Fixed #13724: Corrected routing of write queries involving managers.

Previously, if a database request spanned a related object manager, the
first manager encountered would cause a request to the router, and this
would bind all subsequent queries to the same database returned by the
router. Unfortunately, the first router query would be performed using
a read request to the router, resulting in bad routing information being
used if the subsequent query was actually a write.

This change defers the call to the router until the final query is acutally
made.

It includes a small *BACKWARDS INCOMPATIBILITY* on an edge case - see the
release notes for details.

Thanks to Paul Collins (@paulcollinsiii) for the excellent debugging
work and patch.

3 files changed, 48 insertions, 24 deletions

diff --git a/django/db/models/fields/related.py b/django/db/models/fields/related.py
index 23959f6b18..6c1e96ad8c 100644
--- a/django/db/models/fields/related.py
+++ b/django/db/models/fields/related.py
@@ -159,9 +159,8 @@ class SingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjectDescri
     def is_cached(self, instance):
         return hasattr(instance, self.cache_name)
 
-    def get_queryset(self, **db_hints):
-        db = router.db_for_read(self.related.model, **db_hints)
-        return self.related.model._base_manager.using(db)
+    def get_queryset(self, **hints):
+        return self.related.model._base_manager.db_manager(hints=hints)
 
     def get_prefetch_queryset(self, instances):
         rel_obj_attr = attrgetter(self.related.field.attname)
@@ -256,15 +255,14 @@ class ReverseSingleRelatedObjectDescriptor(six.with_metaclass(RenameRelatedObjec
     def is_cached(self, instance):
         return hasattr(instance, self.cache_name)
 
-    def get_queryset(self, **db_hints):
-        db = router.db_for_read(self.field.rel.to, **db_hints)
-        rel_mgr = self.field.rel.to._default_manager
+    def get_queryset(self, **hints):
+        rel_mgr = self.field.rel.to._default_manager.db_manager(hints=hints)
         # If the related manager indicates that it should be used for
         # related fields, respect that.
         if getattr(rel_mgr, 'use_for_related_fields', False):
-            return rel_mgr.using(db)
+            return rel_mgr
         else:
-            return QuerySet(self.field.rel.to).using(db)
+            return QuerySet(self.field.rel.to, hints=hints)
 
     def get_prefetch_queryset(self, instances):
         rel_obj_attr = self.field.get_foreign_related_value
@@ -385,8 +383,12 @@ def create_foreign_related_manager(superclass, rel_field, rel_model):
                 return self.instance._prefetched_objects_cache[rel_field.related_query_name()]
             except (AttributeError, KeyError):
                 db = self._db or router.db_for_read(self.model, instance=self.instance)
-                qs = super(RelatedManager, self).get_queryset().using(db).filter(**self.core_filters)
                 empty_strings_as_null = connections[db].features.interprets_empty_strings_as_nulls
+                qs = super(RelatedManager, self).get_queryset()
+                qs._add_hints(instance=self.instance)
+                if self._db:
+                    qs = qs.using(self._db)
+                qs = qs.filter(**self.core_filters)
                 for field in rel_field.foreign_related_fields:
                     val = getattr(self.instance, field.attname)
                     if val is None or (val == '' and empty_strings_as_null):
@@ -398,9 +400,12 @@ def create_foreign_related_manager(superclass, rel_field, rel_model):
             rel_obj_attr = rel_field.get_local_related_value
             instance_attr = rel_field.get_foreign_related_value
             instances_dict = dict((instance_attr(inst), inst) for inst in instances)
-            db = self._db or router.db_for_read(self.model, instance=instances[0])
             query = {'%s__in' % rel_field.name: instances}
-            qs = super(RelatedManager, self).get_queryset().using(db).filter(**query)
+            qs = super(RelatedManager, self).get_queryset()
+            qs._add_hints(instance=instances[0])
+            if self._db:
+                qs = qs.using(self._db)
+            qs = qs.filter(**query)
             # Since we just bypassed this class' get_queryset(), we must manage
             # the reverse relation manually.
             for rel_obj in qs:
@@ -545,14 +550,21 @@ def create_many_related_manager(superclass, rel):
             try:
                 return self.instance._prefetched_objects_cache[self.prefetch_cache_name]
             except (AttributeError, KeyError):
-                db = self._db or router.db_for_read(self.instance.__class__, instance=self.instance)
-                return super(ManyRelatedManager, self).get_queryset().using(db)._next_is_sticky().filter(**self.core_filters)
+                qs = super(ManyRelatedManager, self).get_queryset()
+                qs._add_hints(instance=self.instance)
+                if self._db:
+                    qs = qs.using(self._db)
+                return qs._next_is_sticky().filter(**self.core_filters)
 
         def get_prefetch_queryset(self, instances):
             instance = instances[0]
             db = self._db or router.db_for_read(instance.__class__, instance=instance)
             query = {'%s__in' % self.query_field_name: instances}
-            qs = super(ManyRelatedManager, self).get_queryset().using(db)._next_is_sticky().filter(**query)
+            qs = super(ManyRelatedManager, self).get_queryset()
+            qs._add_hints(instance=instance)
+            if self._db:
+                qs = qs.using(db)
+            qs = qs._next_is_sticky().filter(**query)
 
             # M2M: need to annotate the query in order to get the primary model
             # that the secondary model was actually related to. We know that
diff --git a/django/db/models/manager.py b/django/db/models/manager.py
index 4f16b5ebfe..48ad3db9cc 100644
--- a/django/db/models/manager.py
+++ b/django/db/models/manager.py
@@ -68,6 +68,7 @@ class BaseManager(six.with_metaclass(RenameManagerMethods)):
         self.model = None
         self._inherited = False
         self._db = None
+        self._hints = {}
 
     @classmethod
     def _get_queryset_methods(cls, queryset_class):
@@ -144,21 +145,22 @@ class BaseManager(six.with_metaclass(RenameManagerMethods)):
         mgr._inherited = True
         return mgr
 
-    def db_manager(self, using):
+    def db_manager(self, using=None, hints=None):
         obj = copy.copy(self)
-        obj._db = using
+        obj._db = using or self._db
+        obj._hints = hints or self._hints
         return obj
 
     @property
     def db(self):
-        return self._db or router.db_for_read(self.model)
+        return self._db or router.db_for_read(self.model, **self._hints)
 
     def get_queryset(self):
         """
         Returns a new QuerySet object.  Subclasses can override this method to
         easily customize the behavior of the Manager.
         """
-        return self._queryset_class(self.model, using=self._db)
+        return self._queryset_class(self.model, using=self._db, hints=self._hints)
 
     def all(self):
         # We can't proxy this method through the `QuerySet` like we do for the
diff --git a/django/db/models/query.py b/django/db/models/query.py
index 2da4b6595f..0aff4d89ad 100644
--- a/django/db/models/query.py
+++ b/django/db/models/query.py
@@ -47,9 +47,10 @@ class QuerySet(object):
     Represents a lazy database lookup for a set of objects.
     """
 
-    def __init__(self, model=None, query=None, using=None):
+    def __init__(self, model=None, query=None, using=None, hints=None):
         self.model = model
         self._db = using
+        self._hints = hints or {}
         self.query = query or sql.Query(self.model)
         self._result_cache = None
         self._sticky_filter = False
@@ -904,8 +905,8 @@ class QuerySet(object):
     def db(self):
         "Return the database that will be used if this query is executed now"
         if self._for_write:
-            return self._db or router.db_for_write(self.model)
-        return self._db or router.db_for_read(self.model)
+            return self._db or router.db_for_write(self.model, **self._hints)
+        return self._db or router.db_for_read(self.model, **self._hints)
 
     ###################
     # PRIVATE METHODS #
@@ -955,7 +956,7 @@ class QuerySet(object):
         query = self.query.clone()
         if self._sticky_filter:
             query.filter_is_sticky = True
-        c = klass(model=self.model, query=query, using=self._db)
+        c = klass(model=self.model, query=query, using=self._db, hints=self._hints)
         c._for_write = self._for_write
         c._prefetch_related_lookups = self._prefetch_related_lookups[:]
         c._known_related_objects = self._known_related_objects
@@ -1025,6 +1026,14 @@ class QuerySet(object):
     # empty" result.
     value_annotation = True
 
+    def _add_hints(self, **hints):
+        """
+        Update hinting information for later use by Routers
+        """
+        # If there is any hinting information, add it to what we already know.
+        # If we have a new hint for an existing key, overwrite with the new value.
+        self._hints.update(hints)
+
 
 class InstanceCheckMeta(type):
     def __instancecheck__(self, instance):
@@ -1485,10 +1494,11 @@ class RawQuerySet(object):
     annotated model instances.
     """
     def __init__(self, raw_query, model=None, query=None, params=None,
-        translations=None, using=None):
+        translations=None, using=None, hints=None):
         self.raw_query = raw_query
         self.model = model
         self._db = using
+        self._hints = hints or {}
         self.query = query or sql.RawQuery(sql=raw_query, using=self.db, params=params)
         self.params = params or ()
         self.translations = translations or {}
@@ -1572,7 +1582,7 @@ class RawQuerySet(object):
     @property
     def db(self):
         "Return the database that will be used if this query is executed now"
-        return self._db or router.db_for_read(self.model)
+        return self._db or router.db_for_read(self.model, **self._hints)
 
     def using(self, alias):
         """