From 9a913882ec621ab3d5131b8b9665bba8d12b51fe Mon Sep 17 00:00:00 2001
From: Jillian Vogel <jill@opencraft.com>
Date: Wed, 28 Jun 2023 21:47:12 +0930
Subject: [PATCH 1/3] feat: uses InheritanceManager for Taxonomy

* Adds dependency on django-model-utils (which is already a dependency
  of edx-platform)
* Updates api.get_taxonomies and adds api.get_taxonomy, which return the correct subclass of Taxonomy
* Adds a simple SystemTaxonomy base class to verify ^
* Updates tests to ensure correct classes are used, and no extra queries are required for this functionality
---
 openedx_tagging/core/tagging/api.py           | 11 ++-
 .../tagging/migrations/0002_systemtaxonomy.py | 30 ++++++++
 openedx_tagging/core/tagging/models.py        | 59 ++++++++++++++-
 requirements/base.in                          |  2 +
 requirements/base.txt                         |  3 +
 requirements/dev.txt                          |  3 +
 requirements/doc.txt                          |  3 +
 requirements/quality.txt                      |  3 +
 requirements/test.txt                         |  3 +
 .../openedx_tagging/core/tagging/test_api.py  | 22 +++---
 .../core/tagging/test_models.py               | 71 +++++++++++--------
 .../core/tagging/test_rules.py                | 15 ++--
 12 files changed, 175 insertions(+), 50 deletions(-)
 create mode 100644 openedx_tagging/core/tagging/migrations/0002_systemtaxonomy.py

diff --git a/openedx_tagging/core/tagging/api.py b/openedx_tagging/core/tagging/api.py
index 6b9bf2a04..7bd869bd0 100644
--- a/openedx_tagging/core/tagging/api.py
+++ b/openedx_tagging/core/tagging/api.py
@@ -10,7 +10,7 @@
 Please look at the models.py file for more information about the kinds of data
 are stored in this app.
 """
-from typing import List, Type
+from typing import List, Type, Union
 
 from django.db.models import QuerySet
 from django.utils.translation import gettext_lazy as _
@@ -45,12 +45,19 @@ def get_taxonomies(enabled=True) -> QuerySet:
     If you want the disabled taxonomies, pass enabled=False.
     If you want all taxonomies (both enabled and disabled), pass enabled=None.
     """
-    queryset = Taxonomy.objects.order_by("name", "id")
+    queryset = Taxonomy.objects.order_by("name", "id").select_subclasses()
     if enabled is None:
         return queryset.all()
     return queryset.filter(enabled=enabled)
 
 
+def get_taxonomy(id: int) -> Union[Taxonomy, None]:
+    """
+    Returns a Taxonomy of the appropriate subclass which has the given ID.
+    """
+    return Taxonomy.objects.filter(id=id).select_subclasses().first()
+
+
 def get_tags(taxonomy: Taxonomy) -> List[Tag]:
     """
     Returns a list of predefined tags for the given taxonomy.
diff --git a/openedx_tagging/core/tagging/migrations/0002_systemtaxonomy.py b/openedx_tagging/core/tagging/migrations/0002_systemtaxonomy.py
new file mode 100644
index 000000000..da4338b41
--- /dev/null
+++ b/openedx_tagging/core/tagging/migrations/0002_systemtaxonomy.py
@@ -0,0 +1,30 @@
+# Generated by Django 3.2.19 on 2023-06-28 10:19
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("oel_tagging", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name="SystemTaxonomy",
+            fields=[
+                (
+                    "taxonomy_ptr",
+                    models.OneToOneField(
+                        auto_created=True,
+                        on_delete=django.db.models.deletion.CASCADE,
+                        parent_link=True,
+                        primary_key=True,
+                        serialize=False,
+                        to="oel_tagging.taxonomy",
+                    ),
+                ),
+            ],
+            bases=("oel_tagging.taxonomy",),
+        ),
+    ]
diff --git a/openedx_tagging/core/tagging/models.py b/openedx_tagging/core/tagging/models.py
index c90f8a34a..a802fdfc0 100644
--- a/openedx_tagging/core/tagging/models.py
+++ b/openedx_tagging/core/tagging/models.py
@@ -3,10 +3,10 @@
 
 from django.db import models
 from django.utils.translation import gettext_lazy as _
+from model_utils.managers import InheritanceManager
 
 from openedx_learning.lib.fields import MultiCollationTextField, case_insensitive_char_field
 
-
 # Maximum depth allowed for a hierarchical taxonomy's tree of tags.
 TAXONOMY_MAX_DEPTH = 3
 
@@ -93,11 +93,19 @@ def get_lineage(self) -> Lineage:
         return lineage
 
 
+class TaxonomyManager(InheritanceManager):
+    """
+    Base Taxonomy class uses InheritanceManager to help instantiate subclasses during queries.
+    """
+
+
 class Taxonomy(models.Model):
     """
     Represents a namespace and rules for a group of tags which can be applied to a particular Open edX object.
     """
 
+    objects = TaxonomyManager()
+
     id = models.BigAutoField(primary_key=True)
     name = case_insensitive_char_field(
         null=False,
@@ -140,6 +148,18 @@ class Taxonomy(models.Model):
     class Meta:
         verbose_name_plural = "Taxonomies"
 
+    def __repr__(self):
+        """
+        Developer-facing representation of a Taxonomy.
+        """
+        return str(self)
+
+    def __str__(self):
+        """
+        User-facing string representation of a Taxonomy.
+        """
+        return f"<{self.__class__.__name__}> {self.name}"
+
     @property
     def system_defined(self) -> bool:
         """
@@ -351,6 +371,18 @@ class Meta:
             models.Index(fields=["taxonomy", "_value"]),
         ]
 
+    def __repr__(self):
+        """
+        Developer-facing representation of an ObjectTag.
+        """
+        return str(self)
+
+    def __str__(self):
+        """
+        User-facing string representation of an ObjectTag.
+        """
+        return f"{self.object_id} ({self.object_type}): {self.name}={self.value}"
+
     @property
     def name(self) -> str:
         """
@@ -402,7 +434,16 @@ def is_valid(self) -> bool:
 
         A valid ObjectTag must be linked to a Taxonomy, and be a valid tag in that taxonomy.
         """
-        return self.taxonomy_id and self.taxonomy.validate_object_tag(self)
+        if self.taxonomy_id:
+            # self.taxonomy doesn't return the correct subclass, so we must fetch it
+            # FIXME:
+            # * cache to prevent re-fetching the taxonomy
+            # * discourage people from using self.taxonomy
+            taxonomy = (
+                Taxonomy.objects.filter(id=self.taxonomy_id).select_subclasses().first()
+            )
+            return taxonomy.validate_object_tag(self)
+        return False
 
     def get_lineage(self) -> Lineage:
         """
@@ -456,3 +497,17 @@ def resync(self) -> bool:
             changed = True
 
         return changed
+
+
+class SystemTaxonomy(Taxonomy):
+    """
+    System-defined taxonomies are not editable by normal users; they're defined by fixtures/migrations, and may have
+    dynamically-determined Tags and ObjectTags.
+    """
+
+    @property
+    def system_defined(self) -> bool:
+        """
+        This is a system-defined taxonomy.
+        """
+        return True
diff --git a/requirements/base.in b/requirements/base.in
index d17e06397..a9e1bfc7b 100644
--- a/requirements/base.in
+++ b/requirements/base.in
@@ -6,3 +6,5 @@ Django<5.0                # Web application framework
 djangorestframework<4.0   # REST API
 
 rules<4.0                 # Django extension for rules-based authorization checks
+
+django-model-utils        # InheritanceManager for openedx-tagging models
diff --git a/requirements/base.txt b/requirements/base.txt
index 8c0f00b63..a5dc5b291 100644
--- a/requirements/base.txt
+++ b/requirements/base.txt
@@ -10,7 +10,10 @@ django==3.2.19
     # via
     #   -c requirements/constraints.txt
     #   -r requirements/base.in
+    #   django-model-utils
     #   djangorestframework
+django-model-utils==4.3.1
+    # via -r requirements/base.in
 djangorestframework==3.14.0
     # via -r requirements/base.in
 pytz==2023.3
diff --git a/requirements/dev.txt b/requirements/dev.txt
index add9cf1de..ccc5c194f 100644
--- a/requirements/dev.txt
+++ b/requirements/dev.txt
@@ -78,10 +78,13 @@ django==3.2.19
     #   -c requirements/constraints.txt
     #   -r requirements/quality.txt
     #   django-debug-toolbar
+    #   django-model-utils
     #   djangorestframework
     #   edx-i18n-tools
 django-debug-toolbar==4.1.0
     # via -r requirements/dev.in
+django-model-utils==4.3.1
+    # via -r requirements/quality.txt
 djangorestframework==3.14.0
     # via -r requirements/quality.txt
 docutils==0.20.1
diff --git a/requirements/doc.txt b/requirements/doc.txt
index d9a4c3c2a..fc8778e62 100644
--- a/requirements/doc.txt
+++ b/requirements/doc.txt
@@ -41,8 +41,11 @@ django==3.2.19
     # via
     #   -c requirements/constraints.txt
     #   -r requirements/test.txt
+    #   django-model-utils
     #   djangorestframework
     #   sphinxcontrib-django
+django-model-utils==4.3.1
+    # via -r requirements/test.txt
 djangorestframework==3.14.0
     # via -r requirements/test.txt
 doc8==1.1.1
diff --git a/requirements/quality.txt b/requirements/quality.txt
index c580bd7e1..e78a0b511 100644
--- a/requirements/quality.txt
+++ b/requirements/quality.txt
@@ -47,7 +47,10 @@ django==3.2.19
     # via
     #   -c requirements/constraints.txt
     #   -r requirements/test.txt
+    #   django-model-utils
     #   djangorestframework
+django-model-utils==4.3.1
+    # via -r requirements/test.txt
 djangorestframework==3.14.0
     # via -r requirements/test.txt
 docutils==0.20.1
diff --git a/requirements/test.txt b/requirements/test.txt
index e64d8190b..7e52fbbfe 100644
--- a/requirements/test.txt
+++ b/requirements/test.txt
@@ -23,7 +23,10 @@ ddt==1.6.0
     # via
     #   -c requirements/constraints.txt
     #   -r requirements/base.txt
+    #   django-model-utils
     #   djangorestframework
+django-model-utils==4.3.1
+    # via -r requirements/base.txt
 djangorestframework==3.14.0
     # via -r requirements/base.txt
 exceptiongroup==1.1.1
diff --git a/tests/openedx_tagging/core/tagging/test_api.py b/tests/openedx_tagging/core/tagging/test_api.py
index 7a418e687..2ba402ac3 100644
--- a/tests/openedx_tagging/core/tagging/test_api.py
+++ b/tests/openedx_tagging/core/tagging/test_api.py
@@ -31,14 +31,20 @@ def test_create_taxonomy(self):
     def test_get_taxonomies(self):
         tax1 = tagging_api.create_taxonomy("Enabled")
         tax2 = tagging_api.create_taxonomy("Disabled", enabled=False)
-        enabled = tagging_api.get_taxonomies()
-        assert list(enabled) == [tax1, self.taxonomy]
-
-        disabled = tagging_api.get_taxonomies(enabled=False)
-        assert list(disabled) == [tax2]
-
-        both = tagging_api.get_taxonomies(enabled=None)
-        assert list(both) == [tax2, tax1, self.taxonomy]
+        with self.assertNumQueries(1):
+            enabled = list(tagging_api.get_taxonomies())
+        assert enabled == [tax1, self.taxonomy, self.system_taxonomy]
+        assert str(enabled[0]) == "<Taxonomy> Enabled"
+        assert str(enabled[1]) == "<Taxonomy> Life on Earth"
+        assert str(enabled[2]) == "<SystemTaxonomy> System Languages"
+
+        with self.assertNumQueries(1):
+            disabled = list(tagging_api.get_taxonomies(enabled=False))
+        assert disabled == [tax2]
+
+        with self.assertNumQueries(1):
+            both = list(tagging_api.get_taxonomies(enabled=None))
+        assert both == [tax2, tax1, self.taxonomy, self.system_taxonomy]
 
     def test_get_tags(self):
         self.setup_tag_depths()
diff --git a/tests/openedx_tagging/core/tagging/test_models.py b/tests/openedx_tagging/core/tagging/test_models.py
index d74c041db..44709febc 100644
--- a/tests/openedx_tagging/core/tagging/test_models.py
+++ b/tests/openedx_tagging/core/tagging/test_models.py
@@ -3,7 +3,7 @@
 import ddt
 from django.test.testcases import TestCase
 
-from openedx_tagging.core.tagging.models import ObjectTag, Tag, Taxonomy
+from openedx_tagging.core.tagging.models import ObjectTag, SystemTaxonomy, Tag, Taxonomy
 
 
 def get_tag(value):
@@ -65,6 +65,8 @@ def setUp(self):
             get_tag("Porifera"),
         ]
 
+        self.system_taxonomy = SystemTaxonomy.objects.create(name="System Languages")
+
     def setup_tag_depths(self):
         """
         Annotate our tags with depth so we can compare them.
@@ -85,10 +87,16 @@ class TestModelTagTaxonomy(TestTagTaxonomyMixin, TestCase):
 
     def test_system_defined(self):
         assert not self.taxonomy.system_defined
+        assert self.system_taxonomy.system_defined
 
     def test_representations(self):
-        assert str(self.bacteria) == "Tag (1) Bacteria"
-        assert repr(self.bacteria) == "Tag (1) Bacteria"
+        assert str(self.taxonomy) == repr(self.taxonomy) == "<Taxonomy> Life on Earth"
+        assert (
+            str(self.system_taxonomy)
+            == repr(self.system_taxonomy)
+            == "<SystemTaxonomy> System Languages"
+        )
+        assert str(self.bacteria) == repr(self.bacteria) == "Tag (1) Bacteria"
 
     @ddt.data(
         # Root tags just return their own value
@@ -136,6 +144,19 @@ class TestModelObjectTag(TestTagTaxonomyMixin, TestCase):
     def setUp(self):
         super().setUp()
         self.tag = self.bacteria
+        self.object_tag = ObjectTag.objects.create(
+            object_id="object:id:1",
+            object_type="life",
+            taxonomy=self.taxonomy,
+            tag=self.tag,
+        )
+
+    def test_representations(self):
+        assert (
+            str(self.object_tag)
+            == repr(self.object_tag)
+            == "object:id:1 (life): Life on Earth=Bacteria"
+        )
 
     def test_object_tag_name(self):
         # ObjectTag's name defaults to its taxonomy's name
@@ -159,50 +180,38 @@ def test_object_tag_name(self):
 
     def test_object_tag_value(self):
         # ObjectTag's value defaults to its tag's value
-        object_tag = ObjectTag.objects.create(
-            object_id="object:id",
-            object_type="any_old_object",
-            taxonomy=self.taxonomy,
-            tag=self.tag,
-        )
-        assert object_tag.value == self.tag.value
+        assert self.object_tag.value == self.tag.value
 
         # Even if we overwrite the value, it still uses the tag's value
-        object_tag.value = "Another tag"
-        assert object_tag.value == self.tag.value
-        object_tag.save()
-        assert object_tag.value == self.tag.value
+        self.object_tag.value = "Another tag"
+        assert self.object_tag.value == self.tag.value
+        self.object_tag.save()
+        assert self.object_tag.value == self.tag.value
 
         # But if the tag is deleted, then the object_tag's value reverts to our cached value
         self.tag.delete()
-        object_tag.refresh_from_db()
-        assert object_tag.value == "Another tag"
+        self.object_tag.refresh_from_db()
+        assert self.object_tag.value == "Another tag"
 
     def test_object_tag_lineage(self):
         # ObjectTag's value defaults to its tag's lineage
-        object_tag = ObjectTag.objects.create(
-            object_id="object:id",
-            object_type="any_old_object",
-            taxonomy=self.taxonomy,
-            tag=self.tag,
-        )
-        assert object_tag.get_lineage() == self.tag.get_lineage()
+        assert self.object_tag.get_lineage() == self.tag.get_lineage()
 
         # Even if we overwrite the value, it still uses the tag's lineage
-        object_tag.value = "Another tag"
-        assert object_tag.get_lineage() == self.tag.get_lineage()
-        object_tag.save()
-        assert object_tag.get_lineage() == self.tag.get_lineage()
+        self.object_tag.value = "Another tag"
+        assert self.object_tag.get_lineage() == self.tag.get_lineage()
+        self.object_tag.save()
+        assert self.object_tag.get_lineage() == self.tag.get_lineage()
 
         # But if the tag is deleted, then the object_tag's lineage reverts to our cached value
         self.tag.delete()
-        object_tag.refresh_from_db()
-        assert object_tag.get_lineage() == ["Another tag"]
+        self.object_tag.refresh_from_db()
+        assert self.object_tag.get_lineage() == ["Another tag"]
 
     def test_object_tag_is_valid(self):
         object_tag = ObjectTag(
             object_id="object:id",
-            object_type="any_old_object",
+            object_type="life",
         )
         assert not object_tag.is_valid
 
@@ -215,6 +224,7 @@ def test_object_tag_is_valid(self):
         # or, we can have no tag, and a free-text taxonomy
         object_tag.tag = None
         self.taxonomy.allow_free_text = True
+        self.taxonomy.save()
         assert object_tag.is_valid
 
     def test_validate_object_tag_invalid(self):
@@ -281,6 +291,7 @@ def test_tag_object(self):
 
     def test_tag_object_free_text(self):
         self.taxonomy.allow_free_text = True
+        self.taxonomy.save()
         object_tags = self.taxonomy.tag_object(
             ["Eukaryota Xenomorph"],
             "biology101",
diff --git a/tests/openedx_tagging/core/tagging/test_rules.py b/tests/openedx_tagging/core/tagging/test_rules.py
index 2c2b4e748..6a50feec4 100644
--- a/tests/openedx_tagging/core/tagging/test_rules.py
+++ b/tests/openedx_tagging/core/tagging/test_rules.py
@@ -3,9 +3,9 @@
 import ddt
 from django.contrib.auth import get_user_model
 from django.test.testcases import TestCase
-from mock import Mock
 
-from openedx_tagging.core.tagging.models import ObjectTag, Tag, Taxonomy
+from openedx_tagging.core.tagging.api import get_taxonomy
+from openedx_tagging.core.tagging.models import ObjectTag, Tag
 
 from .test_models import TestTagTaxonomyMixin
 
@@ -50,12 +50,13 @@ def setUp(self):
     )
     def test_add_change_taxonomy(self, perm):
         """Taxonomy administrators can create or modify any Taxonomy"""
+        taxonomy = get_taxonomy(self.taxonomy.id)
         assert self.superuser.has_perm(perm)
-        assert self.superuser.has_perm(perm, self.taxonomy)
+        assert self.superuser.has_perm(perm, taxonomy)
         assert self.staff.has_perm(perm)
-        assert self.staff.has_perm(perm, self.taxonomy)
+        assert self.staff.has_perm(perm, taxonomy)
         assert not self.learner.has_perm(perm)
-        assert not self.learner.has_perm(perm, self.taxonomy)
+        assert not self.learner.has_perm(perm, taxonomy)
 
     @ddt.data(
         "oel_tagging.add_taxonomy",
@@ -64,9 +65,7 @@ def test_add_change_taxonomy(self, perm):
     )
     def test_system_taxonomy(self, perm):
         """Taxonomy administrators cannot edit system taxonomies"""
-        # TODO: use SystemTaxonomy when available
-        system_taxonomy = Mock(spec=Taxonomy)
-        system_taxonomy.system_defined.return_value = True
+        system_taxonomy = get_taxonomy(self.system_taxonomy.id)
         assert self.superuser.has_perm(perm, system_taxonomy)
         assert not self.staff.has_perm(perm, system_taxonomy)
         assert not self.learner.has_perm(perm, system_taxonomy)

From 1a6e3ce81651459b72376cd790165f78108a8e72 Mon Sep 17 00:00:00 2001
From: Jillian Vogel <jill@opencraft.com>
Date: Wed, 28 Jun 2023 22:17:02 +0930
Subject: [PATCH 2/3] style: adds type annotations to rules

---
 openedx_tagging/core/tagging/rules.py | 14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

diff --git a/openedx_tagging/core/tagging/rules.py b/openedx_tagging/core/tagging/rules.py
index 7ef984b86..72b9fa6b4 100644
--- a/openedx_tagging/core/tagging/rules.py
+++ b/openedx_tagging/core/tagging/rules.py
@@ -1,6 +1,12 @@
 """Django rules-based permissions for tagging"""
 
 import rules
+from django.contrib.auth import get_user_model
+
+from .models import ObjectTag, Tag, Taxonomy
+
+User = get_user_model()
+
 
 # Global staff are taxonomy admins.
 # (Superusers can already do anything)
@@ -8,7 +14,7 @@
 
 
 @rules.predicate
-def can_view_taxonomy(user, taxonomy=None):
+def can_view_taxonomy(user: User, taxonomy: Taxonomy = None) -> bool:
     """
     Anyone can view an enabled taxonomy,
     but only taxonomy admins can view a disabled taxonomy.
@@ -17,7 +23,7 @@ def can_view_taxonomy(user, taxonomy=None):
 
 
 @rules.predicate
-def can_change_taxonomy(user, taxonomy=None):
+def can_change_taxonomy(user: User, taxonomy: Taxonomy = None) -> bool:
     """
     Even taxonomy admins cannot change system taxonomies.
     """
@@ -27,7 +33,7 @@ def can_change_taxonomy(user, taxonomy=None):
 
 
 @rules.predicate
-def can_change_taxonomy_tag(user, tag=None):
+def can_change_taxonomy_tag(user: User, tag: Tag = None) -> bool:
     """
     Even taxonomy admins cannot add tags to system taxonomies (their tags are system-defined), or free-text taxonomies
     (these don't have predefined tags).
@@ -44,7 +50,7 @@ def can_change_taxonomy_tag(user, tag=None):
 
 
 @rules.predicate
-def can_change_object_tag(user, object_tag=None):
+def can_change_object_tag(user: User, object_tag: ObjectTag = None) -> bool:
     """
     Taxonomy admins can create or modify object tags on enabled taxonomies.
     """

From 799ad2a92d5f5e0542b318c6d1ef2785fdfda598 Mon Sep 17 00:00:00 2001
From: Jillian Vogel <jill@opencraft.com>
Date: Tue, 4 Jul 2023 13:36:17 +0930
Subject: [PATCH 3/3] refactor: renames ObjectTag.taxonomy to
 ObjectTag._taxonomy

so we can ensure that the ObjectTag.taxonomy returned is of the correct subclass.
---
 openedx_tagging/core/tagging/api.py           |  4 +-
 .../migrations/0003_objecttag__taxonomy.py    | 28 ++++++++
 openedx_tagging/core/tagging/models.py        | 65 ++++++++++++++-----
 .../core/tagging/test_models.py               | 27 +++++++-
 .../core/tagging/test_rules.py                |  4 +-
 5 files changed, 105 insertions(+), 23 deletions(-)
 create mode 100644 openedx_tagging/core/tagging/migrations/0003_objecttag__taxonomy.py

diff --git a/openedx_tagging/core/tagging/api.py b/openedx_tagging/core/tagging/api.py
index 7bd869bd0..59438f183 100644
--- a/openedx_tagging/core/tagging/api.py
+++ b/openedx_tagging/core/tagging/api.py
@@ -94,8 +94,8 @@ def get_object_tags(
     Pass valid_only=False when displaying tags to content authors, so they can see invalid tags too.
     Invalid tags will likely be hidden from learners.
     """
-    tags = ObjectTag.objects.filter(
-        taxonomy=taxonomy, object_id=object_id, object_type=object_type
+    tags = taxonomy.objecttag_set.filter(
+        object_id=object_id, object_type=object_type
     ).order_by("id")
     return [tag for tag in tags if not valid_only or taxonomy.validate_object_tag(tag)]
 
diff --git a/openedx_tagging/core/tagging/migrations/0003_objecttag__taxonomy.py b/openedx_tagging/core/tagging/migrations/0003_objecttag__taxonomy.py
new file mode 100644
index 000000000..1dbc3b762
--- /dev/null
+++ b/openedx_tagging/core/tagging/migrations/0003_objecttag__taxonomy.py
@@ -0,0 +1,28 @@
+# Generated by Django 3.2.19 on 2023-07-04 02:20
+
+import django.db.models.deletion
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("oel_tagging", "0002_systemtaxonomy"),
+    ]
+
+    operations = [
+        migrations.RemoveIndex(
+            model_name="objecttag",
+            name="oel_tagging_taxonom_3668ec_idx",
+        ),
+        migrations.RenameField(
+            model_name="objecttag",
+            old_name="taxonomy",
+            new_name="_taxonomy",
+        ),
+        migrations.AddIndex(
+            model_name="objecttag",
+            index=models.Index(
+                fields=["_taxonomy", "_value"], name="oel_tagging__taxono_af4c8a_idx"
+            ),
+        ),
+    ]
diff --git a/openedx_tagging/core/tagging/models.py b/openedx_tagging/core/tagging/models.py
index a802fdfc0..acee7fb3f 100644
--- a/openedx_tagging/core/tagging/models.py
+++ b/openedx_tagging/core/tagging/models.py
@@ -224,7 +224,7 @@ def validate_object_tag(
         """
         # Must be linked to this taxonomy
         if check_taxonomy and (
-            not object_tag.taxonomy_id or object_tag.taxonomy_id != self.id
+            not object_tag.taxonomy or object_tag.taxonomy.id != self.id
         ):
             return False
 
@@ -261,8 +261,8 @@ def tag_object(
 
         current_tags = {
             tag.tag_ref: tag
-            for tag in ObjectTag.objects.filter(
-                taxonomy=self, object_id=object_id, object_type=object_type
+            for tag in self.objecttag_set.filter(
+                object_id=object_id, object_type=object_type
             )
         }
         updated_tags = []
@@ -330,7 +330,7 @@ class ObjectTag(models.Model):
         max_length=255,
         help_text=_("Type of object being tagged"),
     )
-    taxonomy = models.ForeignKey(
+    _taxonomy = models.ForeignKey(
         Taxonomy,
         null=True,
         default=None,
@@ -368,9 +368,16 @@ class ObjectTag(models.Model):
 
     class Meta:
         indexes = [
-            models.Index(fields=["taxonomy", "_value"]),
+            models.Index(fields=["_taxonomy", "_value"]),
         ]
 
+    def __init__(self, *args, **kwargs):
+        """
+        Initializes the cached taxonomy instance.
+        """
+        super().__init__(*args, **kwargs)
+        self._cached_taxonomy = None
+
     def __repr__(self):
         """
         Developer-facing representation of an ObjectTag.
@@ -383,6 +390,35 @@ def __str__(self):
         """
         return f"{self.object_id} ({self.object_type}): {self.name}={self.value}"
 
+    @property
+    def taxonomy(self) -> str:
+        """
+        Returns this tag's taxonomy object, instantiated as the correct Taxonomy subclass.
+
+        This instance is cached so that subsequent calls to self.taxonomy don't re-fetch the value.
+
+        Returns None if taxonomy_id is not set.
+        """
+        if not self._taxonomy_id:
+            self._cached_taxonomy = None
+
+        elif not self._cached_taxonomy:
+            self._cached_taxonomy = (
+                Taxonomy.objects.filter(id=self._taxonomy_id)
+                .select_subclasses()
+                .first()
+            )
+
+        return self._cached_taxonomy
+
+    @taxonomy.setter
+    def taxonomy(self, taxonomy: Taxonomy):
+        """
+        Stores to the _taxonomy field.
+        """
+        self._taxonomy = taxonomy
+        self._cached_taxonomy = taxonomy
+
     @property
     def name(self) -> str:
         """
@@ -391,7 +427,7 @@ def name(self) -> str:
         If taxonomy is set, then returns its name.
         Otherwise, returns the cached _name field.
         """
-        return self.taxonomy.name if self.taxonomy_id else self._name
+        return self.taxonomy.name if self._taxonomy_id else self._name
 
     @name.setter
     def name(self, name: str):
@@ -434,15 +470,8 @@ def is_valid(self) -> bool:
 
         A valid ObjectTag must be linked to a Taxonomy, and be a valid tag in that taxonomy.
         """
-        if self.taxonomy_id:
-            # self.taxonomy doesn't return the correct subclass, so we must fetch it
-            # FIXME:
-            # * cache to prevent re-fetching the taxonomy
-            # * discourage people from using self.taxonomy
-            taxonomy = (
-                Taxonomy.objects.filter(id=self.taxonomy_id).select_subclasses().first()
-            )
-            return taxonomy.validate_object_tag(self)
+        if self._taxonomy_id:
+            return self.taxonomy.validate_object_tag(self)
         return False
 
     def get_lineage(self) -> Lineage:
@@ -468,8 +497,10 @@ def resync(self) -> bool:
         changed = False
 
         # Locate a taxonomy matching _name
-        if not self.taxonomy_id:
-            for taxonomy in Taxonomy.objects.filter(name=self.name, enabled=True):
+        if not self._taxonomy_id:
+            for taxonomy in Taxonomy.objects.filter(
+                name=self.name, enabled=True
+            ).select_subclasses():
                 # Make sure this taxonomy will accept object tags like this.
                 self.taxonomy = taxonomy
                 if taxonomy.validate_object_tag(self, check_tag=False):
diff --git a/tests/openedx_tagging/core/tagging/test_models.py b/tests/openedx_tagging/core/tagging/test_models.py
index 44709febc..39dd013b1 100644
--- a/tests/openedx_tagging/core/tagging/test_models.py
+++ b/tests/openedx_tagging/core/tagging/test_models.py
@@ -158,6 +158,30 @@ def test_representations(self):
             == "object:id:1 (life): Life on Earth=Bacteria"
         )
 
+    def test_change_taxonomy(self):
+        assert self.object_tag.taxonomy == self.taxonomy
+        assert self.object_tag.taxonomy.id == self.taxonomy.id
+
+        # Check that taxonomy can be cleared
+        self.object_tag.taxonomy = None
+        self.object_tag.save()
+        self.object_tag.refresh_from_db()
+        assert self.object_tag.taxonomy is None
+        assert (
+            # pylint: disable=protected-access
+            self.object_tag._taxonomy_id
+            is None
+        )
+
+        # Check that taxonomy can be changed
+        self.object_tag.taxonomy = self.system_taxonomy
+        assert self.object_tag.taxonomy == self.system_taxonomy
+        assert (
+            # pylint: disable=protected-access
+            self.object_tag._taxonomy_id
+            == self.system_taxonomy.id
+        )
+
     def test_object_tag_name(self):
         # ObjectTag's name defaults to its taxonomy's name
         object_tag = ObjectTag.objects.create(
@@ -274,8 +298,7 @@ def test_tag_object(self):
             )
 
             # Ensure the expected number of tags exist in the database
-            assert ObjectTag.objects.filter(
-                taxonomy=self.taxonomy,
+            assert self.taxonomy.objecttag_set.filter(
                 object_id="biology101",
                 object_type="course",
             ).count() == len(tag_list)
diff --git a/tests/openedx_tagging/core/tagging/test_rules.py b/tests/openedx_tagging/core/tagging/test_rules.py
index 6a50feec4..f2873ff2a 100644
--- a/tests/openedx_tagging/core/tagging/test_rules.py
+++ b/tests/openedx_tagging/core/tagging/test_rules.py
@@ -184,8 +184,8 @@ def test_add_change_object_tag(self, perm):
     )
     def test_object_tag_disabled_taxonomy(self, perm):
         """Taxonomy administrators cannot create/edit an ObjectTag with a disabled Taxonomy"""
-        self.taxonomy.enabled = False
-        self.taxonomy.save()
+        self.object_tag.taxonomy.enabled = False
+        self.object_tag.taxonomy.save()
         assert self.superuser.has_perm(perm, self.object_tag)
         assert not self.staff.has_perm(perm, self.object_tag)
         assert not self.learner.has_perm(perm, self.object_tag)