Similar to $map (and in combination with possible $reduce SERVER-17258) adding $filter would complete the generic framework for handling any sort of array processing during projection in agg.
Refactor LINQ infrastructure to enable support of new aggregation operators in server 3.2.
Support projecting multiple array values in a projection like the $elemMatch projection specifier
Optimize $match $unwind $match sequence in pipeline
support $elemMatch in aggregation $project operator
Typo in aggregation $map and $filter error messages
add $filter expression to $project to work with $map and $reduce
Aggregation Builder Support for 3.2