I mentioned yesterday that I am keeping the best for today. What I am going to show you is how you can use Eval Patching for keeping track of denormalized references.

In this case, we have Users & Posts. Each Post contains a UserName property as well as the user id. When the user changes his name, we need to update all of the relevant posts.

Here is how you can do this:

store.DatabaseCommands.UpdateByIndex("Posts/ByUser",
    new IndexQuery{Query = "UserId:" + userId},
    new AdvancedPatchRequest
        {
            Script = @"
var user = LoadDocument(this.UserId);
this.UserName = user.Name;
"
        });

And this is a really simple scenario, the options that this opens, the ability to load a separate document and modify the current document based on its value is really powerful.

Oh, wait, that is actually Eval Patching.

From the very start, RavenDB supported the ability to patch documents. To send a command to the server with some instructions about how to modify a document or a set of documents. For example, we have this:

documentStore.DatabaseCommands.Patch(
    "blogposts/1234",
     new[]
                    {
                        new PatchRequest
                            {
                                Type = PatchCommandType.Add,
                                Name = "Comments",
                                Value = RavenJObject.FromObject(comment)
                            }
                    });

This approach works, is easy to understand and support, and is quite simple to implement.

Unfortunately, it is limited. Users have all sort of very complex scenarios that they want to run that we aren’t really suitable for. For example, if a user wanted to move from a FirstName, LastName properties to FullName, this won’t give that to you.

Enter Matt Warren, who has contributed some really nice features to RavenDB (like facets), and who contributed the ability to do patching by sending a JavaScript function to the server.

Here is how it works using the new syntax:

store.DatabaseCommands.Patch("blogposts/1234",
    new AdvancedPatchRequest
    {
        Script = "this.Comments.push(newComment)",
        Values = {{"newComment", comment}}
    });

Note that you can send variables to the server and they are exposed to your script.

How about our previous example of moving from FirstName, LastName to FullName? Let us see:

store.DatabaseCommands.UpdateByIndex("Raven/DocumentsByEntityName",
 new IndexQuery{Query = "Tag:Users"},
 new AdvancedPatchRequest
    {
        Script = @"
this.FullName = this.FirstName + ' ' + this.LastName;
delete this.FirstName;
delete this.LastName;
"
    }
);

So we support full computation abilities during the patch . So now you can just modify things pretty much as you feel like.

Here are a few other interesting things you can do.

Remove an item by value from an array:

store.DatabaseCommands.Patch("blogposts/1234",
    new AdvancedPatchRequest
    {
        Script = "this.Tags.Remove(tagToRemove)"
        Values = {{"tagToRemove", "Interesting"}}
    });

Remove an item using a condition:

store.DatabaseCommands.Patch("blogposts/1234",
    new AdvancedPatchRequest
    {
        Script = "this.Comments.RemoveWhere(function(comment) { return comment.Spam; });"
    });

This isn’t all, mind, but I’ll keep the really cool part for my next post.