Persisting and fetching DateTime values with MongoDB

28 03 2012

The C# driver for MongoDB serializes data by default from C# objects to a bson representation which is stored inside MongoDB. The DateTime type is a bit special which I want to demonstrate with the following test.


public abstract class RepositorySubjectwhere T : Repository
{
    public static T Subject { get; set; }

    public RepositorySubject()
    {
        var mongoDb = new MongoDB();
        Subject = (T)Activator.CreateInstance(typeof(T), mongoDb);
    }
}


public static class BlogRepositorySpecs
{
    [Subject(typeof(BlogRepository))]
    public class When_refetching_persisted_data : RepositorySubject
    {
        private static BlogPost blogEntry;

        private static BlogPost result;

        Establish context = () =>
        {
            blogEntry = new BlogPost()
            {
                Author = "Test Author",
                Comment = "My Comment",
                CreationDate = new DateTime(2012, 4, 12),
                Id = ObjectId.GenerateNewId()
            };
        };

        Because of = () =>
        {
            Subject.Save(blogEntry);
            result = Subject.FindById(blogEntry.Id.ToString());
        };

        Cleanup after = () => Subject.Drop();

        It should_have_the_correct_creationdate = () => {
           result.CreationDate.ShouldEqual(blogEntry.CreationDate);
        }

        It should_have_the_correct_author = () => {
            result.Author.ShouldEqual(blogEntry.Author);
        }

        It should_have_the_correct_comment = () => {
            result.Comment.ShouldEqual(blogEntry.Comment);
        }
    }
}

This simple test is written with mspec. What the tests does is create a BlogPost-object with a DateTime value and 2 string values. This object is persisted inside MongoDB. Afterwards the object is retrieved and the expected data is compared to the retrieved data.
The abstract RepositorySubject is a helper which can be used for different mspec tests which are written to test different repository functionality.
The Repository class by itself is an abstract class holding the most important CRUD-operations for the usage with MongoDB. BlogRepository is inherited from Repository. Additional implementations can be added here. Inside the MongoDB class the connection to the MongoDB database is established and the commands are executed.

Running the Test

When we run the test on a machine where the current time is set to something else than UTC time the test will fail. Why will the test fail? DateTime values are stored as UTC time inside the database. When we retrieve the data from the database the DateTime value is fetched as a UTC DateTime value. Therefore we compare a local DateTime with a UTC DateTime value.

Set Serialization Options for DateTimes

What can we do to fix this problem? We can register a serialization option for the DateTime value. With this option we can specify that the value will be converted to the local DateTime when we retrieve the object from the database. The following code snippet shows how to transfer the CreationDate from UTC to the local DateTime value.


public class RegisterSerializer
{
    public void Setup()
    {
        BsonClassMap.RegisterClassMap(cm => {
            cm.AutoMap();
            cm.GetMemberMap(c => c.CreationDate).SetSerializationOptions(
                    new DateTimeSerializationOptions(DateTimeKind.Local));
        });
     }
 }

After adding this class we only need to call the method which can be done inside the RepositorySubject by adding the following 2 lines of code.


var registerSerializer = new RegisterSerializer();
registerSerializer.Setup();

Now we can run the tests again. The result should be as expected – green.

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