Offer special field projection $

[theodorDiaconu]

Meteor doesn't currently support this, but we can do it.

Problem

Collection.find(teamId, { fields: { name: true, users: { $elemMatch: { _id: this.userId } } } });
AnotherCollection.find({ 'sharedByUrl.hash': hash }, { fields: { title: true, 'sharedByUrl.$': true } });

This type of projection only returns you the first field it finds that matches the solution. This looks like a very useful thing to have.

Solution

We need to do the following:

Exclude the ObservableCollections that contain special fields projection from the initial find if and only if a projected field has changed. In the example above if:

1. "users" - changed ($addToSet, $pull, $remove)
2. "sharedByUrl.hash" - changed (we strip down ".$." when sending modified fields, if you do an projected update)

When a processor is dealing with this type of ObservableCollection, it needs to do a proper fetch for that element id with the proper projection. (The old way of doing things)

Implementation

In the RedisSubscriptionManager we need to split redis event handling into 2 phases, we process the same way the observableCollections that don't have special field projection. The other ones need to pass an extra argument to the processor (called preFetch) and pass doc as a simple {doc: {doc._id}}

Make sure to only fetch it with things that have changed, not all the fields we currently have.

Conclusions

This may feel like bloating a bit the code, but it's something very important and often used and we should treat it as such.