Fix pagination when sorted by expanded foreign key

Closes #489

datasette/views/table.py

@@ -629,6 +629,8 @@ class TableView(RowTableShared):
             # If there's a sort or sort_desc, add that value as a prefix
             if (sort or sort_desc) and not is_view:
                 prefix = rows[-2][sort or sort_desc]
+                if isinstance(prefix, dict) and "value" in prefix:
+                    prefix = prefix["value"]
                 if prefix is None:
                     prefix = "$null"
                 else:

tests/test_api.py

@@ -753,6 +753,8 @@ def test_table_with_reserved_word_name(app_client):
         ("/fixtures/123_starts_with_digits.json", 0, 1),
         # Ensure faceting doesn't break pagination:
         ("/fixtures/compound_three_primary_keys.json?_facet=pk1", 1001, 21),
+        # Paginating while sorted by an expanded foreign key should work
+        ("/fixtures/roadside_attraction_characteristics.json?_size=2&_sort=attraction_id&_labels=on", 5, 3),
     ],
 )
 def test_paginate_tables_and_views(app_client, path, expected_rows, expected_pages):