Revert commit 88c7640

This commit reverts 88c7640 from PR #6430

We found a problem with `from_url` that are smaller than 5 char
length. I'm reverting this here and adding a new commit that will
workaround this problem.

Author: humitos

readthedocs/redirects/querysets.py

@@ -1,6 +1,8 @@
 """Queryset for the redirects app."""
 
 from django.db import models
+from django.db.models import Value, CharField, IntegerField, Q, F, ExpressionWrapper
+from django.db.models.functions import Substr, Length
 
 from readthedocs.core.utils.extend import SettingsOverrideObject
 
@@ -25,7 +27,79 @@ def api(self, user=None, detail=True):
         return queryset
 
     def get_redirect_path_with_status(self, path, full_path=None, language=None, version_slug=None):
-        for redirect in self.select_related('project'):
+        # add extra fields with the ``path`` and ``full_path`` to perform a
+        # filter at db level instead with Python
+        queryset = self.annotate(
+            path=Value(
+                path,
+                output_field=CharField(),
+            ),
+            full_path=Value(
+                full_path,
+                output_field=CharField(),
+            ),
+
+            from_url_length=ExpressionWrapper(
+                Length('from_url'),
+                output_field=IntegerField(),
+            ),
+
+            # 1-indexed
+            from_url_without_rest=Substr(
+                'from_url',
+                1,
+                F('from_url_length') - 5,  # Strip "$rest"
+                output_field=CharField(),
+            ),
+
+            # 1-indexed
+            full_path_without_rest=Substr(
+                'full_path',
+                1,
+                F('from_url_length') - 5,  # Strip "$rest"
+                output_field=CharField(),
+            ),
+        )
+        prefix = Q(
+            redirect_type='prefix',
+            path__startswith=F('from_url'),
+        )
+        page = Q(
+            redirect_type='page',
+            path__iexact=F('from_url'),
+        )
+        exact = (
+            Q(
+                redirect_type='exact',
+                from_url__endswith='$rest',
+                # This works around a bug in Django doing a substr and an endswith,
+                # so instead we do 2 substrs and an exact
+                # https://code.djangoproject.com/ticket/29155
+                full_path_without_rest=F('from_url_without_rest'),
+            ) | Q(
+                redirect_type='exact',
+                full_path__iexact=F('from_url'),
+            )
+        )
+        sphinx_html = (
+            Q(
+                redirect_type='sphinx_html',
+                path__endswith='/',
+            ) | Q(
+                redirect_type='sphinx_html',
+                path__endswith='/index.html',
+            )
+        )
+        sphinx_htmldir = Q(
+            redirect_type='sphinx_htmldir',
+            path__endswith='.html',
+        )
+
+        # There should be one and only one redirect returned by this query. I
+        # can't think in a case where there can be more at this point. I'm
+        # leaving the loop just in case for now
+        queryset = queryset.filter(prefix | page | exact | sphinx_html | sphinx_htmldir)
+        for redirect in queryset.select_related('project'):
             new_path = redirect.get_redirect_path(
                 path=path,
                 language=language,