What is the quickest way to determine why an integration test failed?

I have the following:

  test "should update deal" do
    patch user_deal_url(@user, @deal), params: { deal: { name: @deal.name+'xxx' } }
    # p @response.body
    # p @deal.errors.full_messages
    assert_select ".alert", false   # should not have alert
    assert_redirected_to dashboard_url
    follow_redirect!
    assert_select ".alert", "Deal was successfully updated."
  end

It only says:

Failure:
DealsControllerTest#test_should_update_deal [C:/Users/Chloe/workspace/test/controllers/deals_controller_test.rb:73]:
Expected response to be a <3XX: redirect>, but was a <200: OK>

I thought I could print @deal.errors.full_messages but it's not the same @deal in the controller. I'm able to print the entire response body and copy it into Notepad++ and search for the error messages, but it's so tedious. I thought I can assert that .alert doesn't exist and it would tell me what it actually was. I added

assert_select "#error_explanation", false   # should not have error

But that only told me that it exists, not why it failed.

Expected exactly 0 elements matching "#error_explanation", found 1..
Expected: 0
  Actual: 1

So how do you quickly determine why a model is failing to save or update in a controller during testing?

Rails 5.0.2

Reference: http://ift.tt/MUotxZ