Fix related resource on inherited polymorphic model (#767)

Fixes #621 

When adding a ResourceRelatedField to a serializer that is included in the polymorphic_serializers
list of a polymorphic serializer, the parent_model is not correctly resolved in get_related_resource_type: instead of resolving to the inherited type, the base type is resolved. This can cause an AttributeError if the field in question is not present on the base model.

We ran into this, and then found the closed and seemingly abandoned issue referenced above. This PR implements the fix suggested by the original author, who deserves all the credit. The example code has been expanded to trigger the error when running the test-suite.

Thanks in advance for your time and please let us know if you have any suggestions for improvement.

Author: 4nickel

AUTHORS

@@ -27,3 +27,4 @@ Stas S. <[email protected]>
 Nathanael Gordon <[email protected]>
 Charlie Allatson <[email protected]>
 Joseba Mendivil <[email protected]>
+Felix Viernickel <[email protected]>

CHANGELOG.md

@@ -21,6 +21,7 @@ any parts of the framework not mentioned in the documentation should generally b
 * Ensure that `409 Conflict` is returned when processing a `PATCH` request in which the resource object’s type and id do not match the server’s endpoint properly as outlined in [JSON:API](https://jsonapi.org/format/#crud-updating-responses-409) spec.
 * Properly return parser error when primary data is of invalid type
 * Pass instance to child serializer when `PolymorphicModelSerializer` inits it in `to_internal_value`
+* Handle serialization of related resources on inherited polymorphic models that are absent on the base model
 
 ## [3.0.0] - 2019-10-14
 

example/migrations/0008_labresults.py

@@ -0,0 +1,23 @@
+# Generated by Django 3.0.3 on 2020-02-06 10:24
+
+from django.db import migrations, models
+import django.db.models.deletion
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('example', '0007_artproject_description'),
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='LabResults',
+            fields=[
+                ('id', models.AutoField(auto_created=True, primary_key=True, serialize=False, verbose_name='ID')),
+                ('date', models.DateField()),
+                ('measurements', models.TextField()),
+                ('research_project', models.ForeignKey(on_delete=django.db.models.deletion.CASCADE, related_name='lab_results', to='example.ResearchProject')),
+            ],
+        ),
+    ]

example/models.py

@@ -151,6 +151,13 @@ class ResearchProject(Project):
     supervisor = models.CharField(max_length=30)
 
 
+class LabResults(models.Model):
+    research_project = models.ForeignKey(
+        ResearchProject, related_name='lab_results', on_delete=models.CASCADE)
+    date = models.DateField()
+    measurements = models.TextField()
+
+
 class Company(models.Model):
     name = models.CharField(max_length=100)
     current_project = models.ForeignKey(

example/serializers.py

@@ -15,6 +15,7 @@
     Comment,
     Company,
     Entry,
+    LabResults,
     Project,
     ProjectType,
     ResearchProject,
@@ -303,11 +304,20 @@ class Meta:
 
 
 class ResearchProjectSerializer(BaseProjectSerializer):
+    # testing exclusive related field on inherited polymorphic model
+    lab_results = relations.ResourceRelatedField(many=True, read_only=True)
+
     class Meta:
         model = ResearchProject
         exclude = ('polymorphic_ctype',)
 
 
+class LabResultsSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = LabResults
+        fields = ('date', 'measurements')
+
+
 class ProjectSerializer(serializers.PolymorphicModelSerializer):
     included_serializers = {
         'project_type': ProjectTypeSerializer,

rest_framework_json_api/utils.py

@@ -143,6 +143,7 @@ def format_resource_type(value, format_type=None, pluralize=None):
 
 
 def get_related_resource_type(relation):
+    from rest_framework_json_api.serializers import PolymorphicModelSerializer
     try:
         return get_resource_type_from_serializer(relation)
     except AttributeError:
@@ -165,7 +166,10 @@ def get_related_resource_type(relation):
     else:
         parent_serializer = relation.parent
         parent_model = None
-        if hasattr(parent_serializer, 'Meta'):
+        if isinstance(parent_serializer, PolymorphicModelSerializer):
+            parent_model = parent_serializer.get_polymorphic_serializer_for_instance(
+                parent_serializer.instance).Meta.model
+        elif hasattr(parent_serializer, 'Meta'):
             parent_model = getattr(parent_serializer.Meta, 'model', None)
         elif hasattr(parent_serializer, 'parent') and hasattr(parent_serializer.parent, 'Meta'):
             parent_model = getattr(parent_serializer.parent.Meta, 'model', None)