chg: handle __in=[] gracefully on Solr

This commit avoids the need to check whether a list is empty to avoid an
error when using it for an `__in` filter.

Closes django-haystack#358
Closes django-haystack#1311

Author: acdha

haystack/backends/solr_backend.py

@@ -581,10 +581,13 @@ def build_query_fragment(self, field, filter_type, value):
             elif filter_type == 'in':
                 in_options = []
 
-                for possible_value in prepared_value:
-                    in_options.append(u'"%s"' % self.backend.conn._from_python(possible_value))
+                if not prepared_value:
+                    query_frag = u'(!*:*)'
+                else:
+                    for possible_value in prepared_value:
+                        in_options.append(u'"%s"' % self.backend.conn._from_python(possible_value))
 
-                query_frag = u"(%s)" % " OR ".join(in_options)
+                    query_frag = u"(%s)" % " OR ".join(in_options)
             elif filter_type == 'range':
                 start = self.backend.conn._from_python(prepared_value[0])
                 end = self.backend.conn._from_python(prepared_value[1])

test_haystack/solr_tests/test_solr_backend.py

@@ -962,6 +962,17 @@ def test_auto_query(self):
         sqs = sqs.filter(tags__in=['cameras', 'electronics'])
         self.assertEqual(len(sqs), 0)
 
+    def test_query__in(self):
+        self.assertGreater(len(self.sqs), 0)
+        sqs = self.sqs.filter(django_ct='core.mockmodel', django_id__in=[1,2])
+        self.assertEqual(len(sqs), 2)
+
+    def test_query__in_empty_list(self):
+        """Confirm that an empty list avoids a Solr exception"""
+        self.assertGreater(len(self.sqs), 0)
+        sqs = self.sqs.filter(id__in=[])
+        self.assertEqual(len(sqs), 0)
+
     # Regressions
 
     def test_regression_proper_start_offsets(self):

test_haystack/solr_tests/test_solr_query.py

@@ -179,3 +179,12 @@ def test_narrow_sq(self):
         self.assertTrue(isinstance(sqs, SearchQuerySet))
         self.assertEqual(len(sqs.query.narrow_queries), 1)
         self.assertEqual(sqs.query.narrow_queries.pop(), 'foo:(moof)')
+
+    def test_query__in(self):
+        sqs = SearchQuerySet(using='solr').filter(id__in=[1,2,3])
+        self.assertEqual(sqs.query.build_query(), u'id:("1" OR "2" OR "3")')
+
+    def test_query__in_empty_list(self):
+        """Confirm that an empty list avoids a Solr exception"""
+        sqs = SearchQuerySet(using='solr').filter(id__in=[])
+        self.assertEqual(sqs.query.build_query(), u'id:(!*:*)')