MongoDB in C#: Mapping IDs

In my previous post about C# and MongoDB, I polluted my domain model with a MongoDB type:

public class Movie
{
  public BsonObjectId Id { get; set; }
  public string Title { get; set; }
  public string Year { get; set; }

  etc
}

When you Insert a document into MongoDB using the C# driver, the driver needs to check to see if the Id has a value. It assigns one if it doesn’t. The above code worked because I used a MongoDB C# driver ID type – the driver could work out what my ID property is. But I don’t really want to use BsonObjectId in this class – it ties me to MongoDB. Instead, you can use a built-in .NET type for the ID property and tell the MongoDB C# driver about it.

First, let’s change the type of the Id property. I’ve gone for a string but there are other types you could use:

public class Movie
{
  public string Id { get; set; }
  public string Title { get; set; }
  public string Year { get; set; }

  etc
}

Now, you need to map that property. I’m doing this code in an NUnit test, so I’m going to do the mapping in the TestFixtureSetUp:

[TestFixtureSetUp]
public void TestFixtureSetUp()
{
  BsonClassMap.RegisterClassMap<Movie>(cm => {
                                              cm.AutoMap();
                                              cm.SetIdMember(cm.GetMemberMap(x => x.Id).SetIdGenerator(StringObjectIdGenerator.Instance));
                                             }
                                       );
}

So, I’ve told the MongoDB C# driver that the ID column is Movie.Id and that it needs to use its built in StringObjectIdGenerator to generate IDs.

There are several ID generators built into the driver:

• BsonObjectIdGenerator
• CombGuidGenerator
• GuidGenerator
• NullIdChecker
• ObjectIdGenerator
• StringObjectIdGenerator
• ZeroIdChecker<T>

Read the CSharp Driver Serialization Tutorial for more information about these generators and mapping them to properties.

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