Finally add Map.create_at field

This field should have been added at the beginning, but this is
like planting trees, isn't it ?

umap/migrations/0014_map_created_at.py

@@ -0,0 +1,23 @@
+# Generated by Django 4.2.2 on 2023-09-27 08:50
+
+from django.db import migrations, models
+import django.utils.timezone
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("umap", "0013_datalayer_edit_status"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="map",
+            name="created_at",
+            field=models.DateTimeField(
+                auto_now_add=True, default=django.utils.timezone.now
+            ),
+            preserve_default=False,
+        ),
+        # Did not find a way to provide default from a column in the field creation
+        migrations.RunSQL("UPDATE umap_map SET created_at=modified_at;"),
+    ]

umap/models.py

@@ -163,6 +163,7 @@ class Map(NamedModel):
         on_delete=models.SET_DEFAULT,
         default=get_default_licence,
     )
+    created_at = models.DateTimeField(auto_now_add=True)
     modified_at = models.DateTimeField(auto_now=True)
     owner = models.ForeignKey(
         settings.AUTH_USER_MODEL,