Add utility for cursor-based pagination

misago/core/cursorpaginator.py

@@ -0,0 +1,43 @@
+from django.core.paginator import EmptyPage, InvalidPage
+
+
+class CursorPaginator:
+    def __init__(self, queryset, order_by, per_page):
+        self.queryset = queryset
+        self.order_by = order_by
+        self.per_page = int(per_page)
+
+    def get_page(self, start=0):
+        if start < 0:
+            raise InvalidPage()
+
+        object_list = self._get_slice(start)
+        if start and not object_list:
+            raise EmptyPage()
+
+        next_cursor = None
+        if len(object_list) > self.per_page:
+            next_slice_first_item = object_list.pop(-1)
+            next_cursor = getattr(next_slice_first_item, self.order_by)
+
+        return Page(start, object_list, next_cursor)
+
+    def _get_slice(self, start):
+        page_len = self.per_page + 1
+        if start:
+            filter_name = "%s__gte" % self.order_by
+            return self.queryset.filter(**{filter_name: start})[:page_len]
+        return self.queryset[:page_len]
+
+
+class Page:
+    def __init__(self, start, object_list, next_):
+        self.start = start or None
+        self.object_list = object_list
+        self.next = next_
+
+    def __len__(self):
+        return len(self.object_list)
+
+    def has_next(self):
+        return bool(self.next)

misago/core/tests/test_cursor_paginator.py

@@ -0,0 +1,79 @@
+import pytest
+
+from ..cursorpaginator import CursorPaginator, EmptyPage, InvalidPage
+
+
+@pytest.fixture
+def mock_objects(mocker):
+    return [mocker.Mock(post=i) for i in range(1, 12)]
+
+
+@pytest.fixture
+def mock_queryset(mocker, mock_objects):
+    return mocker.Mock(
+        filter=mocker.Mock(return_value=mock_objects)
+    )
+
+
+def test_paginator_returns_first_page(mock_objects):
+    paginator = CursorPaginator(mock_objects, "post", 6)
+    assert paginator.get_page()
+
+
+def test_first_page_has_no_start(mock_objects):
+    paginator = CursorPaginator(mock_objects, "post", 6)
+    assert paginator.get_page().start is None
+
+
+def test_first_page_has_correct_length(mock_objects):
+    paginator = CursorPaginator(mock_objects, "post", 6)
+    assert len(paginator.get_page().object_list) == 6
+
+
+def test_first_page_has_correct_items(mock_objects):
+    paginator = CursorPaginator(mock_objects, "post", 6)
+    assert paginator.get_page().object_list == mock_objects[:6]
+
+
+def test_page_has_next_attr_pointing_to_first_item_of_next_page(mock_objects):
+    paginator = CursorPaginator(mock_objects, "post", 6)
+    assert paginator.get_page().next == 7
+
+
+def test_page_can_be_tested_to_see_if_next_page_exists(mock_objects):
+    paginator = CursorPaginator(mock_objects, "post", 6)
+    assert paginator.get_page().has_next()
+
+
+def test_paginator_returns_empty_first_page_without_errors():
+    paginator = CursorPaginator([], "post", 6)
+    assert paginator.get_page().object_list == []
+
+
+def test_paginator_returns_page_starting_at_requested_address(mock_queryset):
+    paginator = CursorPaginator(mock_queryset, "post", 6)
+    assert paginator.get_page(7)
+
+
+def test_requesting_next_page_filters_queryset_using_filter_name(mock_queryset):
+    paginator = CursorPaginator(mock_queryset, "post", 6)
+    paginator.get_page(7)
+    mock_queryset.filter.assert_called_once_with(post__gte=7)
+
+
+def test_requesting_next_page_limits_queryset_to_specified_length(mock_queryset):
+    paginator = CursorPaginator(mock_queryset, "post", 6)
+    assert len(paginator.get_page(7).object_list) == 6
+
+
+def test_paginator_raises_empty_page_error_if_nth_page_is_empty(mocker):
+    queryset = mocker.Mock(filter=lambda **_: [])
+    paginator = CursorPaginator(queryset, "post", 6)
+    with pytest.raises(EmptyPage):
+        paginator.get_page(20)
+
+
+def test_paginator_raises_invalid_page_error_if_starting_position_is_negative():
+    paginator = CursorPaginator(None, None, 0)
+    with pytest.raises(InvalidPage):
+        paginator.get_page(-1)