Schema validation of mandatory fields on a blueprint with a HTTP PATCH method containing only modified fields

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I have a simple CRUD class (based on `flask.views.MethodView`).
The Schema class is based on `marshmallow_sqlalchemy.SQLAlchemyAutoSchema`.

```python
@_bp.arguments(AccountGroupModelSchema)
@_bp.response(200, AccountGroupModelSchema)
def patch(self, updated, account_group_id):
"""Update an existing object."""
item = AccountGroup.query.get_or_404(account_group_id)

item.name = updated.name or item.name

db.session.commit()
return item
```

When submitting a HTTP PATCH, the JSON payload contains only those fields being modified, e.g. ```{ "name": "new_name" }```. This means the Schema validation fails, because mandatory fields are not present.

Is there anyway to handle validating the fields that are present AND rejecting any fields that are unknown but NOT failing validation for missing (as not being modified) fields in a PATCH? Or do I just have to take the JSON directly, update the entity, then validate prior to updated in the ORM?

Start at the @_bp.arguments(AccountGroupModelSchema) decorator and trace how the PATCH payload is validated. Compare the requested partial-field behavior with existing framework behavior, documentation, or tests; done means present fields are validated, unknown fields are rejected, and omitted mandatory fields do not fail validation.

Written by the indexing model from the issue text.