Database
[{name:"Chat 1",_id:"56646564987"},{name:"Chat 2",_id:"56646548997"},{name:"Chat 3",_id:"56646000007"}]
Query
db.collection.aggregate([{$match:{_id:{$in:["56646564987","56646548997","56646000007"]}}},{$group:{_id:"$_id",chat:{$first:"$$ROOT"}}},{$group:{_id:null,chats:{$push:"$chat"}}},{$addFields:{object:{$arrayToObject:{$map:{input:"$chats",as:"chat",in:{k:"$$chat._id",v:"$$chat"}}}}}},{$project:{_id:0,object:1}},{$replaceRoot:{newRoot:"$object"}}])