Mongo Schema-less Collections & C#

Question

I'm exploring Mongo as an alternative to relational databases but I'm running into a problem with the concept of schemaless collections.

In theory it sounds great, but as soon as you tie a model to a collection, the model becomes your defacto schema. You can no longer just add or remove fields from your model and expect it to continue to work. I see the same problems here managing changes as you have with a relational database in that you need some sort of script to migrate from one version of the database schema to the other.

Am I approaching this from the wrong angle? What approaches do members here take to ensure that their collection items stay in sync with their domain model when making updates to their domain model?

Edit: It's worth noting that these problems obviously exist in relational databases as well, but I'm asking specifically for strategies in mitigating the problem using schemaless databases and more specifically Mongo. Thanks!

Answer 1

Schema migration with MongoDB is actually a lot less painful than with, say, SQL server.

Adding a new field is easy, old records will come in with it set to null or you can use attributes to control the default value [BsonDefaultValue("abc", SerializeDefaultValue = false)]

The [BsonIgnoreIfNull] attribute is also handy for omitting objects that are null from the document when it is serialized.

Removing a field is fairly easy too, you can use [BSonExtraElements] (see docs) to collect them up and preserve them or you can use [BsonIgnoreExtraElements] to simply throw them away.

With these in place there really is no need to go convert every record to the new schema, you can do it lazily as needed when records are updated, or slowly in the background.

---

PS, since you are also interested in using dynamic with Mongo, here's an experiment I tried along those lines. And here's an updated post with a complete serializer and deserializer for dynamic objects.

Answer 2

My current thought on this is to use the same sort of implementation I would using a relational database. Have a database version collection which stores the current version of the database.

My repositories would have a minimum required version which they require to accurately serialize and deserialize the collections items. If the current db version is lower than the required version, I just throw an exception. Then use migrations which would do all the conversion necessary to update the collections to the required state to be deserialized and update the database version number.

Answer 3

With statically-typed languages like C#, whenever an object gets serialised somewhere, then its original class changes, then it's deserialised back into the new class, you're probably going to run in problems somewhere along the line. It's fairly unavoidable whether it's MongoDB, WCF, XmlSerializer or whatever.

You've usually got some flexibility with serialization options, for example with Mongo you can change a class property name but still have its value map to the same field name (e.g. using the BsonElement attribute). Or you can tell the deserializer to ignore Mongo fields that don't have a corresponding class property, using the BsonIgnoreExtraElements attribute, so deleting a property won't cause an exception when the old field is loaded from Mongo.

Overall though, for any structural schema changes you'll probably need to reload the data or run a migration script. The other alternative is to use C# dynamic variables, although that doesn't really solve the underlying problem, you'll just get fewer serialization errors.

Answer 4

I've been using mongodb for a little over a year now though not for very large projects. I use hugo's csmongo or the fork here. I like the dynamic approach it introduces. This is especially useful for projects where the database structure is volatile.