For field-level access control type applications.
Document $redact stage in aggregation
$cond operator should allow $match as a boolean expression
access control per collection
aggregation: need an expression to give the size of an array in projections
$project should have numeric type coercion operators
Implement $addFields aggregation stage for using expression language to add new fields to a document
Add ability to project key names as values and values as key names
aggregation framework should have an $in expression
Aggregation operators should have well defined implicit type coercion behavior.
Add set/array operations to $unwind in new aggregation framework
Use $regex as the expression in a $cond