Add search_description to search index

The search_description field is path of wagtail core.
"SEO-crafted description of the content, used for search
indexing. This is also suitable for the page’s
<meta name="description"> HTML tag." [1]

Strangely it is not included in the search index by default.

Add the search_description field to the search index.

[1] http://docs.wagtail.io/en/v2.7/reference/pages/model_reference.html

mpicms/base/mixins.py

@@ -54,6 +54,7 @@ class BodyMixin(Page):
 
     search_fields = [
         index.SearchField('body'),
+        index.SearchField("search_description"),
     ]
 
     api_fields = [