Question

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 1

As sbridges wrote in this post it is a bad idea not to have a dedicated service (sometimes also known as repository or DAO) which abstracts the data access from the logic. Then you could test the logic by providing a mock of the DAO.

Another approach which I do is to create a Mock of the Mongo object (e.g. PowerMockito) and then return the appropriate results.
This because you don’t have to test if the database works in unit tests but more over you should test if the right query was sent to the databse.

Mongo mongo = PowerMockito.mock(Mongo.class);
DB db = PowerMockito.mock(DB.class);
DBCollection dbCollection = PowerMockito.mock(DBCollection.class);

PowerMockito.when(mongo.getDB("foo")).thenReturn(db);
PowerMockito.when(db.getCollection("bar")).thenReturn(dbCollection);

MyService svc = new MyService(mongo); // Use some kind of dependency injection
svc.getObjectById(1);

PowerMockito.verify(dbCollection).findOne(new BasicDBObject("_id", 1));

That would also be an option. Of course the creation of the mocks and returning of the appropriate objects is just coded as an example above.