Make sure that EE foreign keys are polymorphic.SET_NULL

Deleting EEs that had been attached to something was failing.

awx/main/models/mixins.py

@@ -22,7 +22,7 @@ from awx.main.models.base import prevent_search
 from awx.main.models.rbac import (
     Role, RoleAncestorEntry, get_roles_on_resource
 )
-from awx.main.utils import parse_yaml_or_json, get_custom_venv_choices, get_licenser
+from awx.main.utils import parse_yaml_or_json, get_custom_venv_choices, get_licenser, polymorphic
 from awx.main.utils.encryption import decrypt_value, get_encryption_key, is_encrypted
 from awx.main.utils.polymorphic import build_polymorphic_ctypes_map
 from awx.main.fields import JSONField, AskForField
@@ -450,7 +450,7 @@ class ExecutionEnvironmentMixin(models.Model):
         null=True,
         blank=True,
         default=None,
-        on_delete=models.SET_NULL,
+        on_delete=polymorphic.SET_NULL,
         related_name='%(class)ss',
         help_text=_('The container image to be used for execution.'),
     )

awx/main/models/organization.py

@@ -27,6 +27,7 @@ from awx.main.models.rbac import (
 )
 from awx.main.models.unified_jobs import UnifiedJob
 from awx.main.models.mixins import ResourceMixin, CustomVirtualEnvMixin, RelatedJobsMixin
+from awx.main.utils import polymorphic
 
 __all__ = ['Organization', 'Team', 'Profile', 'UserSessionMembership']
 
@@ -66,7 +67,7 @@ class Organization(CommonModel, NotificationFieldsModel, ResourceMixin, CustomVi
         null=True,
         blank=True,
         default=None,
-        on_delete=models.SET_NULL,
+        on_delete=polymorphic.SET_NULL,
         related_name='+',
         help_text=_('The default execution environment for jobs run by this organization.'),
     )

awx/main/models/projects.py

@@ -35,7 +35,7 @@ from awx.main.models.mixins import (
     CustomVirtualEnvMixin,
     RelatedJobsMixin
 )
-from awx.main.utils import update_scm_url
+from awx.main.utils import update_scm_url, polymorphic
 from awx.main.utils.ansible import skip_directory, could_be_inventory, could_be_playbook
 from awx.main.fields import ImplicitRoleField
 from awx.main.models.rbac import (
@@ -272,7 +272,7 @@ class Project(UnifiedJobTemplate, ProjectOptions, ResourceMixin, CustomVirtualEn
         null=True,
         blank=True,
         default=None,
-        on_delete=models.SET_NULL,
+        on_delete=polymorphic.SET_NULL,
         related_name='+',
         help_text=_('The default execution environment for jobs run using this project.'),
     )