Fix exception in 0002 migration (#579)

constance/migrations/0002_migrate_from_old_table.py

@@ -1,7 +1,6 @@
 from logging import getLogger
 
 from django.core.management.color import no_style
-from django.db import DatabaseError
 from django.db import migrations
 
 logger = getLogger(__name__)
@@ -14,15 +13,16 @@ def _migrate_from_old_table(apps, schema_editor) -> None:
     """
     connection = schema_editor.connection
     quoted_string = ', '.join([connection.ops.quote_name(item) for item in ['id', 'key', 'value']])
-    try:
-        with connection.cursor() as cursor:
-            cursor.execute(
-                f'INSERT INTO constance_constance ( {quoted_string} ) SELECT {quoted_string} FROM constance_config',  # noqa: S608
-                [],
-            )
-            cursor.execute('DROP TABLE constance_config', [])
-    except DatabaseError:
-        logger.exception('copy data from old constance table to a new one')
+    old_table_name = 'constance_config'
+    with connection.cursor() as cursor:
+        if old_table_name not in connection.introspection.table_names():
+            logger.info('Old table does not exist, skipping')
+            return
+        cursor.execute(
+            f'INSERT INTO constance_constance ( {quoted_string} ) SELECT {quoted_string} FROM {old_table_name}',  # noqa: S608
+            [],
+        )
+        cursor.execute(f'DROP TABLE {old_table_name}', [])
 
     Constance = apps.get_model('constance', 'Constance')
     sequence_sql = connection.ops.sequence_reset_sql(no_style(), [Constance])