For example in the following query, we should be able push the $match past the $project if the $lookup collection has multikeyness metadata on 'bar' [\{$lookup: {..., as: "foo"}}, \{$unwind: {path: "$foo"}}, project: \{"a": "$foo.bar"}, \{$match: {"a": 1}}]. We'll probably need to special case lookup+unwind somehow.