Remove system-defined taxonomies, replacing with "read_only" flag

src/openedx_tagging/api.py

@@ -37,7 +37,7 @@ def create_taxonomy(  # pylint: disable=too-many-positional-arguments
     enabled=True,
     allow_multiple=True,
     allow_free_text=False,
-    taxonomy_class: type[Taxonomy] | None = None,
+    read_only=False,
     export_id: str | None = None,
 ) -> Taxonomy:
     """
@@ -52,38 +52,34 @@ def create_taxonomy(  # pylint: disable=too-many-positional-arguments
         enabled=enabled,
         allow_multiple=allow_multiple,
         allow_free_text=allow_free_text,
+        read_only=read_only,
         export_id=export_id,
     )
-    if taxonomy_class:
-        taxonomy.taxonomy_class = taxonomy_class
 
     taxonomy.full_clean()
     taxonomy.save()
-    return taxonomy.cast()
+    return taxonomy
 
 
 def get_taxonomy(taxonomy_id: int) -> Taxonomy | None:
     """
-    Returns a Taxonomy cast to the appropriate subclass which has the given ID.
+    Returns the Taxonomy which has the given ID, or None if not found.
     """
-    taxonomy = Taxonomy.objects.filter(pk=taxonomy_id).first()
-    return taxonomy.cast() if taxonomy else None
+    return Taxonomy.objects.filter(pk=taxonomy_id).first()
 
 
 def get_taxonomy_by_export_id(taxonomy_export_id: str) -> Taxonomy | None:
     """
-    Returns a Taxonomy cast to the appropriate subclass which has the given export ID.
+    Returns the Taxonomy which has the given export ID, or None if not found.
     """
-    taxonomy = Taxonomy.objects.filter(export_id=taxonomy_export_id).first()
-    return taxonomy.cast() if taxonomy else None
+    return Taxonomy.objects.filter(export_id=taxonomy_export_id).first()
 
 
 def get_taxonomies(enabled=True) -> QuerySet[Taxonomy]:
     """
     Returns a queryset containing the enabled taxonomies, sorted by name.
 
     We return a QuerySet here for ease of use with Django Rest Framework and other query-based use cases.
-    So be sure to use `Taxonomy.cast()` to cast these instances to the appropriate subclass before use.
 
     If you want the disabled taxonomies, pass enabled=False.
     If you want all taxonomies (both enabled and disabled), pass enabled=None.
@@ -101,7 +97,7 @@ def get_tags(taxonomy: Taxonomy) -> TagDataQuerySet:
     Note that if the taxonomy is dynamic or free-text, only tags that have
     already been applied to some object will be returned.
     """
-    return taxonomy.cast().get_filtered_tags()
+    return taxonomy.get_filtered_tags()
 
 
 def get_root_tags(taxonomy: Taxonomy) -> TagDataQuerySet:
@@ -110,7 +106,7 @@ def get_root_tags(taxonomy: Taxonomy) -> TagDataQuerySet:
 
     Note that if the taxonomy allows free-text tags, then the returned list will be empty.
     """
-    return taxonomy.cast().get_filtered_tags(depth=1)
+    return taxonomy.get_filtered_tags(depth=1)
 
 
 def search_tags(
@@ -135,7 +131,7 @@ def search_tags(
                 "_value", flat=True
             )
         )
-    qs = taxonomy.cast().get_filtered_tags(
+    qs = taxonomy.get_filtered_tags(
         search_term=search_term,
         excluded_values=excluded_values,
     )
@@ -151,7 +147,7 @@ def get_children_tags(
 
     Note that if the taxonomy allows free-text tags, then the returned list will be empty.
     """
-    return taxonomy.cast().get_filtered_tags(parent_tag_value=parent_tag_value, depth=1)
+    return taxonomy.get_filtered_tags(parent_tag_value=parent_tag_value, depth=1)
 
 
 def resync_object_tags(object_tags: QuerySet | None = None) -> int:
@@ -354,9 +350,7 @@ def tag_object(  # pylint: disable=too-many-positional-arguments
     ObjectTagClass = object_tag_class
     tags = list(dict.fromkeys(tags))  # Remove duplicates preserving order
 
-    if taxonomy:
-        taxonomy = taxonomy.cast()  # Make sure we're using the right subclass. This is a no-op if we are already.
-    elif not taxonomy_export_id:
+    if not taxonomy and not taxonomy_export_id:
         raise ValueError("`taxonomy_export_id` can't be None if `taxonomy` is None")
 
     _check_new_tag_count(len(tags), taxonomy, object_id, taxonomy_export_id)
@@ -444,7 +438,6 @@ def add_tag_to_taxonomy(
     Taxonomy, an exception is raised, otherwise the newly created
     Tag is returned
     """
-    taxonomy = taxonomy.cast()
     new_tag = taxonomy.add_tag(tag, parent_tag_value, external_id)
 
     # Resync all related ObjectTags after creating new Tag to
@@ -463,7 +456,6 @@ def update_tag_in_taxonomy(taxonomy: Taxonomy, tag: str, new_value: str):
 
     Currently only supports updating the Tag value.
     """
-    taxonomy = taxonomy.cast()
     updated_tag = taxonomy.update_tag(tag, new_value)
 
     # Resync all related ObjectTags to update to the new Tag value
@@ -483,7 +475,6 @@ def delete_tags_from_taxonomy(
     the `with_subtags` is not set to `True` it will fail, otherwise
     the sub-tags will be deleted as well.
     """
-    taxonomy = taxonomy.cast()
     taxonomy.delete_tags(tags, with_subtags)
 
 

src/openedx_tagging/import_export/api.py

@@ -216,17 +216,16 @@ def _import_validations(taxonomy: Taxonomy):
     """
     Validates if the taxonomy is allowed to import tags
     """
-    taxonomy = taxonomy.cast()
     if taxonomy.allow_free_text:
         raise ValueError(
             _(
-                "Invalid taxonomy ({id}): You cannot import a free-form taxonomy."
+                "Invalid taxonomy ({id}): You cannot import to a free-text taxonomy."
             ).format(id=taxonomy.id)
         )
 
-    if taxonomy.system_defined:
+    if taxonomy.read_only:
         raise ValueError(
             _(
-                "Invalid taxonomy ({id}): You cannot import a system-defined taxonomy."
+                "Invalid taxonomy ({id}): You cannot import to a read-only taxonomy."
             ).format(id=taxonomy.id)
         )

src/openedx_tagging/migrations/0021_remove_system_defined_add_read_only.py

@@ -0,0 +1,89 @@
+"""
+Remove the "system-defined" taxonomy machinery (Taxonomy subclasses and the
+``_taxonomy_class`` casting column) in favour of a simple ``read_only`` flag.
+
+See https://github.com/openedx/openedx-core/issues/634 for the rationale.
+
+This migration:
+
+* Adds the ``read_only`` boolean field to Taxonomy.
+* Marks any taxonomy that used to be "system-defined" (i.e. had a
+  ``_taxonomy_class`` set) as ``read_only=True``, so that its tags remain
+  immutable as they were before.
+* Handles the auto-created "Languages" taxonomy (``id=-1``), which was created
+  by ``0012_language_taxonomy`` and is no longer supported: if it has been used
+  (any related object tags exist) it is converted into a regular, editable
+  taxonomy; otherwise it is deleted.
+* Removes the now-unused ``_taxonomy_class`` column and the proxy models.
+"""
+
+from django.db import migrations, models
+
+# The Languages taxonomy was auto-created with this fixed id by 0012_language_taxonomy.
+LANGUAGE_TAXONOMY_ID = -1
+
+
+def forwards(apps, schema_editor):
+    """
+    Migrate system-defined taxonomies to the new read_only flag, and either
+    convert or drop the auto-created Languages taxonomy.
+    """
+    Taxonomy = apps.get_model("oel_tagging", "Taxonomy")
+    ObjectTag = apps.get_model("oel_tagging", "ObjectTag")
+
+    language_taxonomy = Taxonomy.objects.filter(id=LANGUAGE_TAXONOMY_ID).first()
+    if language_taxonomy:
+        if ObjectTag.objects.filter(taxonomy_id=LANGUAGE_TAXONOMY_ID).exists():
+            # It's in use, so convert it into a regular, editable taxonomy,
+            # keeping whatever language Tags have already been created.
+            language_taxonomy._taxonomy_class = None
+            language_taxonomy.read_only = False
+            language_taxonomy.save()
+        else:
+            # Unused, so remove it (and its tags) entirely.
+            language_taxonomy.delete()
+
+    # Any remaining taxonomy that was backed by a subclass was "system-defined",
+    # meaning its tags could not be modified. Preserve that by marking it read-only.
+    Taxonomy.objects.exclude(_taxonomy_class__isnull=True).exclude(_taxonomy_class="").update(read_only=True)
+
+
+def backwards(apps, schema_editor):
+    """
+    Nothing to undo here: the deleted Languages taxonomy and the subclass
+    information cannot be restored, because the subclasses no longer exist.
+    The ``read_only`` column is dropped by the reversal of the AddField
+    operation.
+    """
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('oel_tagging', '0020_tag_depth_and_lineage'),
+    ]
+
+    operations = [
+        migrations.DeleteModel(
+            name='LanguageTaxonomy',
+        ),
+        migrations.DeleteModel(
+            name='ModelSystemDefinedTaxonomy',
+        ),
+        migrations.DeleteModel(
+            name='SystemDefinedTaxonomy',
+        ),
+        migrations.DeleteModel(
+            name='UserSystemDefinedTaxonomy',
+        ),
+        migrations.AddField(
+            model_name='taxonomy',
+            name='read_only',
+            field=models.BooleanField(default=False, help_text='Indicates that the tags in this taxonomy are maintained by the system or an external integration; taxonomy admins will not be permitted to add, edit, or delete its tags.'),
+        ),
+        migrations.RunPython(forwards, backwards),
+        migrations.RemoveField(
+            model_name='taxonomy',
+            name='_taxonomy_class',
+        ),
+    ]

src/openedx_tagging/models/__init__.py

@@ -3,4 +3,3 @@
 """
 from .base import ObjectTag, Tag, Taxonomy
 from .import_export import TagImportTask, TagImportTaskState
-from .system_defined import LanguageTaxonomy, ModelSystemDefinedTaxonomy, UserSystemDefinedTaxonomy