[{"fieldOnCollection":"foo"},{"fieldOnCollection":"notfoo"},{"anotherField":"bar"},{"anotherField":"notbar"}]

db.collection.aggregate([{"$match":{"$expr":{"$cond":[{"$ne":[{"$type":"$fieldOnCollection"},"missing"]},{"$eq":["$fieldOnCollection","foo"]},{"$eq":["$anotherField","bar"]}]}}}])