Fix annotate_score followed by slice (#3518)

This fixes the bug reported by @nimasmi in
https://github.com/wagtail/wagtail/issues/3431#issuecomment-288051751

Slicing creates a new SearchResults object but the ``score_field`` (set
by ``.annotate_score()`` wasn't being passed along to the new
SearchResults causing the score to not be annotated.

CHANGELOG.txt

@@ -48,6 +48,7 @@ Changelog
  * Fix: The page type usage listing now have a translatable page title (Ramon de Jezus)
  * Fix: Styles for submission filtering form now have a consistent height. (Thijs Kramer)
  * Fix: Custom user models with a primary key type requiring `get_db_prep_value` conversion are now supported (thenewguy)
+ * Fix: Slicing a search result set no longer loses the annotation added by `annotate_score` (Karl Hobley)
 
 
 1.9 (16.02.2017)

docs/releases/1.10.rst

@@ -59,6 +59,7 @@ Bug fixes
  * List-based fields within form builder form submissions are now displayed as comma-separated strings rather than as Python lists (Christine Ho, Matt Westcott)
  * The page type usage listing now have a translatable page title (Ramon de Jezus)
  * Styles for submission filtering form now have a consistent height. (Thijs Kramer)
+ * Slicing a search result set no longer loses the annotation added by `annotate_score` (Karl Hobley)
 
 
 Upgrade considerations

wagtail/wagtailsearch/backends/base.py

@@ -124,6 +124,7 @@ class BaseSearchResults(object):
         new = klass(self.backend, self.query, prefetch_related=self.prefetch_related)
         new.start = self.start
         new.stop = self.stop
+        new._score_field = self._score_field
         return new
 
     def _do_search(self):

wagtail/wagtailsearch/tests/test_elasticsearch_backend.py

@@ -254,6 +254,13 @@ class TestElasticsearchSearchBackend(BackendTests, TestCase):
         for result in results:
             self.assertIsInstance(result._score, float)
 
+    def test_annotate_score_with_slice(self):
+        # #3431 - Annotate score wasn't being passed to new queryset when slicing
+        results = self.backend.search("Hello", models.SearchTest).annotate_score('_score')[:10]
+
+        for result in results:
+            self.assertIsInstance(result._score, float)
+
 
 class TestElasticsearchSearchQuery(TestCase):
     def assertDictEqual(self, a, b):