findAndModify

[trapperkeeper]

There doesn't seem to be a way to update a value and read the updated value in the same operation. What I want to do is increment a sequence and return the new value, in one operation. I am trying to create an auto-incrementing field and I want to ensure that each incremented value is read only once. I generated a sequence in MongoDB, and a function that increments and returns the value, but this function is not accessible via RestHeart. I know that MongoDB has a findAndModify function that will modify a values and return it in one op, but there doesn't seem to be a RestHeart wrapper for this. Is there any way to force an update operation (PATCH) to return the new value? Is there any other way of doing this in RestHeart?

[ujibang]

To achieve this you can create a transformer that injects the new value in The PATCH response

[trapperkeeper]

Just to clarify the approach you suggest, can you fill in a bit of detail? Would I need to create a MongoClient, within the transformer, and perform a separate read operation? If so, wouldn't it still be a two operation process? Or do I create a client and do a findAndModify? If the latter, can I then prevent the increment that the request would have done (without my transformer)?

[ujibang]

you can get the updated document from the transformer without an additional query.

have a look at the code of the example SnooperHook, specifically at this point

if (context.getDbOperationResult() != null) {
            BsonDocument newData = context
                    .getDbOperationResult()
                    .getNewData();
}

Basically the method context.getDbOperationResult().getNewData()should return the updated document. In it you will find your auto-incremented value.

[trapperkeeper]

Excellent, I'll give it a try. Thank you very much!

[trapperkeeper]

Thanks again for your help. I was able to get it working using the built-in transformer org.restheart.metadata.transformers.WriteResultTransformer. For anyone else who stumbles across this thread, I have a collection called 'sequences'. Documents in the collection store the next available number in a field called 'sequence_value'. I put a WriteResultTransformer on the collection and now get oldData and newData in the write (PATCH) response when I do an $inc of said value.