db={"one":[{obj:{a:123,b:"ABC"}}],"two":[{arr:[{a:123,b:"ABC"},{a:234,b:"BCD"}]},{arr:[{a:123,b:"BCD"},{a:234,b:"ABC"}]}]}

db.one.aggregate([{$lookup:{from:"two",let:{obj:"$obj"},pipeline:[{$match:{$expr:{$in:["$$obj","$arr"]}}}],as:"matchFromTwo"}}])