Fixed bug with Model.delete() which did not always delete objects in the right order.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7722 bcc190cf-cafb-0310-a4f2-bffc1f526a37

django/db/models/query.py

@@ -16,6 +16,92 @@ ITER_CHUNK_SIZE = CHUNK_SIZE
 # Pull into this namespace for backwards compatibility
 EmptyResultSet = sql.EmptyResultSet
 
+class CyclicDependency(Exception):
+    pass
+
+class CollectedObjects(object):
+    """
+    A container that stores keys and lists of values along with
+    remembering the parent objects for all the keys.
+
+    This is used for the database object deletion routines so that we
+    can calculate the 'leaf' objects which should be deleted first.
+    """
+
+    def __init__(self):
+        self.data = {}
+        self.children = {}
+
+    def add(self, model, pk, obj, parent_model, nullable=False):
+        """
+        Adds an item.
+        model is the class of the object being added,
+        pk is the primary key, obj is the object itself, 
+        parent_model is the model of the parent object
+        that this object was reached through, nullable should
+        be True if this relation is nullable.
+
+        If the item already existed in the structure,
+        returns true, otherwise false.
+        """
+        d = self.data.setdefault(model, SortedDict())
+        retval = pk in d
+        d[pk] = obj
+        # Nullable relationships can be ignored -- they
+        # are nulled out before deleting, and therefore
+        # do not affect the order in which objects have
+        # to be deleted.
+        if parent_model is not None and not nullable:
+            self.children.setdefault(parent_model, []).append(model)
+
+        return retval
+
+    def __contains__(self, key):
+        return self.data.__contains__(key)
+
+    def __getitem__(self, key):
+        return self.data[key]
+
+    def __nonzero__(self):
+        return bool(self.data)
+
+    def iteritems(self):
+        for k in self.ordered_keys():
+            yield k, self[k]
+
+    def items(self):
+        return list(self.iteritems())
+
+    def keys(self):
+        return self.ordered_keys()
+
+    def ordered_keys(self):
+        """
+        Returns the models in the order that they should be 
+        dealth with i.e. models with no dependencies first.
+        """
+        dealt_with = SortedDict()
+        # Start with items that have no children
+        models = self.data.keys()
+        while len(dealt_with) < len(models):
+            found = False
+            for model in models:
+                children = self.children.setdefault(model, [])
+                if len([c for c in children if c not in dealt_with]) == 0:
+                    dealt_with[model] = None
+                    found = True
+            if not found:
+                raise CyclicDependency("There is a cyclic dependency of items to be processed.")
+            
+        return dealt_with.keys()
+
+    def unordered_keys(self):
+        """
+        Fallback for the case where is a cyclic dependency but we
+        don't care.
+        """
+        return self.data.keys()
+
 class QuerySet(object):
     "Represents a lazy database lookup for a set of objects"
     def __init__(self, model=None, query=None):
@@ -275,7 +361,7 @@ class QuerySet(object):
         while 1:
             # Collect all the objects to be deleted in this chunk, and all the
             # objects that are related to the objects that are to be deleted.
-            seen_objs = SortedDict()
+            seen_objs = CollectedObjects()
             for object in del_query[:CHUNK_SIZE]:
                 object._collect_sub_objects(seen_objs)
 
@@ -682,19 +768,27 @@ def delete_objects(seen_objs):
     Iterate through a list of seen classes, and remove any instances that are
     referred to.
     """
-    ordered_classes = seen_objs.keys()
-    ordered_classes.reverse()
+    try:
+        ordered_classes = seen_objs.keys()
+    except CyclicDependency:
+        # if there is a cyclic dependency, we cannot in general delete
+        # the objects.  However, if an appropriate transaction is set
+        # up, or if the database is lax enough, it will succeed. 
+        # So for now, we go ahead and try anway.
+        ordered_classes = seen_objs.unordered_keys()
 
+    obj_pairs = {}
     for cls in ordered_classes:
-        seen_objs[cls] = seen_objs[cls].items()
-        seen_objs[cls].sort()
+        items = seen_objs[cls].items()
+        items.sort()
+        obj_pairs[cls] = items
 
         # Pre notify all instances to be deleted
-        for pk_val, instance in seen_objs[cls]:
+        for pk_val, instance in items:
             dispatcher.send(signal=signals.pre_delete, sender=cls,
                     instance=instance)
 
-        pk_list = [pk for pk,instance in seen_objs[cls]]
+        pk_list = [pk for pk,instance in items]
         del_query = sql.DeleteQuery(cls, connection)
         del_query.delete_batch_related(pk_list)
 
@@ -705,15 +799,17 @@ def delete_objects(seen_objs):
 
     # Now delete the actual data
     for cls in ordered_classes:
-        seen_objs[cls].reverse()
-        pk_list = [pk for pk,instance in seen_objs[cls]]
+        items = obj_pairs[cls]
+        items.reverse()
+
+        pk_list = [pk for pk,instance in items]
         del_query = sql.DeleteQuery(cls, connection)
         del_query.delete_batch(pk_list)
 
         # Last cleanup; set NULLs where there once was a reference to the
         # object, NULL the primary key of the found objects, and perform
         # post-notification.
-        for pk_val, instance in seen_objs[cls]:
+        for pk_val, instance in items:
             for field in cls._meta.fields:
                 if field.rel and field.null and field.rel.to in seen_objs:
                     setattr(instance, field.attname, None)