Add example for using swagger_schema_fields for a Field (#494)

* Add example for using swagger_schema_fields for a Field
* Mention that Meta class can be added to fields as well
* Reference the DRF docs on how to add validation to serializers

docs/custom_spec.rst

@@ -212,7 +212,8 @@ Schema generation of ``serializers.SerializerMethodField`` is supported in two w
 Serializer ``Meta`` nested class
 ********************************
 
-You can define some per-serializer options by adding a ``Meta`` class to your serializer, e.g.:
+You can define some per-serializer or per-field options by adding a ``Meta`` class to your ``Serializer`` or
+serializer ``Field``, e.g.:
 
 .. code-block:: python
 
@@ -236,6 +237,64 @@ The available options are:
      which are converted to Swagger ``Schema`` attribute names according to :func:`.make_swagger_name`.
      Attribute names and values must conform to the `OpenAPI 2.0 specification <https://github.com/OAI/OpenAPI-Specification/blob/master/versions/2.0.md#schemaObject>`_.
 
+Suppose you wanted to model an email using a `JSONField` to store the subject and body for performance reasons:
+
+.. code-block:: python
+
+   from django.contrib.postgres.fields import JSONField
+
+   class Email(models.Model):
+       # Store data as JSON, but the data should be made up of
+       # an object that has two properties, "subject" and "body"
+       # Example:
+       # {
+       #   "subject": "My Title",
+       #   "body": "The body of the message.",
+       # }
+       message = JSONField()
+
+To instruct ``drf-yasg`` to output an OpenAPI schema that matches this, create a custom ``JSONField``:
+
+.. code-block:: python
+
+   class EmailMessageField(serializers.JSONField):
+       class Meta:
+           swagger_schema_fields = {
+               "type": openapi.TYPE_OBJECT,
+               "title": "Email",
+               "properties": {
+                   "subject": openapi.Schema(
+                       title="Email subject",
+                       type=openapi.TYPE_STRING,
+                   ),
+                   "body": openapi.Schema(
+                       title="Email body",
+                       type=openapi.TYPE_STRING,
+                   ),
+               },
+               "required": ["subject", "body"],
+            }
+
+   class EmailSerializer(ModelSerializer):
+       class Meta:
+           model = Email
+           fields = "__all__"
+
+       message = EmailMessageField()
+
+.. Warning::
+
+   Overriding a default ``Field`` generated by a ``ModelSerializer`` will also override automatically
+   generated validators for that ``Field``.  To add ``Serializer`` validation back in manually, see the relevant
+   `DRF Validators`_ and `DRF Fields`_ documentation.
+
+   One example way to do this is to set the ``default_validators`` attribute on a field.
+
+   .. code-block:: python
+
+      class EmailMessageField(serializers.JSONField):
+         default_validators = [my_custom_email_validator]
+         ...
 
 *************************
 Subclassing and extending
@@ -389,3 +448,5 @@ A second example, of a :class:`~.inspectors.FieldInspector` that removes the ``t
 
 
 .. _Python 3 type hinting: https://docs.python.org/3/library/typing.html
+.. _DRF Validators: https://www.django-rest-framework.org/api-guide/validators/
+.. _DRF Fields: https://www.django-rest-framework.org/api-guide/fields/#validators