Slicing ESResults now returns a new ESResults object

Previously, slicing an ESResults object made it run a query against ElasticSearch and return the results
This commit changes this by making slice return a new ESResults object with start and stop limits applied. To get results, you now have to iterate the ESResults object.

wagtail/wagtailsearch/backends/elasticsearch.py

@@ -116,24 +116,47 @@ class ElasticSearchResults(object):
     def __init__(self, backend, query):
         self.backend = backend
         self.query = query
+        self.start = 0
+        self.stop = None
 
-    def _do_search(self, offset=0, limit=None):
+    def _set_limits(self, start=None, stop=None):
+        if stop is not None:
+            if self.stop is not None:
+                self.stop = min(self.stop, self.start + stop)
+            else:
+                self.stop = self.start + stop
+
+        if start is not None:
+            if self.stop is not None:
+                self.start = min(self.stop, self.start + start)
+            else:
+                self.start = self.start + start
+
+    def _clone(self):
+        klass = self.__class__
+        new = klass(self.backend, self.query)
+        new.start = self.start
+        new.stop = self.stop
+        return new
+
+    def _do_search(self):
         # Params for elasticsearch query
         params = dict(
             index=self.backend.es_index,
             body=dict(query=self.query.to_es()),
             _source=False,
             fields='pk',
-            from_=offset,
+            from_=self.start,
         )
 
-        # Add limit if set
-        if limit is not None:
-            params['size'] = limit
+        # Add size if set
+        if self.stop is not None:
+            params['size'] = self.stop - self.start
 
         # Send to ElasticSearch
         hits = self.backend.es.search(**params)
 
+        # Get pks from results
         pks = [hit['fields']['pk'] for hit in hits['hits']['hits']]
 
         # ElasticSearch 1.x likes to pack pks into lists, unpack them if this has happened
@@ -151,6 +174,7 @@ class ElasticSearchResults(object):
         return [results[str(pk)] for pk in pks if results[str(pk)]]
 
     def _do_count(self):
+        # Get query
         query = self.query.to_es()
 
         # Elasticsearch 1.x
@@ -166,15 +190,33 @@ class ElasticSearchResults(object):
                 body=query,
             )
 
-        return count['count']
+        # Get count
+        hit_count = count['count']
+
+        # Add limits
+        hit_count -= self.start
+        if self.stop is not None:
+            hit_count = min(hit_count, self.stop - self.start)
+
+        return max(hit_count, 0)
 
     def __getitem__(self, key):
+        new = self._clone()
+
         if isinstance(key, slice):
-            # Run query
-            return self._do_search(key.start, key.stop - key.start)
+            # Set limits
+            start = int(key.start) if key.start else None
+            stop = int(key.stop) if key.stop else None
+            new._set_limits(start, stop)
+
+            return new
         else:
-            # Return a single item
-            return self._do_search(key, key + 1)[0]
+            new.start = key
+            new.stop = key + 1
+            return list(new)[0]
+
+    def __iter__(self):
+        return iter(self._do_search())
 
     def __len__(self):
         return self._do_count()