在Django中外键'self'关系中继承特定父模型属性

Question

我有一个Django模型：

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)

我希望当inherit不为空时，属性attribute1、attribute2等等能够从父对象inherit那里继承，这样当我访问这些属性时，就能得到父对象的值。我并不关心在子对象中设置这些值。

我考虑过使用模型方法，比如：

_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)

但这样看起来不太好，因为我大约有10个属性。有没有更好的方法来实现这个？

Answer 1

我之前也遇到过类似的问题。我的目标是创建一个类，让某些字段可以继承组的值。下面的代码为你的模型引入了这些功能：

1. 如果你想让某个字段继承组的值，只需要创建一个名为 "inherit_group_" 的布尔字段。
2. 当你给一个可以继承的字段设置值时，"inherit_group_" 的值会自动变成 False（因为你已经给它设置了明确的值）。
3. 如果你把 "inherit_group_" 设置为 True，那么这个字段的值就会恢复为组的值。
4. 其他的事情，比如保存和初始化，都会自动处理。

有一点需要注意 - 如果你在使用这个模型的 ModelForm，你需要重写保存函数，这样所有的 "inherit_group_" 属性会最后被设置。因为当相关的地点字段被设置时，"inherit_group_" 会像上面说的那样被设置为 False。下面也有这段代码。其实最好是通过创建新的类 - InheritableModel 和 InheritableModelForm 来处理这一切。不过我当时太懒了，没有这样做 :）。

这是模型的代码：

_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)]

这是 ModelForm 的代码：

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 2

你可以使用一个描述符：

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)

模型看起来会是这样的：

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')

这可能还可以通过一个元类来增强，这样可以自动把模型的字段隐藏在描述符后面。

Answer 3

也许使用 __getattr__ 和 __setattr__ 是个不错的选择。

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)

免责声明：我没有尝试运行这个