def forwards(self, orm):
        # If there are duplicated documents, we'll have an error when we try to
        # create this index. So to protect against that, we should delete those
        # documents before we create the index.

        # We need to wrap the data migration and alter operation in separate transactions for PostgreSQL
        # See: http://south.readthedocs.org/en/latest/migrationstructure.html#transactions
        try:
            db.start_transaction()
            duplicated_records = Document.objects \
                .values('content_type_id', 'object_id') \
                .annotate(id_count=models.Count('id')) \
                .filter(id_count__gt=1)

            # Delete all but the first document.
            for record in duplicated_records:
                docs = Document.objects \
                    .values_list('id', flat=True) \
                    .filter(
                        content_type_id=record['content_type_id'],
                        object_id=record['object_id'],
                    )[1:]

                docs = list(docs)

                logging.warn('Deleting documents %s' % docs)

                Document.objects.filter(id__in=docs).delete()
            db.commit_transaction()
        except Exception, e:
            db.rollback_transaction()
            raise e

    def test_alter_column_postgres_multiword(self):
        """
        Tests altering columns with multiple words in Postgres types (issue #125)
        e.g. 'datetime with time zone', look at django/db/backends/postgresql/creation.py
        """
        db.create_table("test_multiword", [
            ('col_datetime', models.DateTimeField(null=True)),
            ('col_integer', models.PositiveIntegerField(null=True)),
            ('col_smallint', models.PositiveSmallIntegerField(null=True)),
            ('col_float', models.FloatField(null=True)),
        ])
        
        # test if 'double precision' is preserved
        db.alter_column('test_multiword', 'col_float', models.FloatField('float', null=True))

        # test if 'CHECK ("%(column)s" >= 0)' is stripped
        db.alter_column('test_multiword', 'col_integer', models.PositiveIntegerField(null=True))
        db.alter_column('test_multiword', 'col_smallint', models.PositiveSmallIntegerField(null=True))

        # test if 'with timezone' is preserved
        if db.backend_name == "postgres":
            db.start_transaction()
            db.execute("INSERT INTO test_multiword (col_datetime) VALUES ('2009-04-24 14:20:55+02')")
            db.alter_column('test_multiword', 'col_datetime', models.DateTimeField(auto_now=True))
            assert db.execute("SELECT col_datetime = '2009-04-24 14:20:55+02' FROM test_multiword")[0][0]
            db.rollback_transaction()

        
        db.delete_table("test_multiword")

def run_backwards(app, migrations, ignore=[], fake=False, db_dry_run=False, silent=False):
    """
    Runs the specified migrations backwards, in order, skipping those
    migrations in 'ignore'.
    """
    for migration in migrations:
        if migration not in ignore:
            app_name = get_app_name(app)
            if not silent:
                print " < %s: %s" % (app_name, migration)
            klass = get_migration(app, migration)
            if fake:
                if not silent:
                    print "   (faked)"
            else:
                if db_dry_run:
                    db.dry_run = True
                
                db.start_transaction()
                try:
                    klass().backwards()
                    db.execute_deferred_sql()
                except:
                    db.rollback_transaction()
                    raise
                else:
                    db.commit_transaction()

            if not db_dry_run:
                # Record us as having not done this
                record = MigrationHistory.for_migration(app_name, migration)
                record.delete()

def run_forwards(app, migrations, fake=False, db_dry_run=False, silent=False):
    """
    Runs the specified migrations forwards, in order.
    """
    for migration in migrations:
        app_name = get_app_name(app)
        if not silent:
            print " > %s: %s" % (app_name, migration)
        klass = get_migration(app, migration)

        if fake:
            if not silent:
                print "   (faked)"
        else:
            if db_dry_run:
                db.dry_run = True
                
            db.start_transaction()
            try:
                klass().forwards()
                db.execute_deferred_sql()
            except:
                db.rollback_transaction()
                raise
            else:
                db.commit_transaction()

        if not db_dry_run:
            # Record us as having done this
            record = MigrationHistory.for_migration(app_name, migration)
            record.applied = datetime.datetime.utcnow()
            record.save()

 def test_dry_rename(self):
     """
     Test column renaming while --dry-run is turned on (should do nothing)
     See ticket #65
     """
     cursor = connection.cursor()
     db.create_table("test_drn", [('spam', models.BooleanField(default=False))])
     # Make sure we can select the column
     cursor.execute("SELECT spam FROM test_drn")
     # Rename it
     db.dry_run = True
     db.rename_column("test_drn", "spam", "eggs")
     db.dry_run = False
     cursor.execute("SELECT spam FROM test_drn")
     db.commit_transaction()
     db.start_transaction()
     try:
         cursor.execute("SELECT eggs FROM test_drn")
     except:
         pass
     else:
         self.fail("Dry-renamed new column could be selected!")
     db.rollback_transaction()
     db.delete_table("test_drn")
     db.start_transaction()

def drop_field(app_name, model_name, field_name):
    """
    Drop the given field from the app's model
    """
    # Table is typically appname_modelname, but it could be different
    #   always best to be sure.
    from .fields import CategoryM2MField, CategoryFKField
    from .settings import FIELD_REGISTRY
    try:
        from south.db import db
    except ImportError:
        raise ImproperlyConfigured(_('%(dependency) must be installed for this command to work') %
                                   {'dependency' : 'South'})
    mdl = models.get_model(app_name, model_name)

    fld = '%s.%s.%s' % (app_name, model_name, field_name)

    if isinstance(FIELD_REGISTRY[fld], CategoryFKField):
        print (_('Dropping ForeignKey %(field_name) from %(model_name)') %
               {'field_name' : field_name, 'model_name' : model_name})
        try:
            db.start_transaction()
            table_name = mdl._meta.db_table
            db.delete_column(table_name, field_name)
            db.commit_transaction()
        except DatabaseError, e:
            db.rollback_transaction()
            raise e

 def test_alter_constraints(self):
     """
     Tests that going from a PostiveIntegerField to an IntegerField drops
     the constraint on the database.
     """
     # Only applies to databases that support CHECK constraints
     if not db.has_check_constraints:
         return
     # Make the test table
     db.create_table("test_alterc", [
         ('num', models.PositiveIntegerField()),
     ])
     # Add in some test values
     db.execute("INSERT INTO test_alterc (num) VALUES (1)")
     db.execute("INSERT INTO test_alterc (num) VALUES (2)")
     # Ensure that adding a negative number is bad
     db.commit_transaction()
     db.start_transaction()
     try:
         db.execute("INSERT INTO test_alterc (num) VALUES (-3)")
     except:
         db.rollback_transaction()
     else:
         self.fail("Could insert a negative integer into a PositiveIntegerField.")
     # Alter it to a normal IntegerField
     db.alter_column("test_alterc", "num", models.IntegerField())
     # It should now work
     db.execute("INSERT INTO test_alterc (num) VALUES (-3)")
     db.delete_table("test_alterc")
     # We need to match up for tearDown
     db.start_transaction()

    def _class_prepared_handler(sender, **kwargs):
        """ Signal handler for class_prepared. 
            This will be run for every model, looking for the moment when all
            dependent models are prepared for the first time. It will then run
            the given function, only once.
        """

        sender_app=sender._meta.app_label.lower()+'.'+sender._meta.object_name
        already_prepared=set([sender_app])
        for app,models in app_cache.app_models.items():
            for model_name,model in models.items():
                already_prepared.add(app.lower()+'.'+model_name)
                
        if all([x in already_prepared for x in dependencies]):
            db.start_transaction()
            try:
                # We need to disconnect, otherwise each new dynamo model generation
                # will trigger it and cause a "maximim recursion error"
                class_prepared.disconnect(_class_prepared_handler,weak=False)                
                fn()
            except DatabaseError, message:
                # If tables are  missing altogether, not much we can do
                # until syncdb/migrate is run. "The code must go on" in this 
                # case, without running our function completely. At least
                # database operations will be rolled back.
                db.rollback_transaction()
                # Better connect again
                if message<>'no such table: dynamo_metamodel':
                    class_prepared.connect(_class_prepared_handler, weak=False)
                else:
                    raise
            else:
                db.commit_transaction()

    def forwards(self, orm):

        # move some models from maps to layers app
        
        # 0. add some missing fields (not for wfp)
        db.start_transaction()
        try:
            # Adding field 'Layer.bbox_top'
            db.add_column('maps_layer', 'bbox_top', self.gf('django.db.models.fields.FloatField')(null=True, blank=True), keep_default=False)
            # Adding field 'Layer.bbox_bottom'
            db.add_column('maps_layer', 'bbox_bottom', self.gf('django.db.models.fields.FloatField')(null=True, blank=True), keep_default=False)
            # Adding field 'Layer.bbox_left'
            db.add_column('maps_layer', 'bbox_left', self.gf('django.db.models.fields.FloatField')(null=True, blank=True), keep_default=False)
            # Adding field 'Layer.bbox_right'
            db.add_column('maps_layer', 'bbox_right', self.gf('django.db.models.fields.FloatField')(null=True, blank=True), keep_default=False)
            db.commit_transaction()
        except:
            print 'No need to create the fields, they are already there'
            db.rollback_transaction()
            
        # 1. layers_layer moved from maps_layer
        db.rename_table('maps_layer', 'layers_layer') 
        if not db.dry_run:
            orm['contenttypes.contenttype'].objects.filter(app_label='maps', model='layer').update(app_label='layers')
            
        # 2. layers_contactrole moved from maps_contactrole
        db.rename_table('maps_contactrole', 'layers_contactrole') 
        if not db.dry_run:
            orm['contenttypes.contenttype'].objects.filter(app_label='maps', model='contactrole').update(app_label='layers')

    def forwards(self, orm):

        # Removing unique constraint on 'BadgeData', fields ['type', 'name']
        db.delete_unique('askbot_badgedata', ['type', 'name'])

        # Deleting field 'BadgeData.multiple'
        db.delete_column('askbot_badgedata', 'multiple')

        # Deleting field 'BadgeData.description'
        db.delete_column('askbot_badgedata', 'description')


        # Deleting field 'BadgeData.type'
        db.delete_column('askbot_badgedata', 'type')

        # Deleting field 'BadgeData.name'
        db.delete_column('askbot_badgedata', 'name')

        # Changing field 'BadgeData.slug'
        db.alter_column('askbot_badgedata', 'slug', self.gf('django.db.models.fields.SlugField')(unique=True, max_length=50))
        # Adding unique constraint on 'BadgeData', fields ['slug']
        return
        try:#work around the South 0.7.3 bug
            db.start_transaction()
            db.create_unique('askbot_badgedata', ['slug'])
            db.commit_transaction()
        except:
            db.rollback_transaction()

 def test_text_default(self):
     """
     MySQL cannot have blank defaults on TEXT columns.
     """
     db.start_transaction()
     try:
         db.create_table("test_textdef", [("textcol", models.TextField(blank=True))])
     finally:
         db.rollback_transaction()

 def done_migrate(self, migration):
     db.start_transaction()
     try:
         # Record us as having done this
         self.record(migration)
     except:
         db.rollback_transaction()
         raise
     else:
         db.commit_transaction()

def migrate_app(app, *args, **kwargs):
    """
    Migrate all models of this app registered
    """
    try:
        from south.db import db
    except ImportError:
        raise ImproperlyConfigured("South must be installed for this command to work")

    # pull the information from the registry
    if not isinstance(app, basestring):
        return
    fields = [fld for fld in FIELD_REGISTRY.keys() if fld.startswith(app)]

    # call the south commands to add the fields/tables
    for fld in fields:
        app_name, model_name, field_name = fld.split('.')

        # Table is typically appname_modelname, but it could be different
        #   always best to be sure.
        mdl = models.get_model(app_name, model_name)

        if isinstance(FIELD_REGISTRY[fld], CategoryFKField):
            print "Adding ForeignKey %s to %s" % (field_name, model_name)
            try:
                db.start_transaction()
                table_name = mdl._meta.db_table
                FIELD_REGISTRY[fld].default = -1
                db.add_column(table_name, field_name, FIELD_REGISTRY[fld], keep_default=False)
                db.commit_transaction()
            except DatabaseError, e:
                db.rollback_transaction()
                if "already exists" in str(e):
                    print "Already exists"
                else:
                    raise e
        elif isinstance(FIELD_REGISTRY[fld], CategoryM2MField):
            print "Adding Many2Many table between %s and %s" % (model_name, 'category')
            table_name = "%s_%s" % (mdl._meta.db_table, 'categories')
            try:
                db.start_transaction()
                db.create_table(table_name, (
                    ('id', models.AutoField(verbose_name='ID', primary_key=True, auto_created=True)),
                    (model_name, models.ForeignKey(mdl, null=False)),
                    ('category', models.ForeignKey(Category, null=False))
                ))
                db.create_unique(table_name, ['%s_id' % model_name, 'category_id'])
                db.commit_transaction()
            except DatabaseError, e:
                db.rollback_transaction()
                if "already exists" in str(e):
                    print "Already exists"
                else:
                    raise e

def ignore_exists(fun, *args, **kwargs):
    try:
        fun(*args, **kwargs)
    except Exception, exc:
        if "exists" in str(exc):
            # don't panic, everything is ok: it's just a hack
            if db.has_ddl_transactions:
                db.rollback_transaction()
                db.start_transaction()
            return False
        raise

 def test_capitalised_constraints(self):
     """
     Under PostgreSQL at least, capitalised constrains must be quoted.
     """
     db.start_transaction()
     try:
         db.create_table("test_capconst", [("SOMECOL", models.PositiveIntegerField(primary_key=True))])
         # Alter it so it's not got the check constraint
         db.alter_column("test_capconst", "SOMECOL", models.IntegerField())
     finally:
         db.rollback_transaction()

 def run_migration(self, migration):
     migration_function = self.direction(migration)
     db.start_transaction()
     try:
         migration_function()
         db.execute_deferred_sql()
     except:
         db.rollback_transaction()
         if not db.has_ddl_transactions:
             print self.run_migration_error(migration)
         raise
     else:
         db.commit_transaction()

    def forwards(self, orm):
        # As opposed to Document1, we can't just delete Document2 documents if
        # there is a duplication because it actually holds data. So instead
        # we'll just find duplications and emit a better error message.
        # Note we reset the `order_by` to make sure to prevent error with mysql:
        # ORDER BY clause is not in GROUP BY clause and contains nonaggregated column.
        duplicated_records = Document2.objects \
         .values('uuid', 'version', 'is_history') \
         .annotate(id_count=models.Count('id')) \
         .filter(id_count__gt=1) \
         .order_by()


        duplicated_records = list(duplicated_records)
        duplicated_ids = []
        duplicated_ids_to_delete = []

        for record in duplicated_records:
             docs = Document2.objects \
                 .values_list('id', flat=True) \
                 .filter(
                     uuid=record['uuid'],
                     version=record['version'],
                     is_history=record['is_history'],
                 ) \
                 .order_by('-last_modified')

             duplicated_ids.extend(docs)
             duplicated_ids_to_delete.extend(docs[1:]) # Keep the most recent duplicate

        if duplicated_ids:
             msg = 'Found duplicated Document2 records! %(duplicated_ids)s.\n' \
                   'This will require manual merging or deletion of the Document2 records with duplicate ids and rerunning the migration.\n' \
                   'For more information on manipulating Hue built in objects, have a look at our blog: http://gethue.com/hue-api-execute-some-builtin-commands/\n' \
                   'For example, to delete Document2 records with the oldest duplicated ids execute the following:\n\n' \
                   'from desktop.models import Document2\n' \
                   'Document2.objects.filter(id__in=%(duplicated_ids_to_delete)s).delete()\n' % {
                       'duplicated_ids': duplicated_ids,
                       'duplicated_ids_to_delete': duplicated_ids_to_delete
                   }
             logging.error(msg)
             raise RuntimeError(msg)

        try:
            db.start_transaction()
            # Adding unique constraint on 'Document2', fields ['uuid', 'version', 'is_history']
            db.create_unique(u'desktop_document2', ['uuid', 'version', 'is_history'])
            db.commit_transaction()
        except Exception, e:
            db.rollback_transaction()
            raise e

    def test_add_unique_fk(self):
        """
        Test adding a ForeignKey with unique=True or a OneToOneField
        """
        db.create_table("test_add_unique_fk", [("spam", models.BooleanField(default=False))])
        db.start_transaction()

        db.add_column(
            "test_add_unique_fk", "mock1", models.ForeignKey(db.mock_model("Mock", "mock"), null=True, unique=True)
        )
        db.add_column("test_add_unique_fk", "mock2", models.OneToOneField(db.mock_model("Mock", "mock"), null=True))

        db.rollback_transaction()
        db.delete_table("test_add_unique_fk")

def safe_add_column(table, column, column_data, keep_default = False):
    """when user calls syncdb with askbot the first time
    the auth_user table will be created together with the patched columns
    so, we need to add these columns here in separate transactions
    and roll back if they fail, if we want we could also record - which columns clash
    """
    try:
        db.start_transaction()
        db.add_column(table, column, column_data, keep_default = keep_default)
        db.commit_transaction()
        return True
    except:
        db.rollback_transaction()
        return False

    def backwards(self, orm):
        "Write your backwards methods here."
        try:
            db.start_transaction()
            entities = orm.Profile.objects.values('entity').all()
            print entities
            found = True
            db.commit_transaction()
        except:
            db.rollback_transaction()
            found = False

        if found:
            db.delete_column('profiles_profile', 'entity_id')