Enable cursor pagination of value querysets. (#4569)

To do `GROUP_BY` queries in django requires one to use `.values()`
eg this groups posts by user getting a count of posts per user.

```
Posts.objects.order_by('user').values('user').annotate(post_count=Count('post'))
```

This would produce a value queryset which serializes its result
objects as dictionaries while `CursorPagination` requires a queryset
with result objects that are model instances.

This commit enables cursor pagination for value querysets.

- had to mangle the tests a bit to test it out. They might need
  some refactoring.
- tried the same for `.values_list()` but it turned out to be
  trickier than I expected since you have to use tuple indexes.

rest_framework/pagination.py

@@ -711,7 +711,11 @@ class CursorPagination(BasePagination):
         return replace_query_param(self.base_url, self.cursor_query_param, encoded)
 
     def _get_position_from_instance(self, instance, ordering):
-        attr = getattr(instance, ordering[0].lstrip('-'))
+        field_name = ordering[0].lstrip('-')
+        if isinstance(instance, dict):
+            attr = instance[field_name]
+        else:
+            attr = getattr(instance, field_name)
         return six.text_type(attr)
 
     def get_paginated_response(self, data):

tests/test_pagination.py

@@ -3,6 +3,8 @@ from __future__ import unicode_literals
 
 import pytest
 from django.core.paginator import Paginator as DjangoPaginator
+from django.db import models
+from django.test import TestCase
 
 from rest_framework import (
     exceptions, filters, generics, pagination, serializers, status
@@ -530,85 +532,7 @@ class TestLimitOffset:
         assert content.get('previous') == prev_url
 
 
-class TestCursorPagination:
+class CursorPaginationTestsMixin:
-    """
-    Unit tests for `pagination.CursorPagination`.
-    """
-
-    def setup(self):
-        class MockObject(object):
-            def __init__(self, idx):
-                self.created = idx
-
-        class MockQuerySet(object):
-            def __init__(self, items):
-                self.items = items
-
-            def filter(self, created__gt=None, created__lt=None):
-                if created__gt is not None:
-                    return MockQuerySet([
-                        item for item in self.items
-                        if item.created > int(created__gt)
-                    ])
-
-                assert created__lt is not None
-                return MockQuerySet([
-                    item for item in self.items
-                    if item.created < int(created__lt)
-                ])
-
-            def order_by(self, *ordering):
-                if ordering[0].startswith('-'):
-                    return MockQuerySet(list(reversed(self.items)))
-                return self
-
-            def __getitem__(self, sliced):
-                return self.items[sliced]
-
-        class ExamplePagination(pagination.CursorPagination):
-            page_size = 5
-            ordering = 'created'
-
-        self.pagination = ExamplePagination()
-        self.queryset = MockQuerySet([
-            MockObject(idx) for idx in [
-                1, 1, 1, 1, 1,
-                1, 2, 3, 4, 4,
-                4, 4, 5, 6, 7,
-                7, 7, 7, 7, 7,
-                7, 7, 7, 8, 9,
-                9, 9, 9, 9, 9
-            ]
-        ])
-
-    def get_pages(self, url):
-        """
-        Given a URL return a tuple of:
-
-        (previous page, current page, next page, previous url, next url)
-        """
-        request = Request(factory.get(url))
-        queryset = self.pagination.paginate_queryset(self.queryset, request)
-        current = [item.created for item in queryset]
-
-        next_url = self.pagination.get_next_link()
-        previous_url = self.pagination.get_previous_link()
-
-        if next_url is not None:
-            request = Request(factory.get(next_url))
-            queryset = self.pagination.paginate_queryset(self.queryset, request)
-            next = [item.created for item in queryset]
-        else:
-            next = None
-
-        if previous_url is not None:
-            request = Request(factory.get(previous_url))
-            queryset = self.pagination.paginate_queryset(self.queryset, request)
-            previous = [item.created for item in queryset]
-        else:
-            previous = None
-
-        return (previous, current, next, previous_url, next_url)
 
     def test_invalid_cursor(self):
         request = Request(factory.get('/', {'cursor': '123'}))
@@ -703,6 +627,145 @@ class TestCursorPagination:
         assert isinstance(self.pagination.to_html(), type(''))
 
 
+class TestCursorPagination(CursorPaginationTestsMixin):
+    """
+    Unit tests for `pagination.CursorPagination`.
+    """
+
+    def setup(self):
+        class MockObject(object):
+            def __init__(self, idx):
+                self.created = idx
+
+        class MockQuerySet(object):
+            def __init__(self, items):
+                self.items = items
+
+            def filter(self, created__gt=None, created__lt=None):
+                if created__gt is not None:
+                    return MockQuerySet([
+                        item for item in self.items
+                        if item.created > int(created__gt)
+                    ])
+
+                assert created__lt is not None
+                return MockQuerySet([
+                    item for item in self.items
+                    if item.created < int(created__lt)
+                ])
+
+            def order_by(self, *ordering):
+                if ordering[0].startswith('-'):
+                    return MockQuerySet(list(reversed(self.items)))
+                return self
+
+            def __getitem__(self, sliced):
+                return self.items[sliced]
+
+        class ExamplePagination(pagination.CursorPagination):
+            page_size = 5
+            ordering = 'created'
+
+        self.pagination = ExamplePagination()
+        self.queryset = MockQuerySet([
+            MockObject(idx) for idx in [
+                1, 1, 1, 1, 1,
+                1, 2, 3, 4, 4,
+                4, 4, 5, 6, 7,
+                7, 7, 7, 7, 7,
+                7, 7, 7, 8, 9,
+                9, 9, 9, 9, 9
+            ]
+        ])
+
+    def get_pages(self, url):
+        """
+        Given a URL return a tuple of:
+
+        (previous page, current page, next page, previous url, next url)
+        """
+        request = Request(factory.get(url))
+        queryset = self.pagination.paginate_queryset(self.queryset, request)
+        current = [item.created for item in queryset]
+
+        next_url = self.pagination.get_next_link()
+        previous_url = self.pagination.get_previous_link()
+
+        if next_url is not None:
+            request = Request(factory.get(next_url))
+            queryset = self.pagination.paginate_queryset(self.queryset, request)
+            next = [item.created for item in queryset]
+        else:
+            next = None
+
+        if previous_url is not None:
+            request = Request(factory.get(previous_url))
+            queryset = self.pagination.paginate_queryset(self.queryset, request)
+            previous = [item.created for item in queryset]
+        else:
+            previous = None
+
+        return (previous, current, next, previous_url, next_url)
+
+
+class CursorPaginationModel(models.Model):
+    created = models.IntegerField()
+
+
+class TestCursorPaginationWithValueQueryset(CursorPaginationTestsMixin, TestCase):
+    """
+    Unit tests for `pagination.CursorPagination` for value querysets.
+    """
+
+    def setUp(self):
+        class ExamplePagination(pagination.CursorPagination):
+            page_size = 5
+            ordering = 'created'
+
+        self.pagination = ExamplePagination()
+        data = [
+            1, 1, 1, 1, 1,
+            1, 2, 3, 4, 4,
+            4, 4, 5, 6, 7,
+            7, 7, 7, 7, 7,
+            7, 7, 7, 8, 9,
+            9, 9, 9, 9, 9
+        ]
+        for idx in data:
+            CursorPaginationModel.objects.create(created=idx)
+
+        self.queryset = CursorPaginationModel.objects.values()
+
+    def get_pages(self, url):
+        """
+        Given a URL return a tuple of:
+
+        (previous page, current page, next page, previous url, next url)
+        """
+        request = Request(factory.get(url))
+        queryset = self.pagination.paginate_queryset(self.queryset, request)
+        current = [item['created'] for item in queryset]
+
+        next_url = self.pagination.get_next_link()
+        previous_url = self.pagination.get_previous_link()
+
+        if next_url is not None:
+            request = Request(factory.get(next_url))
+            queryset = self.pagination.paginate_queryset(self.queryset, request)
+            next = [item['created'] for item in queryset]
+        else:
+            next = None
+
+        if previous_url is not None:
+            request = Request(factory.get(previous_url))
+            queryset = self.pagination.paginate_queryset(self.queryset, request)
+            previous = [item['created'] for item in queryset]
+        else:
+            previous = None
+
+        return (previous, current, next, previous_url, next_url)
+
+
 def test_get_displayed_page_numbers():
     """
     Test our contextual page display function.