How to test database failure with Minitest? - ruby-on-rails

Question

I'm targetting 100% coverage on a fresh new rails application. I only have one resource, auto-generated.
For creation and update operations, the generated source code contains a "else" case, triggered on database failure (save has failed).
# POST /projects
# POST /projects.json
def create
#project = Project.new(project_params)
#project.user = current_user
respond_to do |format|
if #project.save
format.html { redirect_to projects_url, notice: 'Project was successfully created.' }
format.json { render :show, status: :created, location: #project }
else
format.html { render :new }
format.json { render json: #project.errors, status: :unprocessable_entity }
end
end
end
As It's part of my controller (and may include dedicated behavior like redirection or error message), I want to test it in unit test, under MiniTest.
I have a beginning of solution with creating a test containing ActiveRecord::Base.remove_connection. I also create a teardown function containing the opposite : ActiveRecord::Base.estabish_connection
test "should not create project" do
assert_equal('Project.count') do
ActiveRecord::Base.remove_connection
post :create, project: { name: #project.name, user_id: #project.user_id }
end
assert_redirected_to projects_path
end
And to restore connection for other tests :
teardown do
ActiveRecord::Base.establish_connection
end
It's not acceptable for (at least) two reasons :
the test finishes in a exception ConnectionNotEstablished which is not the goal of the test, and after that I can't verify the behavior of the controler itself (redirection)
establish_connection is performed on every test (useless 90% of time)
Thanks for your suggestions !
EDIT : added code source

Answer 1

As I mentioned in my comment, I think you want to use a mock instead of disconnecting your database. Since you are using #project.save to check if the save worked, we want it to return false instead of raising a ConnectionNotEstablished error.
We can try to use MiniTest::Mock to accomplish this, but other people might use an external mocking library like Mocha.
Note: I don't use minitest, so this is my best guess
test "should not create project" do
Project.expect(:find, #project) do
#project.expect(:save, false) do
assert_equal('Project.count') do
post :create, project: { name: #project.name, user_id: #project.user_id }
end
end
end
assert_redirected_to projects_path
end
If you use mocha, it might look something like this:
test "should not create project" do
Project.any_instance.stubs(:save).returns(false)
assert_equal('Project.count') do
post :create, project: { name: #project.name, user_id: #project.user_id }
end
assert_redirected_to projects_path
end