--- a/schema.py	Wed Aug 19 19:08:59 2009 +0200
+++ b/schema.py	Thu Aug 20 17:55:08 2009 +0200
@@ -16,6 +16,7 @@
 from logilab.common.decorators import cached, clear_cache, monkeypatch
 from logilab.common.logging_ext import set_log_methods
 from logilab.common.deprecation import deprecated
+from logilab.common.graph import get_cycles
 from logilab.common.compat import any
 
 from yams import BadSchemaDefinition, buildobjs as ybo
@@ -59,6 +60,38 @@
 ybo.RTYPE_PROPERTIES += ('eid',)
 ybo.RDEF_PROPERTIES += ('eid',)
 
+
+# XXX same algorithm as in reorder_cubes and probably other place,
+# may probably extract a generic function
+def order_eschemas(eschemas):
+    """return entity schemas ordered such that entity types which specializes an
+    other one appears after that one
+    """
+    graph = {}
+    for eschema in eschemas:
+        if eschema.specializes():
+            graph[eschema] = set((eschema.specializes(),))
+        else:
+            graph[eschema] = set()
+    cycles = get_cycles(graph)
+    if cycles:
+        cycles = '\n'.join(' -> '.join(cycle) for cycle in cycles)
+        raise Exception('cycles in entity schema specialization: %s'
+                        % cycles)
+    eschemas = []
+    while graph:
+        # sorted to get predictable results
+        for eschema, deps in sorted(graph.items()):
+            if not deps:
+                eschemas.append(eschema)
+                del graph[eschema]
+                for deps in graph.itervalues():
+                    try:
+                        deps.remove(eschema)
+                    except KeyError:
+                        continue
+    return eschemas
+
 def bw_normalize_etype(etype):
     if etype in ETYPE_NAME_MAP:
         msg = '%s has been renamed to %s, please update your code' % (
@@ -804,6 +837,7 @@
 PyFileReader.context['RRQLExpression'] = yobsolete(RRQLExpression)
 
 # workflow extensions #########################################################
+
 from yams.buildobjs import _add_relation as yams_add_relation
 
 class workflowable_definition(ybo.metadefinition):

--- a/server/migractions.py	Wed Aug 19 19:08:59 2009 +0200
+++ b/server/migractions.py	Thu Aug 20 17:55:08 2009 +0200
@@ -33,7 +33,8 @@
 from yams.schema2sql import eschema2sql, rschema2sql
 
 from cubicweb import AuthenticationError, ETYPE_NAME_MAP
-from cubicweb.schema import META_RTYPES, VIRTUAL_RTYPES, CubicWebRelationSchema
+from cubicweb.schema import (META_RTYPES, VIRTUAL_RTYPES,
+                             CubicWebRelationSchema, order_eschemas)
 from cubicweb.dbapi import get_repository, repo_connect
 from cubicweb.common.migration import MigrationHelper, yes
 
@@ -518,10 +519,11 @@
             if not rschema in self.repo.schema:
                 self.cmd_add_relation_type(rschema.type)
                 new.add(rschema.type)
-        for eschema in newcubes_schema.entities():
-            if not eschema in self.repo.schema:
-                self.cmd_add_entity_type(eschema.type)
-                new.add(eschema.type)
+        toadd = [eschema for eschema in newcubes_schema.entities()
+                 if not eschema in self.repo.schema]
+        for eschema in order_eschemas(toadd):
+            self.cmd_add_entity_type(eschema.type)
+            new.add(eschema.type)
         # check if attributes has been added to existing entities
         for rschema in newcubes_schema.relations():
             existingschema = self.repo.schema.rschema(rschema.type)
@@ -553,9 +555,11 @@
         for rschema in fsschema.relations():
             if not rschema in removedcubes_schema and rschema in reposchema:
                 self.cmd_drop_relation_type(rschema.type)
-        for eschema in fsschema.entities():
-            if not eschema in removedcubes_schema and eschema in reposchema:
-                self.cmd_drop_entity_type(eschema.type)
+        toremove = [eschema for eschema in fsschema.entities()
+                    if not eschema in removedcubes_schema
+                    and eschema in reposchema]
+        for eschema in reversed(order_eschemas(toremove)):
+            self.cmd_drop_entity_type(eschema.type)
         for rschema in fsschema.relations():
             if rschema in removedcubes_schema and rschema in reposchema:
                 # check if attributes/relations has been added to entities from

--- a/test/unittest_schema.py	Wed Aug 19 19:08:59 2009 +0200
+++ b/test/unittest_schema.py	Thu Aug 20 17:55:08 2009 +0200
@@ -20,7 +20,7 @@
 
 from cubicweb.schema import CubicWebSchema, CubicWebEntitySchema, \
      RQLConstraint, CubicWebSchemaLoader, ERQLExpression, RRQLExpression, \
-     normalize_expression
+     normalize_expression, order_eschemas
 from cubicweb.devtools import TestServerConfiguration as TestConfiguration
 
 DATADIR = join(dirname(__file__), 'data')
@@ -126,12 +126,18 @@
         expr = RRQLExpression('U has_update_permission O')
         self.assertEquals(str(expr), 'Any O WHERE U has_update_permission O, O eid %(o)s, U eid %(u)s')
 
-
 loader = CubicWebSchemaLoader()
 config = TestConfiguration('data')
 config.bootstrap_cubes()
 
-class SQLSchemaReaderClassTest(TestCase):
+class SchemaReaderClassTest(TestCase):
+
+    def test_order_eschemas(self):
+        schema = loader.load(config)
+        self.assertEquals(order_eschemas([schema['Note'], schema['SubNote']]),
+                                         [schema['Note'], schema['SubNote']])
+        self.assertEquals(order_eschemas([schema['SubNote'], schema['Note']]),
+                                         [schema['Note'], schema['SubNote']])
 
     def test_knownValues_load_schema(self):
         schema = loader.load(config)