How to return “already exists” error in Flask-restless?

Question

I would like to do some handler for exception. I'm using a combination of Flask-restless and SQLAlchemy in python.

My problem:

When I send request to api with object that already exists in DB, SQLAlchemy shows exception:

IntegrityError: (IntegrityError) column <column_name> is not unique u'INSERT INTO ...

So I have tried to add attribute validation_exceptions into create_api method:

manager.create_api( ... , validation_exceptions=[IntegrityError])

But response json contains:

{
    "validation_errors": "Could not determine specific validation errors"
} 

and server api shows exception :

Traceback (most recent call last):
  File "C:\Python27\lib\site-packages\flask_restless\views.py", line 797, in _extract_error_messages
    left, right = str(exception).rsplit(':', 1)
ValueError: need more than 1 value to unpack

Exception validation in Flask-restless doesn't work with this type of exception (IntegrityError)

What should I do? Is it possible to create some handler for exception and to return my own error message in json?

Answer 1

The documentation (v0.17.0 to date of this posting) states:

Currently, Flask-Restless expects that an instance of a specified validation error will have a errors attribute, which is a dictionary mapping field name to error description (note: one error per field).

So to change the content of validation_errors your exception needs an errors attribute that contains a dictionary. The content of this dictionary will appear in the servers response as validation_errors.

From flask-restless/tests/test_validation.py:

class TestSimpleValidation(ManagerTestBase):
"""Tests for validation errors raised by the SQLAlchemy's simple built-in
validation.
For more information about this functionality, see the documentation for
:func:`sqlalchemy.orm.validates`.
"""

def setup(self):
    """Create APIs for the validated models."""
    super(TestSimpleValidation, self).setup()

    class Person(self.Base):
        __tablename__ = 'person'
        id = Column(Integer, primary_key=True)
        age = Column(Integer, nullable=False)

        @validates('age')
        def validate_age(self, key, number):
            if not 0 <= number <= 150:
                exception = CoolValidationError()
                exception.errors = dict(age='Must be between 0 and 150')
                raise exception
            return number

        @validates('articles')
        def validate_articles(self, key, article):
            if article.title is not None and len(article.title) == 0:
                exception = CoolValidationError()
                exception.errors = {'articles': 'empty title not allowed'}
                raise exception
            return article

    class Article(self.Base):
        __tablename__ = 'article'
        id = Column(Integer, primary_key=True)
        title = Column(Unicode)
        author_id = Column(Integer, ForeignKey('person.id'))
        author = relationship('Person', backref=backref('articles'))

    self.Article = Article
    self.Person = Person
    self.Base.metadata.create_all()
    self.manager.create_api(Article)
    self.manager.create_api(Person, methods=['POST', 'PATCH'],
                            validation_exceptions=[CoolValidationError])

Request:

data = dict(data=dict(type='person', age=-1))
response = self.app.post('/api/person', data=dumps(data))

Response:

HTTP/1.1 400 Bad Request

{ "validation_errors":
    {
      "age": "Must be between 0 and 150",
    }
}

Answer 2

You can use preprocessors for capturing validation errors.

def validation_preprocessor(data, *args, **kwargs):
    # validate data by any of your cool-validation-frameworks
    if errors:
        raise ProcessingException(description='Something went wrong', code=400)

manager.create_api(
    Model,
    methods=['POST'],
    preprocessors=dict(
        POST=[validation_preprocessor]
    )
)

But I'm not sure that it is a good way to do that.