How I Mock Mongo Collection?

- I have been working with Mongo quite alot recently and writing my service classes using mongo without testing is such a pain in the neck. So today, I'll post about how I mock mongo collection for my unit testings.
- Given that I have a ReminderService like below:

public class ReminderService
{
    private readonly MongoCollection<User> _mongoCollection;
    private readonly INotificationService _notificationService;

    public ReminderService(MongoCollection<User> mongoCollection, INotificationService notificationService)
    {
        _mongoCollection = mongoCollection;
        _notificationService = notificationService;
    }

    public void Send()
    {
        var notActivatedUsers = _mongoCollection.FindAs<User>(Query.EQ("activated", false)).ToList();
        notActivatedUsers.ForEach(user => _notificationService.Send(new ActivationReminder(user)));
    }
}

- It has 1 method Send which needs to be unit tested. This method will find all users who have not their activated account and then send reminder emails to them. This service class is oviously designed for unit testing as it has 2 dependencies: a mongo collection and an interface of INotificationService. To mock the interface, it's straightforward , isn't it? So our job today is mocking this mongo collection.

- Before mocking a class like mongo collection, I often check whether it has virtual methods because the thing we need to mock here is the method FindAs. By pressing F12 on the MongoCollection class to go to assembly metadata, we can easily see that this method is virtual and return a MongoCursor.

// Summary:
//     Returns a cursor that can be used to find all documents in this collection
//     that match the query as TDocuments.
//
// Parameters:
//   query:
//     The query (usually a QueryDocument or constructed using the Query builder).
//
// Type parameters:
//   TDocument:
//     The type to deserialize the documents as.
//
// Returns:
//     A MongoDB.Driver.MongoCursor<TDocument>.
public virtual MongoCursor<TDocument> FindAs<TDocument>(IMongoQuery query);

- Hmm, it's getting more complicated here because this method does not return an enumerable or collection but a MongoCursor<TDocument> and that type inherits from IEnumerable<TDocument>. So it's telling me that when we call mongoCursor.ToList(), behind the scene, it will call GetEnumerator(). And certainly, checking class MongoCursor, I see that it's methods are all virtual so it's really open for unit testing.

- To wrap up, the idea will be creating a mock cursor to return expected items and let it be the result of method FindAs on our mock collection. There are other things to note here: mocking a class is not similar to mocking an interface since it has constructor. That means we have to provide the constructor parameters to create a mock object of MongoCollection class. The parameters here are MongoDatabase and MongoCollectionSettings<TDefaultDocument>:

// Summary:
//     Creates a new instance of MongoCollection. Normally you would call one of
//     the indexers or GetCollection methods of MongoDatabase instead.
//
// Parameters:
//   database:
//     The database that contains this collection.
//
//   settings:
//     The settings to use to access this collection.
public MongoCollection(MongoDatabase database, MongoCollectionSettings<TDefaultDocument> settings);

- To have a MongoDatabase we need MongoServer and MongoDatabaseSettings and so on. So honestly it's not really simple. I don't want my unit test to talk to real mongo server while running so I must ensure everything runs in memory. I had to create a TestHelper class for these thing, ofcourse for reuse reason:

public static class MongoTestHelpers
{
    private static readonly MongoServerSettings ServerSettings;
    private static readonly MongoServer Server;
    private static readonly MongoDatabaseSettings DatabaseSettings;
    private static readonly MongoDatabase Database;

    static MongoTestHelpers()
    {
        ServerSettings = new MongoServerSettings
        {
            Servers = new List<MongoServerAddress>
            {
                new MongoServerAddress("unittest")
            }
        };
        Server = new MongoServer(ServerSettings);
        DatabaseSettings = new MongoDatabaseSettings("databaseName", new MongoCredentials("", ""), GuidRepresentation.Standard, SafeMode.True, true);
        Database = new MongoDatabase(Server, DatabaseSettings);
    }

    /// <summary>
    /// Creates a mock of mongo collection
    /// </summary>
    /// <typeparam name="T"></typeparam>
    /// <returns></returns>
    /// <remarks></remarks>
    public static MongoCollection<T> CreateMockCollection<T>()
    {
        var collectionSetting = new MongoCollectionSettings<T>(Database, typeof(T).Name);
        var m = Substitute.For<MongoCollection<T>>(Database, collectionSetting);
        m.Database.Returns(Database);
        m.Settings.Returns(collectionSetting);
        return m;
    }

    public static MongoCollection<T> ReturnsCollection<T>(this MongoCollection<T> collection, IEnumerable<T> enumerable)
    {
        var cursor = Substitute.For<MongoCursor<T>>(collection, Substitute.For<IMongoQuery>());
        cursor.GetEnumerator().Returns(enumerable.GetEnumerator());
        cursor.When(x => x.GetEnumerator())
              .Do(callInfo => enumerable.GetEnumerator().Reset());// Reset Enumerator, incase the method is called multiple times

        cursor.SetSortOrder(Arg.Any<IMongoSortBy>()).Returns(cursor);
        cursor.SetLimit(Arg.Any<int>()).Returns(cursor);
        cursor.SetFields(Arg.Any<IMongoFields>()).Returns(cursor);
        cursor.SetFields(Arg.Any<string[]>()).Returns(cursor);
        cursor.SetFields(Arg.Any<string>()).Returns(cursor);
        cursor.SetSkip(Arg.Any<int>()).Returns(cursor);

        collection.Find(Arg.Any<IMongoQuery>()).Returns(cursor);
        collection.FindAs<T>(Arg.Any<IMongoQuery>()).Returns(cursor);
        collection.FindAll().Returns(cursor);
        // You properly need to setup more methods of cursor here
        
        return collection;
    }
}

- There are some interesting points here:

1. I'm using NSubstitute, hope you understand the syntax. I think the syntax is similar to other frameworks like Moq so it's not a problem if you use different library.
2. The code will never talk to real database as I used "unittest" as server address.
3. Method ReturnsCollection is an extension method of MongoCollection<T>. We'll use it to specify the items we simulate the result from mongo query. In practice, we might use fluent syntax on mongo cursor object such as SetSortOrder, SetLimit, etc so we definitely don't want these things alter our result. That's why you see there're alot of lines to mock the result for those methods to return the same cursor.

- So with this helper, the unit test will look like this:

[TestFixture]
public class ReminderServiceTests
{
    [Test]
    public void Send_should_send_notification_to_unactivated_users()
    {
        // Arrange
        var mockCollection = MongoTestHelpers.CreateMockCollection<User>();
        mockCollection.ReturnsCollection(new List<User>
        {
            new User {IsActivated = false}, 
            new User {IsActivated = false}
        });
        var notificationService = Substitute.For<INotificationService>();
        var service = new ReminderService(mockCollection, notificationService);

        // Action
        service.Send();

        // Assert
        notificationService.Received(2).Send(Arg.Any<ActivationReminder>());
    }
}

- Well, The purpose of this test is guaranteeing that the notification service will send reminders to all unactivated users from database. Because I'm using mongo so the final thing we want to mock is the result set returned by Mongo, we're not testing mongo query here. Hope this help.


Cheers.