db.Foo.aggregate( {$unwind: "$bars"}, {$lookup: { from:"bar", localField: "bars", foreignField: "_id", as: "bar" }}, {$match: { "bar.testprop": true }} )
db.Foo.aggregate( {$unwind: "$bars"}, {$lookup: { from:"bar", localField: "bars", foreignField: "_id", as: "bar" }}, {$match: { "bar.testprop": true }} )