Use django.db.models.functions.Replace (from 2.2) for exact redirect

`Replace` allows us to modify a field in the row. We use it to remove
the `$rest` part of the URL and be able to compare the beginning of
the from URL (without the `$rest`) against the `full_path`.

Author: humitos

readthedocs/redirects/querysets.py

@@ -5,6 +5,8 @@
 
 from readthedocs.core.utils.extend import SettingsOverrideObject
 
+from .vendor import Replace
+
 
 class RedirectQuerySetBase(models.QuerySet):
 
@@ -37,6 +39,11 @@ def get_redirect_path_with_status(self, path, full_path=None, language=None, ver
                 full_path,
                 output_field=CharField(),
             ),
+            # This ``Replace`` could be slow if there too many redirects
+            from_url_without_rest__startswith=Replace(
+                'from_url',
+                Value('$rest'),
+            ),
         )
         prefix = Q(
             redirect_type='prefix',
@@ -50,6 +57,7 @@ def get_redirect_path_with_status(self, path, full_path=None, language=None, ver
             Q(
                 redirect_type='exact',
                 from_url__endswith='$rest',  # Python implementation does "in"
+                full_path__startswith=F('from_url_without_rest__startswith'),
             ) | Q(
                 redirect_type='exact',
                 full_path__iexact=F('from_url'),

readthedocs/redirects/vendor.py

@@ -0,0 +1,10 @@
+from django.db.models.expressions import Func, Value
+
+
+# This function is available on Django 2.1
+# We are copying it here and can be removed once we migrate to Django 2.2
+class Replace(Func):
+    function = 'REPLACE'
+
+    def __init__(self, expression, text, replacement=Value(''), **extra):
+        super().__init__(expression, text, replacement, **extra)