Add isEmpty to cursors

[mitar]

Please provide a isEmpty method on cursors. Currently there are two ways to do it: use count() or fetch one element and see what you get. But isEmpty could probably be much simpler and faster than any of those. Counting exact count just to know if there is any element is not the best either.

[ankurp]

What do you exactly mean by isEmpty method on cursors? What cursors? Do you want to check if element has any children?

[awatson1978]

+1
isEmpty would be handy to have around. And also maybe indexOf() and next().

@ankurp - Mitar is talking about the minimongo cursors defined by the Collections API.

[ankurp]

@awatson1978 Thanks for clarifying. Let me take a look at it as it seems something I can add.

[mitar]

OK, but this is needed to do it performance wise. Current approach (calling count or fetching one document) is simply too much work to just know if collection is empty or not. This is not just a syntactic sugar because it cannot be implemented without support in Meteor.

[Slava]

@mitar doesn't cursor.findOne() accomplish the job in a fairly optimal way?

[glasser]

How do you think that the framework would implement "isEmpty" other than as "findOne, no fields"?

[mitar]

MongoDB has hasNext call. I think this would be best to use it? If it hasNext initially, it is not empty.

[mitar]

For findOne you need both to set fields to none and disable any possibly defined transformation. Additionally, it is also question how reactivity is then applied. You would want that reactivity is triggered only when empty state changes and not necessary when first document changes or when count changes.

[glasser]

hasNext is not part of the Node driver interface. And I'm reasonably sure that as far as efficiency goes, it's equivalent to findOne.

[mitar]

Hm, you are right.

[jagill]

@glasser I can't find the syntax for "no fields" in the meteor documentation or the mongo documentation. Naive attempts like {fields:null} or {fields:{}} don't work, of course. This is topical because I'm getting an infinite autorun loop caused by the reactivity associated with the extra fields on an object I get by findOne. There are hackier ways around it, of course, but "no fields" (or isEmpty) sure would be nice!

[Slava]

@jagill cursor.findOne({}, {fields: {_id: 1}}).

[jagill]

@Slava that's what I've been doing, but reactivity will be triggered if findOne finds a different document, right?

I think that's an acceptable cost most of the time, tho.

[Slava]

@jagill right, it can retrigger if it finds a different document, however it is easy to work-around this behavior if you store the previous result of !!cursor.findOne({}, {fields:{_id:1}}) and compare it on every invocation.

[jagill]

@Slava cool, thanks!