Server side previous page call not working - python-3.x

Question

I configured the server-side processing of datatables. On the server side I use python3 and mongodb.
I think my paging logic is good as you can see from the code:
PYTHON:
#bp.route('/_ajax_products', methods=['GET', 'POST'])
#login_required
def ajax_products():
num = int(request.args.get('page_num')) + 1
total_items = product_db.count()
items_to_show = 100
result = {"draw": num, "recordsTotal": total_items, "recordsFiltered": total_items}
list_prod = product_db.find().sort([("Code", 1)]).skip(items_to_show * (num - 1)).limit(items_to_show)
final_list = []
for i in list_prod:
# iteration on products and addition to the final list
result['data'] = final_list
return jsonify(result)
DATATABLE INITIALISATION:
$('#ProductsList').DataTable({
"dom": 'Brlf<t><"clear">p',
"pageLength": 100,
select: true,
"processing": true,
"serverSide": true,
"ajax": {
url:"/_ajax_products",
data: function ( d ) {
var datatable = $('#ProductsList').DataTable();
var currentPage = datatable.page.info().page;
d.page_num = currentPage;
}
},
"columns":[...]
...
})
The data loads well in my datatable. When I call the next page either there is no problem.
The problem appears when I call a previous page.
The display starts on page 1. when I press for example the pagination button 3, I can see in my console:
"GET /_ajax_products?draw=3&
But when I try to go back to page 1, the draw parameter goes to 4:
"GET /_ajax_products?draw=4&
... and it continues to increment.
On the server side the good data are found but they are not displayed in the datatable.
How can I solve this problem?

Answer 1

I finally found a solution. The error came from a misunderstanding of what the draw option was doing.
Indeed I thought that the value of draw corresponded to the page to be displayed, which is not the case.
Here is the new version of the code in case it can help someone:
#bp.route('/_ajax_products', methods=['GET', 'POST'])
#login_required
def ajax_products():
num = int(request.args.get('page_num')) + 1
total_items = product_db.count()
items_to_show = 100
result = {"recordsTotal": total_items, "recordsFiltered": total_items}
list_prod = product_db.find().sort([("Code", 1)]).skip(items_to_show * (num - 1)).limit(items_to_show)
final_list = []
for i in list_prod:
# iteration on products and addition to the final list
result['data'] = final_list
return jsonify(result)