DATAMONGO-3587 - Support Evaluation Query Operators $expr.

[divyajnu08]

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[mp911de]

Care to rebase your pull request? There are several commits that do not belong into this PR.

[divyajnu08]

Hi Mark

The changes I made in MatchOperation were since I thought the now we can create a Criteria using the expression Criteria.expr(AggregationExpression) that can be passed to match operation existing constructor to create the match operation.
So there is no need to explicitly add an expression field in the MatchOperation class for "$expr" operator( which I changed for issue#3790 ) since it can be done with criteria itself. So I made the changes. I have removed those and now only changes related to Criteria class are there.

Thanks

[divyajnu08]

Accidently it got deleted. Please find the PR for this.

[christophstrobl]

Thanks for taking the time to craft the PR and sorry for long radio silence. Unfortunately the expression gets evaluated right away without considering the context the query may be run in. This leads to invalid field references, because the QueryMapper considers values prefixed with $ keywords and will simply go over them.
I'll close this one in favor of #4316