Unit testing with MongoDB
My database of choice is MongoDB. I'm writing a data-layer API to abstract implementation details from client applications - that is, I'm essentially providing a single public interface (an object which acts as an IDL).
I'm testing my logic as I go in a TDD manner. Before each unit test, an @Before method is called to create a database singleton, after which, when the test completes, an @After method is called to drop the database. This helps to promote independence among unit tests.
Nearly all unit tests, i.e. performing a contextual query, require some kind of insertion logic to occur before hand. My public interface provides an insert method - yet, it seems incorrect to use this method as precursor logic to each unit test.
Really I need some kind of mocking mechanism, yet, I haven't had much experience with mocking frameworks, and it seems that Google returns nothing re a mocking framework one might use with MongoDB.
What do others do in these situations? That is, how do people unit test code that interacts with a database?
Also, my public interface connects to a database defined in a external configuration file - it seems incorrect to use this connection for my unit testing - again, a situation that would benefit from some kind of mocking?
Answer
Technically tests that talk to a database (nosql or otherwise) are not unit tests, as the tests are testing interactions with an external system, and not just testing an isolated unit of code. However tests that talk to a database are often extremely useful, and are often fast enough to run with the other unit tests.
Usually I have a Service interface (eg UserService) which encapsulates all the logic for dealing with the database. Code that relies on UserService can use a mocked version of UserService and is easily tested.
When testing the implementation of the Service that talks to Mongo, (eg MongoUserService) it is easiest to write some java code that will start/stop a mongo process on the local machine, and have your MongoUserService connect to that, see this question for some notes.
You could try to mock the functionality of the database while testing MongoUserService, but generally that is too error prone, and doesn't test what you really want to test, which is interaction with a real database. So when writing tests for MongoUserService, you set up a database state for each test. Look at DbUnit for an example of a framework for doing so with a database.