MongoDB aggregate sorting different response on same values - database

Question

when i was pagination with mongodb aggregation, i found a problem. I'll detail this a little bit. This problem only happens when sorting. I made a pattern to keep it simple.
Model have a count (number) random generated for sorting and have id (number) for so that we can do it visually and it is unique.
Aggregate pipeline like
db.getCollection('test').aggregate([{
$sort:{
count:-1
}
},
{
$skip : 0
},
{
$limit :2
}])
Example limit 2
Returned data
/* 1 */
{
"_id" : ObjectId("6027005ffba493078dca3580"),
"count" : 9,
"id" : 38
}
/* 2 */
{
"_id" : ObjectId("6027005ffba493078dca3565"),
"count" : 9,
"id" : 11
}
When limit example 3
Returned data
/* 1 */
{
"_id" : ObjectId("6027005ffba493078dca3587"),
"count" : 9,
"id" : 45
}
/* 2 */
{
"_id" : ObjectId("6027005ffba493078dca3580"),
"count" : 9,
"id" : 38
}
/* 3 */
{
"_id" : ObjectId("6027005ffba493078dca3565"),
"count" : 9,
"id" : 11
}
For limit 2 first element id = 38,
For limit 3 first element id = 45
And its returned always different response for different limit skip value, for same value sorting.
This problem is preventing me from pagination properly.
If I add a second sorting example
$sort:{
count:-1
_id: 1
}
}
Its a resolved.
What is the reason? and another solution?

Answer 1

This is expected behavior when document with id:45 is inserted immediately after the limit:2 aggregation example was executed , the limit:3 example will show you the document with id:45 on the top place since the documents/indexes are loaded for sorting based on the natural insertion order by document _id and the _id is monotonically increasing already sorted.
Based on the above I don't think it make any difference if you add also the _id in the sort stage since documents will be automatically sorted based on the _id if it is not specified and the count value is same for all documents ...