db={savedposts:[{postid:1,saved:"Somebody",info:"Some info"},{postid:2,saved:"Anybody",info:"other info"}],posts:[{_id:1,test:1,savedby:"Somebody"},{_id:2,test:2,savedby:"Anybody"},{_id:3,test:3,savedby:"Not matching1"},{_id:4,test:4,savedby:"Not matching2"}]}

db.posts.aggregate([{$lookup:{from:"savedposts",let:{first:"$_id",second:"$savedby"},pipeline:[{$match:{$expr:{$and:[{$eq:["$postid","$$first"]},{$eq:["$saved","$$second"]}]}}}],as:"result"}}])