rasmustg/mongo-csharp-migrations

MongoDB Incremental Schema Migrations

MongoDB Incremental Migrations

Not production code, just a proof of concept.

Points to be demonstrated:

• what should be done to achieve migration capability without taking database offline?
• what is the overhead (extra object metadata, any slowdowns during deserialization)?
• what should be changed on the C# driver?

How?

In order to make the migration process work, [a couple of extension points] (https://github.com/darkiri/mongo-csharp-driver/compare/master...migration) are needed on the official C# driver. It means that micro framework relies on a custom build made from the [forked repository] (https://github.com/darkiri/mongo-csharp-driver/tree/migration).

// setup
BsonSerializer.RegisterSerializationProvider(new MigrationSerializationProvider());

[Migration(typeof (MigrationTo_1_0))]
private class Customer
{
    public ObjectId Id { get; set; }
    public string FirstName { get; set; }
    public string LastName { get; set; }
}

private class MigrationTo_1_0 : IMigration<Customer>
{
    public Version To
    {
        get { return new Version(1, 0); }
    }

    public void Upgrade(Customer obj, IDictionary<string, object> extraElements)
    {
        var fullName = (string) extraElements["Name"];

        obj.LastName = fullName.Split().Last();
        obj.FirstName = fullName
                .Substring(0, fullName.Length - obj.LastName.Length)
                .Trim();
        extraElements.Remove("Name");
    }
}

In one of the earlier application versions Customer had the only property Name. In v1.0 the Name was splitted into the FirstName and LastName. MigrationTo_1_0 is applied to the Customer and contains code migrating data from all older versions to the version 1.0. Migration class contains an Upgrade method. It means that it is not possible to downgrade the data to the earlier versions.