Inheriting specific parent model attributes in foreignkey->'self' relationship in Django

Question

I have a Django model:

class Foo(models.Model):
    name = models.CharField(unique=True)
    attribute1 = models.FloatField(null=True, blank=True)
    attribute2 = models.FloatField(null=True, blank=True)
    attribute3 = models.BooleanField(null=True, blank=True)
    attribute4 = models.CharField(null=True, blank=True)
    inherit = models.ForeignKey('self', related_name='children', null=True, blank=True)

I'd like that when inherit is not null/blank, that attribute1 and attribute2 etc. are inherited from the parent object inherit so that when I access the attributes I get the values of the parent. I dont care about setting values in the child.

I thought about using model methods eg.:

_attribute1 = models.FloatField(null=True, blank=True)
get_attribute1(self):
    if self.inherit:
        return self.inherit._attribute1
    else:
        return self._attribute1
set_attribute1(self, value):
    if not self.inherit:
        self._attribute1 = value
attribute1 = property(get_attribute1, set_attribute1)

But it seems ugly since I have about 10 attributes. Is there a better way to do this?

Answer 1

Perhaps the __getattr__ and __setattr__ are a good choice here.

class Foo(models.Model):
    name = models.CharField(unique=True)
    inherit = models.ForeignKey('self', related_name='children', null=True, blank=True)

    _attribute1 = models.FloatField(null=True, blank=True)
    _attribute2 = models.FloatField(null=True, blank=True)
    _attribute3 = models.BooleanField(null=True, blank=True)
    _attribute4 = models.CharField(null=True, blank=True)

    def __getattr__(self, name):
        if self.inherit and hasattr(self.inherit, name):
            return getattr(self.inherit, name, None)
        elif hasattr(self, '_'+name):
            return getattr(self, '_'+name, None)
        return super(Foo, self).__getattr__(name)

    def __setattr__(self, name, value):
        if self.inherit and hasattr(self.inherit, name):
            return setattr(self.inherit, name, value)
        elif hasattr(self, '_'+name):
            return self.__dict__[name] = value
        return super(Foo, self).__setattr__(name, value)

Disclaimer: I didn't try running this

Answer 2

I had a similar problem. The goal was to have a class that allowed for certain fields to take on the value of the group. The code below introduces these features to your model:

1. If you want to make a field inherit the group value, you only have to create another BooleanField with the name "inherit_group_".
2. If you set the value of an inheritable field, the value of "inherit_group_" is automatically set to False (since you've set an explicit value).
3. If you set "inherit_group_" to True, the value of the associated field is returned back to the group's value.
4. Everything else is handled including saving and initialization.

One warning - if you're using a ModelForm with this Model, you'll need to override the save function so that all the "inherit_group_ attributes are set last since, when the associated location fields are set, "inherit_group_" will be set to False as described above. That code is below as well. This whole thing is probably best handled by creating new classes - InheritableModel and InheritableModelForm. I was too lazy to do that though :).

Here's the code for the Model:

_inheritable_fields = []

def __init__(self, *args, **kwargs):
    super(Location, self).__init__(*args, **kwargs)
    # Change iheritable field values to group value if current value is
    # None
    self._inheritable_fields = [ 
        fname for fname in dir(self)
        if hasattr(self, 'inherit_group_%s' % fname)]

    # Make sure that all fields are in the correct state given the
    # inherit_group values
    [setattr(self, 'inherit_group_%s' % fname, getattr(self,
        'inherit_group_%s' % fname))
        for fname in self._inheritable_fields]

def __setattr__(self, name, val):
    super(Location, self).__setattr__(name, val)

    if name == "group" and val:
        # A new group was specified. Update all fields that are currently
        # inheriting from the group
        [models.Model.__setattr__(self, fname, getattr(self.group, fname))
            for fname in self._inheritable_fields
            if getattr(self, 'inherit_group_%s' % fname)]

    elif name in self._inheritable_fields:
        # An inheritable field value changed. Update its inheritance state
        models.Model.__setattr__(self, 'inherit_group_%s' % name, False)

    elif name.startswith('inherit_group_'):
        field_name = re.sub('^inherit_group_', '', name)
        if val and field_name in self._inheritable_fields:
            # An inheritance state (e.g., inherit_group_name) was changed.
            # Change value back to group value
            if hasattr(self, 'group'):
                models.Model.__setattr__(self, field_name, 
                    getattr(self.group, field_name))
            else:
                models.Model.__setattr__(self, field_name, None)

def save(self, *args, **kwargs):
    # Set all fields using the inherited value to None for DB storage.
    val_from_group = [ 
        fname for fname in self._inheritable_fields
        if getattr(self, 'inherit_group_%s' % fname)]
    [models.Model.__setattr__(self, fname, None) for fname in val_from_group]

    super(Location, self).save(*args, **kwargs)

    # Return the fields changed above back to the group values.
    [models.Model.__setattr__(self, fname, getattr(self.group, fname))
        for fname in self._inheritable_fields
        if getattr(self, 'inherit_group_%s' % fname)]

Here's the the code for the ModelForm:

def save(self, commit=True):
    location = super(LocationForm, self).save(commit=False)

    # location.inherit_group_x has to be set last as it'll be set to
    # False when it's associated field is set
    [setattr(location, 'inherit_group_%s' % fname,
        self.cleaned_data['inherit_group_%s' % fname])
        for fname in location._inheritable_fields]

    if commit:
        location = super(LocationForm, self).save()

    return location

Answer 3

You could use a Descriptor:

class InheritedAttribute(object):
    def __init__(self, name):
        self.attname = '_' + name

    def __get__(self, instance, owner):
        if instance.inherit:
            return getattr(instance.inherit, self.attname)
        else:
            return getattr(instance, self.attname)

    def __set__(self, instance, value):
        setattr(instance, self.attname, value)

the Model would look like this:

class Foo(models.Model)
    name = models.CharField(unique=True)
    _attribute1 = models.FloatField(null=True, blank=True)
    _attribute2 = models.FloatField(null=True, blank=True)
    _attribute3 = models.BooleanField(null=True, blank=True)
    _attribute4 = models.CharField(null=True, blank=True)
    inherit = models.ForeignKey('self', related_name='children', null=True, blank=True)
    attribute1 = InheritedAttribute('attribute1')
    attribute2 = InheritedAttribute('attribute2')
    attribute3 = InheritedAttribute('attribute3')
    attribute1 = InheritedAttribute('attribute4')

This could probably be enhanced with a Metaclass that automatically hides the model fields behind a descriptor.