[{"createdByUserId":ObjectId("5e6c2dddad72870c84f8476b")},{"createdByUserId":ObjectId("5e6c2dddad72870c84f84762")}]

db.collection.aggregate([{$match:{$expr:{$eq:["5e6c2dddad72870c84f8476b",{$toString:"$createdByUserId"}]}}}])