how to set up a calculated field in Django?

[Eileen Bauer]

Hi,

i have the following items in my model:

    mother_alive = models.IntegerField(choices=YES_NO_CHOICES, blank=True, null=True, default=1)

    father_alive = models.IntegerField(choices=YES_NO_CHOICES, blank=True, null=True, default=1)

and I'd like to set up a generated field for them so I'd be able to detect whether the child is an orphan or not. In MySQL i believe it'd look like this:

    orphan varchar(101) GENERATED ALWAYS AS (mother_alive+father_alive) VIRTUAL,

I don't know how to change my model to do that...

Any help?

-Eileen

[nm]

One way I can think of is to add a property to your model. I believe something like this should work:

```

## in your model class:

    mother_alive = models.IntegerField(choices=YES_NO_CHOICES, blank=True, null=True, default=1)

    father_alive = models.IntegerField(choices=YES_NO_CHOICES, blank=True, null=True, default=1)

   

    @property

    def is_orphan(self):

        return self.mother_alive == 0 and self.father_alive == 0  # not sure what the "no" value is, you have to set it appropriately

```

You'll then be able to access it as an attribute of the model:

```

Child.objects.get(id=1).is_orphan

# True or False

```

This value will not be stored in the database, though, and you won't be able to filter children by the `is_orphan` column, because there'll be no such column.

Another approach would be to use annotations. With annotations, you can do aggregations, just like with any other model field.

[wd]

hi,

Besides the solution provided by @nm,  maybe you can do it by using database trigger ...

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[Tyler Lynch]

If you're using the newest postgres 12, you could also use their new generated column feature (https://www.postgresql.org/docs/current/ddl-generated-columns.html). You can implement that in Django by editing the SQL in the migration setting up your table. Though I do look forward to this feature being integrated more directly into Django as a postgres-specific option, prly won't be too long until it's incorporated.