Support for some additional query Expressions

[hjardines]

Feature Request

MongoDB v3.2 introduced $arrayElemAt as an Aggregation expression (among many others), which is very useful to filter for Arrays of Embedded documents at a given position, typically "first" or "last".
Ref: https://docs.mongodb.com/manual/reference/operator/aggregation/arrayElemAt/

Then later MongoDB v3.6 introduced the ability of using the same "operators" for regular queries.
Quote: "The $expr allows the use of aggregation expressions within the query language."
Ref : https://docs.mongodb.com/manual/release-notes/3.6/

Q	A
New Feature	yes
RFC	yes
BC Break	no

Summary

I use Query Builder a lot, and prefer it a lot over the Aggregation Builder whenever is possible.

Is this something being looked upon on? Thanks a lot for all the hard work.

[malarzm]

We know we're lagging a lot with operators and at this point I'm not sure we'll be able to add all of them at once like we always wanted. I think we need do start adding them as requests pop up, so @hjardines if you have some time on your hands we'd greatly appreciate a PR adding them!