Database
db={"puzzles":[{"_id":ObjectId("5f08f168a9416800013a6259")},{"_id":ObjectId("5f1360a4abebae0001774d73")}],"scores":[{"_id":ObjectId("5f43d5998aa28c00017c2a1e"),"pId":ObjectId("5f08f168a9416800013a6259"),"u":{"_id":ObjectId("5694fe24a57168f5fa3bbdaa")},"points":75}]}
Query
db.scores.aggregate([{$match:{"u._id":ObjectId("5694fe24a57168f5fa3bbdaa")}},{$lookup:{from:"puzzles",let:{pId:"$pId"},pipeline:[{$match:{$expr:{$ne:["$$pId","$_id"]}}}],as:"notMatched"}}])