[Answered ]-Django custom queryset class slicing not working

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Querysets are evaluated only when you slice them. In other terms the database is hit each time you perform a slicing operation (cf. Django documentation regarding this point).

Then, the question is: why isn’t the effect of your order_by method the same for every evaluation? There might be several (non exclusive) explanations, depending on the content of your User database table and on your variable sort_array.

  • The database has been updated between 2 evaluations of the queryset,
  • *sort_array corresponds to a tuple which is not long, precise enough for the order to be fixed and unique. From the documentation:

A particular ordering is guaranteed only when ordering by a set of fields that uniquely identify each object in the results. For example, if a name field isn’t unique, ordering by it won’t guarantee objects with the same name always appear in the same order.

A simple solution would be to force the evaluation of your queryset before to perform slicing, by calling list() on it:

my_list = list(User.pas1_objects.select_related("userlogindetails", "usermeta", "tenant")
    .filter(data_filter)
    .exclude(userrolepermissions__role_id__in=["PG", "PD"], tenant_id__in=tenant_ids)
    .order_by(*sort_array)
)
# And then you can slice your list without any risk of inconsistency
my_list[0:1]
👤scūriolus

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