for nil:NilClass

This test appears somewhat moot because it would be covered upstream by a validity test. Still, the curiosity of possibly bypassing the validation test interrupt is strong...

Can unprocessable_entity be tested under standard rails framework?