Trevor asked a really interesting question in the mailing list. Assume that we have the following model:
And what we want to do is to be able to add a note to a book. The API looks like so:
public void SubmitNote(string isbn, string note);
The requirements are simple:
- There is one book per ISBN
- Notes on the book shouldn’t be duplicated
- The Book document may be large, and we don’t actually care about it, just want to add it
- If there is a note on an non existent book, we need to create it
The key observation here is that Trevor doesn’t want to load the document, modify it and save it back. What he is looking for is a way to send the change to the database server and have it happen there.
Luckily, RavenDB has the right set of features for this, the Patching API. Here is how you’ll write the code to update the document without having to load it:
We can send the update to the server, have the change happen there, with no need to load and save the document. And we get strongly typed API and compiler validation, joy all around.
This is great, but it misses something. If we’ll run this code twice, we’ll have a duplicated comment, which is something that we don’t want to do.
Luckily, we have more options. The strongly typed API we just wrote is sitting on top of the actual patch API, which is much more powerful. Let’s see how we can tackle this requirement, shall we?
Now, we send a script to the server, which will execute it. If the note already exists, that means that we will not modify the document. So we got that out of the way. But we are still missing a piece of the puzzle, as you can probably see from the title of the post. What happens if the Book document does not exists?
Luckily, we thought about this issue and RavenDB is ready to help here as well. When you send a patch request to RavenDB, you can send a single script, or two. The second one will be executed if the document does not exists, like so:
If the document exists, we will run the patch script and modify the document. If the document does not exists, we will create an empty document and then run the patchIfMissing script to populate it. The code above handles all of the stated requirements, and we can call it a day.
But as long as we are here, let’s add another requirement. The only information we have from the SubmitNote(isbn, note) call is the ISBN of the book. Presumably we want to do things to this book, for example, figure out what the title is. Using this mechanism, how do we do this? When we return from the SaveChanges call, there is no way to tell if the document was newly created or already there?
The answer here is to ask RavenDB to do so. And we can modify our patchIfMissing a bit to do so. Note the changes in that are in the code:
If we need to execute the missing patch code, we’ll create the new document and in the same transaction, we’ll also create a task document to fetch additional information about this book. Your code will usually subscribe to the Tasks collection and execute additional work as a result of that.
And this is it, everything we need to, in a single operation. Note that in many cases, you don’t actually have a single such call, though. Let’s say that you are getting a batch of work all at once. Your API will actually look like:
public void SumbitNotes(Dictionary<string, string> isbnToNotes);
In this case, we are going to execute the code above for all the items that we got in the call, but call SaveChanges once. All of them would operate as a single transaction and a single round trip to the server.