Add more functionalities to OrderingFilter

rest_framework/filters.py

@@ -5,6 +5,7 @@ returned by list views.
 import operator
 import warnings
 from functools import reduce
+from typing import Iterable
 
 from django.core.exceptions import FieldDoesNotExist, ImproperlyConfigured
 from django.db import models
@@ -217,7 +218,30 @@ class SearchFilter(BaseFilterBackend):
         ]
 
 
+class OrderingExpressionFactory:
+
+    def __init__(self, queryset_value: str = '', nulls_as_low: bool = True):
+        self.queryset_value = queryset_value
+        self.nulls_as_low = nulls_as_low
+
+    def get_expression(self, given_value: str):
+        queryset_value = self.queryset_value or given_value
+        if given_value.startswith('-'):
+            return models.F(queryset_value.strip('-')).desc(nulls_last=self.nulls_as_low)
+        else:
+            return models.F(queryset_value).asc(nulls_first=self.nulls_as_low)
+
+
 class OrderingFilter(BaseFilterBackend):
+    '''
+    If you use this class, the ordering fields can be a tuple of
+    (<query_value>, <OrderingExpressionFactory instance>). Using `OrderingExpressionFactory`,
+    you can map query_param values to certain values in the queryset and whether `null` values
+    are considered as low or high.
+
+    Using this class, you cas set `ordering_prefix (Iterable)` attribute to the view. This prefix would be
+    considered as the first ordering expressions.
+    '''
     # The URL query parameter used for the ordering.
     ordering_param = api_settings.ORDERING_PARAM
     ordering_fields = None
@@ -225,7 +249,7 @@ class OrderingFilter(BaseFilterBackend):
     ordering_description = _('Which field to use when ordering the results.')
     template = 'rest_framework/filters/ordering.html'
 
-    def get_ordering(self, request, queryset, view):
+    def get_ordering(self, request, queryset, view):  # returns an iterable of expressions for ordering
         """
         Ordering is set by a comma delimited ?ordering=... query parameter.
 
@@ -233,16 +257,29 @@ class OrderingFilter(BaseFilterBackend):
         the `ordering_param` value on the OrderingFilter or by
         specifying an `ORDERING_PARAM` value in the API settings.
         """
-        params = request.query_params.get(self.ordering_param)
-        if params:
-            fields = [param.strip() for param in params.split(',')]
-            ordering = self.remove_invalid_fields(queryset, fields, view, request)
-            if ordering:
-                return ordering
+        params = request.query_params.get(self.ordering_param, '')
+        if params or getattr(view, 'ordering_prefix', []):
+            params = [param.strip() for param in params.split(',')]
+            ordering_expressions = self.get_ordering_expressions(queryset, params, view, request)
+            if ordering_expressions:
+                return ordering_expressions
 
         # No ordering was included, or all the ordering fields were invalid
         return self.get_default_ordering(view)
 
+    def get_ordering_expressions(self, queryset, fields_in_query: Iterable[str], view, request):
+        valid_fields = dict(self.get_valid_fields(queryset, view, {'request': request}))
+
+        ordering_expressions = list(getattr(view, 'ordering_prefix', [])) or []
+        for field in fields_in_query:
+            exp = valid_fields.get(field, None) or valid_fields.get(field[1:], None)
+            if exp:
+                ordering_expressions.append(
+                    exp.get_expression(field) if isinstance(exp, OrderingExpressionFactory)
+                    else field
+                )
+        return ordering_expressions
+
     def get_default_ordering(self, view):
         ordering = getattr(view, 'ordering', None)
         if isinstance(ordering, str):