Database
[{"_id":"5f86da4b5bb9a62742371409","status":true,"doc":[{"_id":ObjectId("5f86cacbe4d7c423f21faf50"),"a":4,"b":null},{"_id":ObjectId("5f86cb01a1ca5124063299c1"),"a":6,"b":null}]}]
Query
db.collection.aggregate([{$set:{doc:{$map:{input:"$doc",as:"d",in:{$mergeObjects:["$$d",{$reduce:{input:[{"_id":ObjectId("5f86cacbe4d7c423f21faf50"),"b":90},{"_id":ObjectId("5f86cb01a1ca5124063299c1"),"b":45}],initialValue:{},in:{$cond:[{$eq:["$$this._id","$$d._id"]},{b:"$$this.b"},"$$value"]}}}]}}}}}])