Django1.1 model field value preprocessing before returning

Question

Hi, all.

I have a model class like this:

class Note(models.Model):
    author = models.ForeignKey(User, related_name='notes')
    content = NoteContentField(max_length=256)

NoteContentField is a custom sub-class of CharField that override the to_python method in purpose of doing some twitter-text-conversion processing.

class NoteContentField(models.CharField):
    __metaclass__ = models.SubfieldBase

    def to_python(self, value):
        value = super(NoteContentField, self).to_python(value)

        from ..utils import linkify
        return mark_safe(linkify(value))

However, this doesn't work.

When I save a Note object like this:

note = Note(author=request.use, 
            content=form.cleaned_data['content'])
note.save()

The conversed value is saved into the database, which is not what I wanna see.

What I'm trying to do is to save the raw content into the database, and only make the conversion when the content attribute is later accessed.

Would you please tell me what's wrong with this?

Thanks to Pierre and Daniel.

I have figured out what's wrong.

I thought the text-conversion code should be in either to_python or get_db_prep_value, and that's wrong.

I should override both of them, make to_python do the conversion and get_db_prep_value return the unconversed value:

from ..utils import linkify 
class NoteContentField(models.CharField):
    __metaclass__ = models.SubfieldBase

    def to_python(self, value):
        self._raw_value = super(NoteContentField, self).to_python(value)
        return mark_safe(linkify(self._raw_value))

    def get_db_prep_value(self, value):
        return self._raw_value

I wonder if there is a better way to implement this?

Answer 1

I think you should provide the reverse function to to_python.

Take a look at Django doc here : Converting Python objects to query value

Answer 2

You seem to have only read half the docs. As Pierre-Jean noted above, and even linked you to the correct part of the document, you need to define the reverse function, which is get_db_prep_value.