Return empty object or null cursor

[fongandrew]

Is there a way for a find function to return an intentionally empty set of documents? For example, I have a parent publication retrieving the User document and a child find function returning a cursor only if the User document contains the requisite permission.

If the child find function is passed an invalid User document, there's currently no good way to process that. When using Meteor.publish, we can return an empty set of values as an empty Array or simply by calling this.ready() if the parent subscription doesn't check out. That don't appear to work within Meteor.publishComposite and result in the follow traceback instead:

I20141105-22:52:42.905(0)? Exception from task: TypeError: Object  has no method '_getCollectionName'
I20141105-22:52:42.907(0)?     at Publication._getCollectionName (packages/reywood:publish-composite/publish_composite.js:171)
I20141105-22:52:42.908(0)?     at Publication.publish (packages/reywood:publish-composite/publish_composite.js:105)
I20141105-22:52:42.909(0)?     at null.<anonymous> (packages/reywood:publish-composite/publish_composite.js:180)
I20141105-22:52:42.910(0)?     at Array.forEach (native)
I20141105-22:52:42.910(0)?     at Function._.each._.forEach (packages/underscore/underscore.js:105)
I20141105-22:52:42.911(0)?     at Publication._publishChildrenOf (packages/reywood:publish-composite/publish_composite.js:177)
I20141105-22:52:42.911(0)?     at Object.observeHandle.cursor.observe.added (packages/reywood:publish-composite/publish_composite.js:118)
I20141105-22:52:42.912(0)?     at observeChangesCallbacks.added (packages/minimongo/observe.js:153)
I20141105-22:52:42.913(0)?     at self.applyChange.added (packages/minimongo/observe.js:53)
I20141105-22:52:42.914(0)?     at packages/mongo/observe_multiplex.js:188
I20141105-22:52:42.944(0)? Exception from sub PLdN4pDMc5NfWi82h TypeError: Object  has no method '_getCollectionName'
I20141105-22:52:42.944(0)?     at Object.Future.wait (/home/vagrant/.meteor/packages/meteor-tool/.1.0.35.ftql1v++os.linux.x86_64+web.browser+web.cordova/meteor-tool-os.linux.x86_64/dev_bundle/lib/node_modules/fibers/future.js:326:15)
I20141105-22:52:42.945(0)?     at _.extend.runTask (packages/meteor/fiber_helpers.js:79)
I20141105-22:52:42.946(0)?     at _.extend.addHandleAndSendInitialAdds (packages/mongo/observe_multiplex.js:47)
I20141105-22:52:42.946(0)?     at MongoConnection._observeChanges (packages/mongo/mongo_driver.js:1108)
I20141105-22:52:42.947(0)?     at Cursor.observeChanges (packages/mongo/mongo_driver.js:783)
I20141105-22:52:42.947(0)?     at Function.LocalCollection._observeFromObserveChanges (packages/minimongo/observe.js:177)
I20141105-22:52:42.947(0)?     at Cursor.observe (packages/mongo/mongo_driver.js:777)
I20141105-22:52:42.948(0)?     at Publication.publish (packages/reywood:publish-composite/publish_composite.js:108)
I20141105-22:52:42.948(0)?     at null._handler (packages/reywood:publish-composite/publish_composite.js:12)
I20141105-22:52:42.948(0)?     at maybeAuditArgumentChecks (packages/ddp/livedata_server.js:1599)
I20141105-22:52:42.949(0)?     - - - - -
I20141105-22:52:42.950(0)?     at Publication._getCollectionName (packages/reywood:publish-composite/publish_composite.js:171)
I20141105-22:52:42.951(0)?     at Publication.publish (packages/reywood:publish-composite/publish_composite.js:105)
I20141105-22:52:42.951(0)?     at null.<anonymous> (packages/reywood:publish-composite/publish_composite.js:180)
I20141105-22:52:42.952(0)?     at Array.forEach (native)
I20141105-22:52:42.953(0)?     at Function._.each._.forEach (packages/underscore/underscore.js:105)
I20141105-22:52:42.954(0)?     at Publication._publishChildrenOf (packages/reywood:publish-composite/publish_composite.js:177)
I20141105-22:52:42.955(0)?     at Object.observeHandle.cursor.observe.added (packages/reywood:publish-composite/publish_composite.js:118)
I20141105-22:52:42.956(0)?     at observeChangesCallbacks.added (packages/minimongo/observe.js:153)
I20141105-22:52:42.957(0)?     at self.applyChange.added (packages/minimongo/observe.js:53)
I20141105-22:52:42.958(0)?     at packages/mongo/observe_multiplex.js:188

[reywood]

The find function can return any falsy value (e.g., null, undefined, false, etc) to short circuit. This effectively returns an empty set. You should never call this.ready() or this.stop() in find functions.

For example:

Meteor.publishComposite("mypub", {
    find: function() {
        if (!this.user) {
            return;
        }

        return MyCollection.find({ ... });
    }
});

[bitomule]

And what happens if you didn't return nothing? Can that cause problems?

[reywood]

Doing return; is equivalent to return undefined; which is perfectly fine.

[bitomule]

Yeah, but what happens if a find doesn't return anything, like:

Find:{
   If(bla){
       return find...
   }
}

[reywood]

Collection.find() always returns a cursor, so that's fine. It doesn't matter if there are records in it or if it's empty.

[bitomule]

Sorry, maybe I didn't explain it well but my question was what happens if bla (in this case) is false so collection.find is never fired. No return is called.

[reywood]

Ah, I see. That is also equivalent to return undefined; so no problem.

[bitomule]

ok thanks :)

[s-devaney]

In the native Meteor publish method, returning an empty cursor (i.e. []) works (although this.stop() is best practice) but doing this in publishComposite results in an error, it might be good to replicate this behavior? Although a return statement works. as described above.