One of the most common operations in RavenDB is to load a document, make a simple change and save it back. Usually, we tell users to just rely on the change tracking on the session and just save the document, but while it is the easiest way, it isn’t always the best. If I have a large document, I might not want to send it all the way back to the server just for a small change. That is why RavenDB has had a Patch operation for a long time. But while we had this feature, it was always a bit clumsy. It either required you to build a patch request using a somewhat cryptic and limited object graph or to write your own inline javascript to make the modifications.

With RavenDB 4.0, we are introducing patching as a first class concept, baked directly into the session, for example:

In this case, we’ll send the server a request to update the last modified date when the SaveChanges method is called. The syntax is not all that I wish it could be, but we have to operate with the limitations that Linq syntax can accept.

A more interesting case is when you want to use patching not to reduce the network load, but to allow multiple concurrent operations on the document. Let us consider the case of adding a comment to this blog post. It is fine if two users post a comment at the same time, and we can express that using:

This gives us an easy way to express such things and expose a RavenDB capability that too few users has taken advantage of. Beneath the scenes, the Linq expressions are turned into JavaScript patches, which is then used to send just the right commands for the server to work with.

It’s a really cool feature, even if I so myself.