Awesome RavenDB Feature of the dayEvil Patching
time to read 6 min | 1025 words

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 Smile. 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.

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More posts in "Awesome RavenDB Feature of the day" series:

  1. (26 Jul 2012) Eval Patching, Part II–Denormalized References
  2. (24 Jul 2012) Evil Patching