Handle when ReadOnlyField has nullable source

drf_spectacular/openapi.py

@@ -898,6 +898,11 @@ def _map_serializer_field(self, field, direction, bypass_extensions=False):
                 return append_meta(build_basic_type(OpenApiTypes.STR), meta)
 
             target = follow_field_source(model, source)
+
+            # Whether a ReadOnlyField might be null is only known after following its source
+            if hasattr(target, 'null') and target.null:
+                meta['nullable'] = True
+
             if (
                 hasattr(target, "_partialmethod")
                 and target._partialmethod.func.__name__ == '_get_FIELD_display'

drf_spectacular/plumbing.py

@@ -1,4 +1,5 @@
 import collections
+import copy
 import functools
 import hashlib
 import inspect
@@ -566,7 +567,7 @@ def append_meta(schema: _SchemaType, meta: _SchemaType) -> _SchemaType:
     return safe_ref({**schema, **meta})
 
 
-def _follow_field_source(model, path: List[str]):
+def _follow_field_source(model, path: List[str], null: bool = False):
     """
         navigate through root model via given navigation path. supports forward/reverse relations.
     """
@@ -598,6 +599,8 @@ def _follow_field_source(model, path: List[str]):
                 # not via the related_name (default: X_set) but the model name.
                 return field.target_field
             else:
+                field = copy.copy(field)
+                field.null = null
                 return field
     else:
         if (
@@ -616,10 +619,11 @@ def _follow_field_source(model, path: List[str]):
                     f'on model {model}. Please add a type hint on the model\'s property/function '
                     f'to enable traversal of the source path "{".".join(path)}".'
                 )
-            return _follow_field_source(target_model, path[1:])
+            return _follow_field_source(target_model, path[1:], null)
         else:
-            target_model = model._meta.get_field(path[0]).related_model
-            return _follow_field_source(target_model, path[1:])
+            field = model._meta.get_field(path[0])
+            target_model = field.related_model
+            return _follow_field_source(target_model, path[1:], null=(null or field.null))
 
 
 def _follow_return_type(a_callable):

tests/test_readonlyfield.py

@@ -0,0 +1,37 @@
+import uuid
+
+from django.db import models
+from rest_framework import serializers, viewsets
+
+from tests import assert_schema, generate_schema
+
+
+class Cubicle(models.Model):
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+
+
+class Employee(models.Model):
+    id = models.UUIDField(primary_key=True, default=uuid.uuid4, editable=False)
+    cubicle = models.ForeignKey(Cubicle, on_delete=models.CASCADE, null=True)
+
+
+class EmployeeSerializer(serializers.ModelSerializer):
+
+    cubicle_id = serializers.ReadOnlyField(source='cubicle.id')
+
+    class Meta:
+        fields = ['id', 'cubicle_id']
+        model = Employee
+
+
+class EmployeeModelViewset(viewsets.ModelViewSet):
+    serializer_class = EmployeeSerializer
+    queryset = Employee.objects.none()
+
+
+def test_readonlyfield(no_warnings, django_transforms):
+    assert_schema(
+        generate_schema('songs', EmployeeModelViewset),
+        'tests/test_readonlyfield.yml',
+        transforms=django_transforms,
+    )

tests/test_readonlyfield.yml

@@ -0,0 +1,197 @@
+openapi: 3.0.3
+info:
+  title: ''
+  version: 0.0.0
+paths:
+  /songs/:
+    get:
+      operationId: songs_list
+      tags:
+      - songs
+      security:
+      - cookieAuth: []
+      - basicAuth: []
+      - {}
+      responses:
+        '200':
+          content:
+            application/json:
+              schema:
+                type: array
+                items:
+                  $ref: '#/components/schemas/Employee'
+          description: ''
+    post:
+      operationId: songs_create
+      tags:
+      - songs
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Employee'
+          application/x-www-form-urlencoded:
+            schema:
+              $ref: '#/components/schemas/Employee'
+          multipart/form-data:
+            schema:
+              $ref: '#/components/schemas/Employee'
+      security:
+      - cookieAuth: []
+      - basicAuth: []
+      - {}
+      responses:
+        '201':
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Employee'
+          description: ''
+  /songs/{id}/:
+    get:
+      operationId: songs_retrieve
+      parameters:
+      - in: path
+        name: id
+        schema:
+          type: string
+          format: uuid
+        description: A UUID string identifying this employee.
+        required: true
+      tags:
+      - songs
+      security:
+      - cookieAuth: []
+      - basicAuth: []
+      - {}
+      responses:
+        '200':
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Employee'
+          description: ''
+    put:
+      operationId: songs_update
+      parameters:
+      - in: path
+        name: id
+        schema:
+          type: string
+          format: uuid
+        description: A UUID string identifying this employee.
+        required: true
+      tags:
+      - songs
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/Employee'
+          application/x-www-form-urlencoded:
+            schema:
+              $ref: '#/components/schemas/Employee'
+          multipart/form-data:
+            schema:
+              $ref: '#/components/schemas/Employee'
+      security:
+      - cookieAuth: []
+      - basicAuth: []
+      - {}
+      responses:
+        '200':
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Employee'
+          description: ''
+    patch:
+      operationId: songs_partial_update
+      parameters:
+      - in: path
+        name: id
+        schema:
+          type: string
+          format: uuid
+        description: A UUID string identifying this employee.
+        required: true
+      tags:
+      - songs
+      requestBody:
+        content:
+          application/json:
+            schema:
+              $ref: '#/components/schemas/PatchedEmployee'
+          application/x-www-form-urlencoded:
+            schema:
+              $ref: '#/components/schemas/PatchedEmployee'
+          multipart/form-data:
+            schema:
+              $ref: '#/components/schemas/PatchedEmployee'
+      security:
+      - cookieAuth: []
+      - basicAuth: []
+      - {}
+      responses:
+        '200':
+          content:
+            application/json:
+              schema:
+                $ref: '#/components/schemas/Employee'
+          description: ''
+    delete:
+      operationId: songs_destroy
+      parameters:
+      - in: path
+        name: id
+        schema:
+          type: string
+          format: uuid
+        description: A UUID string identifying this employee.
+        required: true
+      tags:
+      - songs
+      security:
+      - cookieAuth: []
+      - basicAuth: []
+      - {}
+      responses:
+        '204':
+          description: No response body
+components:
+  schemas:
+    Employee:
+      type: object
+      properties:
+        id:
+          type: string
+          format: uuid
+          readOnly: true
+        cubicle_id:
+          type: string
+          format: uuid
+          readOnly: true
+          nullable: true
+      required:
+      - cubicle_id
+      - id
+    PatchedEmployee:
+      type: object
+      properties:
+        id:
+          type: string
+          format: uuid
+          readOnly: true
+        cubicle_id:
+          type: string
+          format: uuid
+          readOnly: true
+          nullable: true
+  securitySchemes:
+    basicAuth:
+      type: http
+      scheme: basic
+    cookieAuth:
+      type: apiKey
+      in: cookie
+      name: sessionid