db={"doc1":[{"id":3,"Whats for dinner":"dinner","What is for dinner tonight":"dinner","Whats for lunch":"lunch"}],"doc2":[{"Id":3,"dinner":"We are having roast!","lunch":"We are having sandwiches"}]}

db.doc1.aggregate([{$project:{data:{$objectToArray:"$$ROOT"}}},{$unwind:"$data"},{$lookup:{from:"doc2",pipeline:[{$project:{data:{$objectToArray:"$$ROOT"}}},],as:"answers"}},{$set:{answers:{$first:{$filter:{input:{$first:"$answers.data"},cond:{$eq:["$$this.k","$data.v"]}}}}}},{$match:{answers:{$exists:true}}},{$project:{data:[{k:"$data.k",v:"$answers.v"}]}},{$replaceWith:{$arrayToObject:"$data"}}])