updating the database enforcing iban type definition and switching from

a one show initial migration to a pre migration check

django_iban/__init__.py

@@ -0,0 +1 @@
+default_app_config = 'django_iban.apps.IBANFieldConfig'

django_iban/apps.py

@@ -0,0 +1,48 @@
+from os.path                  import join
+from os.path                  import dirname
+from os.path                  import abspath
+from django.db                import connection
+from itertools                import chain
+from django.apps              import AppConfig
+from django.conf              import settings
+from django.db.models.signals import pre_migrate
+
+from django_iban.fields import IBANField
+
+
+class IBANFieldConfig(AppConfig):
+    name         = 'django_iban'
+    verbose_name = 'Django IBAN Field'
+
+    def ready(self):
+        pre_migrate.connect(self.enforce_database_constraint_callback)
+
+    def enforce_database_constraint_callback(self, sender, **kwargs):
+        database_backend = settings.DATABASES['default']['ENGINE']
+        enforcing_status = any([
+            iban_field.enforce_database_constraint
+            for iban_field in chain.from_iterable([
+                model._meta.fields for model in sender.get_models()])
+            if isinstance(iban_field, IBANField)])
+
+        if database_backend in (
+            'django.db.backends.postgresql_psycopg2',
+            'django.contrib.gis.db.backends.postgis'
+        ) and enforcing_status:
+            cursor    = connection.cursor()
+            base_path = dirname(abspath(__file__))
+
+            cursor.execute("select * from pg_available_extensions where name = 'plpythonu'")
+            if cursor.rowcount == 1:
+                cursor.execute(
+                    """
+                    SELECT p.proname FROM pg_catalog.pg_namespace n JOIN pg_catalog.pg_proc p
+                    ON p.pronamespace = n.oid
+                    WHERE n.nspname = 'public' AND p.proname = 'is_valid_iban';
+                    """)
+
+                if cursor.rowcount != 1:
+                    with open(join(base_path, 'pg_iban_validator.sql')) as iban_type_definition:
+                        cursor.execute(iban_type_definition.read())
+            else:
+                raise Exception('django_iban requested database enforcing but plpython extension not installed!')