Propagate 'default' from model_field to serializer field.

[rnshaikh]

This is PR for changes requested in PR #8130.
default fields propagated from model to serializers.

1. Generate 'default' for serializer as generated for openapi schema.
2. Updated OPTIONS response to accommodate default field there. created Testcase to check default is generated for OPTIONS response for different field.

fixes /~https://github.com/encode/django-rest-framework/issues/7469

[auvipy]

I think we should also document the intended side effect for this change

[rnshaikh]

@auvipy Okay. should i create list of side effects?

[auvipy]

that would be great

[auvipy]

Hey Rizwan! would be great if you could revisit this and add some docs on the intended side effects topic here

[auvipy]

this is the only remaining issue to get merged for 3.15 release /~https://github.com/encode/django-rest-framework/milestones/3.15

[rnshaikh]

@auvipy apologize for delay.
I have gone through the effects of these changes. find following points.

1. all default value of model will be pass as default to serializer and to the options method.
2. if the default is callable then it is not propagated in options.
3. if the default is callable is not propagated but value will be evaluated each time it is used.
4. if the value is not given then default value will be present in serializer also avaliable in serializer's methods.
5. if the value is not given specified validation on given field will be evaluted on default value as that field will be present in serializer.

[auvipy]

can you please add them as a note in the relevant doc section?

[rnshaikh]

@auvipy I have added notes regarding default value propagation from model to serializer in default argument section of serializer field

[vignirvignir]

Hi!

After upgrading from DRF 3.14 to 3.15 our tests broke when a model serializer that allows required=False for a non-nullable field in the corresponding model is now ignored so the endpoint that uses the serializer to create the object throws an error claiming you need to provide the value for the field.

However, the value that eventually goes into the field of the model might be managed in the create method of the serializer and set there.

In this example there is an entity_id field that refers to an entity field on the model. The entity_id does not need to be provided by the user as other information are used to create the Entity objects and assign to the entity field of the Customer model in the create method of the serializer.

class CustomerDetailSerializer(serializers.ModelSerializer):
    entity_id = EntityForeignKey(source="entity", write_only=True, allow_null=True, required=False)


class Customer(models.Model):
    entity = models.ForeignKey("users.Entity", related_name="customers", on_delete=models.PROTECT)