How do I easily create a calculated field in a Django model?

Question

^{No, I don't want a property. I really don't. What I want is what I asked.}

Due to subclassing requirements, I'm looking for a way to generate one field from a set of two others and store this computation in the database. Not a property in Python, not an SQL calculation, a pre-calculated field that is updated on save and stored, as is, in the database.

For example:

class Help(models.Model):
    title = models.TextField()
    body = models.TextField()

class SoftwareHelp(Help):
    about_software = models.ForeignKey('Software')

Regardless of what a user enters in the title field, I want it to say "Help for " once save is clicked. In reality, the code has more fields, but this explains the principle.

I know its possible to do this by overriding the save() method, but wanted to make sure I wasn't saving to the database twice, and want to know if there is another better way.

What do you mean with your last sentence? Why would override the `save` method save the to the database twice? — ilse2005, Mar 13 '16 at 23:37
One answer I saw had a `save` method that called `super.save` twice for some reason. — , Mar 13 '16 at 23:45
Its one of the two from the prologue - you might not be able to see the deleted answer - http://stackoverflow.com/a/17682852/764357 — , Mar 14 '16 at 00:56
The first `save` call is unnecessary. Maybe that's one of the reasons the post is deleted :) — ilse2005, Mar 14 '16 at 10:38

score 2 · Accepted Answer · answered Mar 13 '16 at 23:51

2

I think the easiest way is to override the save method. I don't see any reason why it should save to the database twice.

class SoftwareHelp(Help):
    about_software = models.ForeignKey('Software')

    def save(self, *args, **kwargs):
        self.about_software = 'Help for %s' % self.title
        return super(SoftwareHelp, self).save(*args, **kwargs)

answered Mar 13 '16 at 23:51

ilse2005

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This was pretty much what I had to do. – Mar 14 '16 at 00:57
Seems like `pre_save` is a better candidate for this. Though, how do I do this if I want `about_software` to be set as soon as I create an instance of that model? Edit: Hmm, never mind, `pre_save` is for a `Field`, not the `Model`. – FMCorz Mar 25 '17 at 08:57

How do I easily create a calculated field in a Django model?

1 Answers1