--- a/migration.py	Mon Oct 25 15:47:01 2010 +0200
+++ b/migration.py	Thu Oct 28 19:37:59 2010 +0200
@@ -33,7 +33,7 @@
 from logilab.common.changelog import Version
 
 from cubicweb import ConfigurationError, ExecutionError
-
+from cubicweb.cwconfig import CubicWebConfiguration as cwcfg
 
 def filter_scripts(config, directory, fromversion, toversion, quiet=True):
     """return a list of paths of migration files to consider to upgrade
@@ -454,8 +454,8 @@
     """
 
     def __init__(self, config):
-        self.cubes = {}
         self.config = config
+        self.cubes = {'cubicweb': cwcfg.cubicweb_version()}
 
     def add_cube(self, name, version):
         self.cubes[name] = version
@@ -463,44 +463,49 @@
     def solve(self):
         self.warnings = []
         self.errors = []
-        self.read_constraints()
-        for cube, versions in sorted(self.constraints.items()):
-            oper, version = None, None
+        self.dependencies = {}
+        self.reverse_dependencies = {}
+        self.constraints = {}
+        # read dependencies
+        for cube in self.cubes:
+            if cube == 'cubicweb': continue
+            self.dependencies[cube] = dict(self.config.cube_dependencies(cube))
+        # compute reverse dependencies
+        for cube, dependencies in self.dependencies.iteritems():
+            for name, constraint in dependencies.iteritems():
+                self.reverse_dependencies.setdefault(name,set())
+                if constraint:
+                    try:
+                        oper, version = constraint.split()
+                        self.reverse_dependencies[name].add( (oper, version, cube) )
+                    except:
+                        self.warnings.append(
+                            'cube %s depends on %s but constraint badly '
+                            'formatted: %s' % (cube, name, constraint))
+        # check consistency
+        for cube, versions in sorted(self.reverse_dependencies.items()):
+            oper, version, source = None, None, None
             # simplify constraints
             if versions:
                 for constraint in versions:
-                    op, ver = constraint
+                    op, ver, src = constraint
                     if oper is None:
                         oper = op
                         version = ver
+                        source = src
                     elif op == '>=' and oper == '>=':
-                        version = max_version(ver, version)
+                        if version_strictly_lower(version, ver):
+                            version = ver
+                            source = src
                     else:
                         print 'unable to handle this case', oper, version, op, ver
             # "solve" constraint satisfaction problem
             if cube not in self.cubes:
-                self.errors.append( ('add', cube, version) )
+                self.errors.append( ('add', cube, version, source) )
             elif versions:
                 lower_strict = version_strictly_lower(self.cubes[cube], version)
                 if oper in ('>=','='):
                     if lower_strict:
-                        self.errors.append( ('update', cube, version) )
+                        self.errors.append( ('update', cube, version, source) )
                 else:
                     print 'unknown operator', oper
-
-    def read_constraints(self):
-        self.constraints = {}
-        self.reverse_constraints = {}
-        for cube in self.cubes:
-            use = self.config.cube_dependencies(cube)
-            for name, constraint in use.iteritems():
-                self.constraints.setdefault(name,set())
-                if constraint:
-                    try:
-                        oper, version = constraint.split()
-                        self.constraints[name].add( (oper, version) )
-                    except:
-                        self.warnings.append(
-                            'cube %s depends on %s but constraint badly '
-                            'formatted: %s' % (cube, name, constraint))
-                self.reverse_constraints.setdefault(name, set()).add(cube)