MongoDB: auto sort documents in ascending order - node.js

Question

I have a list of documents in a database collection. The documents ordered get changed by a front-end drag-drop functionality. order field of a document sort all documents in ascending order. For example,
[{
order: 1,
no: 1
......
},
{
order: 2,
no: 2
......
},
{
order: 3,
no: 3
......
}]
Now. when drag-drop is applied it may change to different formats. For example -
[{
order: 3,
no: 1
......
},
{
order: 1,
no: 2
......
},
{
order: 2,
no: 3
......
}]
Now it is not an issue to sort the list from the front end. Later by clicking a button, a new record is added to the top of the list. It's not a problem at all to sort again to the ascending order from the client side.
Actual Issue
I have pagination on the page so every time I don't have all items to sort from the client-side programming. There should have only/best way to sort them from the MongoDB query. For example, using the following query I can sort documents -
db.collection.aggregate([
{
"$sort": {
order: 1
}
}
])
But there is a problem if order field has duplicate numbers or doesn't have any number the above query can't sort it. Is there any query to sort the order field automatically in ascending order regardless have duplicate numbers or no value at all?
For example, if we have documents in the following format -
[
{
order: null,
no: 1,
...
},
{
order: 1,
no: 2,
...
},
{
order: 1,
no: 3,
...
},
{
order: 3,
no: 4,
...
},
]
it should be converted to -
[
{
order: 1,
no: 1,
...
},
{
order: 2,
no: 2,
...
},
{
order: 3,
no: 3,
...
},
{
order: 4,
no: 4,
...
},
]

Answer 1

We can use $addFields and $ifNull to set a default value of nullable field on query
db.collection.aggregate([
{
$addFields: {
order: { $ifNull: ["$order", "$order", -1] }, // order will be -1 if it is null
},
}
])
.sort({ order: 1 });