db={users:[{"_id":ObjectId("65c35a039773c8593fc45ddb"),"email":"email","name":"name","groups":[{"_id":ObjectId("65c1b7716f130b7c4190104a"),"name":"testgroup"}]},{"_id":ObjectId("65c35a039773c8593fc45ddf"),"email":"email","name":"name","groups":[{"_id":ObjectId("65c1b7716f130b7c4190104b"),"name":"testgroup1"}]},{"_id":ObjectId("65c35a039773c8593fc45ddc"),"email":"email","name":"name","groups":[]}],groups:[{"_id":ObjectId("65c1b7716f130b7c4190104a"),"name":"testgroup"},{"_id":ObjectId("65c1b7716f130b7c4190104f"),"name":"testgroup2"}]}

db.users.aggregate([{$match:{"groups.0":{$exists:true}}},{$lookup:{from:"groups",localField:"groups._id",foreignField:"_id",as:"matchingDocuments",pipeline:[{$project:{_id:1}}]}},{$match:{$expr:{$ne:[{$size:"$groups"},{$size:"$matchingDocuments"}]}}}])