Trappings: An easier way to do functional testing

I’ve spent the last couple weeks piecing together a testing utility to fill a need. The problem is that we need to run functional and integration tests that hit the database, but it’s actually quite difficult. There’s a few techniques that are traditionally used for setting up test data for automated tests.

One possible solution is you can setup a script that populates the database before all tests run. But this has the pesky problem of causing interdependent tests. One test might update an object that another test makes assertions about, and suddenly you have false test failures that you have to spend time to debug.

Our case was even worse – we were using our API to setup test data. Use the API to insert a user at the beginning of the test and delete it at the end. When the User INSERT or User DELETE operations went haywire we got a whole ton of false test failures. You really should only test one thing with a test, and our tests were getting way out of control.

The craziness drove me to write Trappings. Trappings provides a clear place for you to create test data for .NET projects and have it torn down at the end of the test. It makes it possible to trivially write functional tests that are independent of each other – failures of one don’t cause failures of another.

How to setup data

Test fixtures are a place to declare data to be setup. Here is the sample from the readme:

class TheRaceTrack : ITestFixtureData
  // A convenient pattern to follow is to make static properties for things
  // you'll access within the test. All of these are completely valid within
  // the using block.
  public static Car Cruze { get; set; }

  public IEnumerable<SetupObject> Setup() 
    // Assign to static field for easy access later
    Cruze = new Car { Make = "Chevy", Model = "Cruze" };

    // cruze will be inserted into the database after this line
    yield return new SetupObject { CollectionName = "cars", Value = Cruze };

    // Since `cruze` has already been inserted, it's ID is already auto-assigned
    var tim = new Driver { Name = "Tim", CarId = Cruze.Id };
    yield return new SetupObject("drivers", tim);

All you have to do is implement ITestFixtureData and not hide the default constructor. Setup returns an IEnumerable which you can really use to your advantage. As each object is yielded, the next one isn’t constructed until the previous one is fully inserted into the database. This means you can take advantage of MongoDB’s ID auto-generation to piece together complex relationships.

Another feature is that classes can be public, private, nested – whatever you need. If you want a fixture to be shared for a lot of tests, make it public. If you want more fixtures for specific use cases, just toss them into nested classes and keep them close to the tests. The only constraints are placed by the compiler. I find this can be very helpful.

A pattern I’ve begun following is to make static properties to hold references to objects I create during Setup(). In the above example I can reference TheRaceTrack.Cruze.Id to get the ID of the Chevy Cruze. For instance:

public void ILoveCars()
    // Database is now setup. You can use code that assumes that documents
    // exist in and db.drivers

    var driver = from driver in drivers.AsQueryable()
                 where driver.CarId == TheRaceTrack.Cruze.Id
                 select driver;

  // objects from TheRaceTrack are no longer accessible in Mongo

Here, we use the FixtureSession to create TheRaceTrack and ensure that the objects it creates will be gone at the end of the using statement. Within the using statement we can do anything we want with these objects – including delete them. This works even for other processes, like a client-server architecture where you’re testing the server from a client. Since the objects exist in the database, they exist globally (they’re even accessible to other computers).


While I haven’t said it explicitly yet, this only currently works for MongoDB. I did it this way because that’s what I use most of the time and, frankly, it’s stinkin easy. But there’s no reason why this couldn’t work for SQL or other databases, it’s just not on my priority list.

I’ve released the package on NuGet under the MIT license. My hope is that everyone can feel free to use it, and contribute back if they find it useful.