Fix related resource on inherited polymorphic model

AUTHORS

@@ -27,3 +27,4 @@ Stas S. <stas@nerd.ro>
 Nathanael Gordon <nathanael.l.gordon@gmail.com>
 Charlie Allatson <charles.allatson@gmail.com>
 Joseba Mendivil <git@jma.email>
+Felix Viernickel <felix@gedankenspieler.org>

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)