Find MongoDB records where array field is not empty

Question

All of my records have a field called "pictures". This field is an array of strings.

I now want the newest 10 records where this array IS NOT empty.

I've googled around, but strangely enough I haven't found much on this. I've read into the $where option, but I was wondering how slow that is to native functions, and if there is a better solution.

And even then, that does not work:

ME.find({$where: 'this.pictures.length > 0'}).sort('-created').limit(10).execFind() 

Returns nothing. Leaving this.pictures without the length bit does work, but then it also returns empty records, of course.

Answer 1

If you also have documents that don't have the key, you can use:

ME.find({ pictures: { $exists: true, $not: {$size: 0} } }) 

MongoDB doesn't use indexes if $size is involved, so here is a better solution:

ME.find({ pictures: { $exists: true, $ne: [] } }) 

If your property can have invalid values (like null boolean or others) , then you an add an additional check using $types as proposed in this answer:

With mongo >= 3.2:

ME.find({ pictures: { $exists: true, $type: 'array', $ne: [] } }) 

With mongo < 3.2:

ME.find({ pictures: { $exists: true, $type: 4, $ne: [] } }) 

Since the MongoDB 2.6 release, you can compare with the operator $gt, but this could lead to unexpected results (you can find a detailed explanation in this answer):

ME.find({ pictures: { $gt: [] } }) 

Answer 2

After some more looking, especially in the mongodb documents, and puzzling bits together, this was the answer:

ME.find({pictures: {$exists: true, $not: {$size: 0}}})