Unique validator in WTForms with SQLAlchemy models

Question

I defined some WTForms forms in an application that uses SQLALchemy to manage database operations.

For example, a form for managing Categories:

class CategoryForm(Form):
    name = TextField(u'name', [validators.Required()])

And here's the corresponding SQLAlchemy model:

class Category(Base):
    __tablename__= 'category'
    id = Column(Integer, primary_key=True)
    name = Column(Unicode(255))

    def __repr__(self):
        return '<Category %i>'% self.id

    def __unicode__(self):
        return self.name

I would like to add a unique constraint on the form validation (not on the model itself).

Reading the WTForms documentation, I found a way to do it with a simple class:

class Unique(object):
    """ validator that checks field uniqueness """
    def __init__(self, model, field, message=None):
        self.model = model
        self.field = field
        if not message:
            message = u'this element already exists'
        self.message = message

    def __call__(self, form, field):         
        check = self.model.query.filter(self.field == field.data).first()
        if check:
            raise ValidationError(self.message)

Now I can add that validator to the CategoryForm like this:

name = TextField(u'name', [validators.Required(), Unique(Category, Category.name)])

This check works great when the user tries to add a category that already exists \o/ BUT it won't work when the user tries to update an existing category (without changing the name attribute).

When you want to update an existing category : you'll instantiate the form with the category attribute to edit:

def category_update(category_id):
    """ update the given category """
    category = Category.query.get(category_id)
    form = CategoryForm(request.form, category)

The main problem is I don't know how to access the existing category object in the validator which would let me exclude the edited object from the query.

Is there a way to do it? Thanks.

Answer 1

In the validation phase, you will have access to all the fields. So the trick here is to pass in the primary key into your edit form, e.g.

class CategoryEditForm(CategoryForm):
    id = IntegerField(widget=HiddenInput())

Then, in the Unique validator, change the if-condition to:

check = self.model.query.filter(self.field == field.data).first()
if 'id' in form:
    id = form.id.data
else:
    id = None
if check and (id is None or id != check.id):