db={"cat":[{"_id":ObjectId("5ee8d0d16e4fec1ad4779249"),"description":ObjectId("5ea9af047d6a4f6480fd42f4")}],"description":[{"_id":ObjectId("5ea9af047d6a4f6480fd42f4"),"color":"ginger"},{"_id":ObjectId("60599bbf641f8f97fa91c0c4"),"color":"pink"}]}

db.cat.aggregate([{$lookup:{from:"description",let:{catId:"$description"},pipeline:[{$match:{$expr:{$and:[{$eq:["$_id","$$catId"]},{$eq:["$color","ginger"]}]}}}],as:"description"}},])