将django默认模型字段值作为父属性值 [英] Get django default model field value to be a parent attribute value

问题描述

我想要将默认modB localField值作为与外键对应的modA的有用字段值。这将给出：

I want the default modB localField value to be the modA wantedField value corresponding to the foreign key. This would gives :

class modA(models.Model):
    wantedField = models.CharField(max_length=9)
    def asDefault():
        return wantedField


class modB(models.Model): 
    moda        = models.ForeignKey(modA)
    localField  = models.CharField(max_length=9,default=moda.asDefault)

但是moda是ForeignKey对象因此没有属性asDefault。
我想做什么？

But moda is a ForeignKey object and as such has no attribute 'asDefault'. Is what i want to do possible ?

这是我现在所在的地方：

This is where i am now:

class modA(models.Model):
    wantedField = models.CharField(max_length=9)
    [ ... other fields ...]
    def asDefault():
        return wantedField

class modB(models.Model): 
    moda        = models.ForeignKey(modA)
    [ ... other fields ...]
    localField  = models.CharField(max_length=9,default=moda.asDefault)
    def save(self, *args, **kwargs):
        if self.pk is None:
            if self.moda and not self.localField:
                self.localField = self.moda.wantedField
        super(modB, self).save(*args, **kwargs)

另外，该方法与任何其他字段类型相同吗？
反正现在返回：

Also, would the approach be the same with any other field type ? Anyway, for the moment it returns:

AttributeError: 'ForeignKey' object has no attribute 'asDefault'

推荐答案

我认为这可以通过覆盖modB的保存方法。

I think this could be done by overriding the save method of modB.

class modB(models.Model): 
    moda        = models.ForeignKey(modA)
    localField  = models.CharField(max_length=9,default=moda.asDefault)

    def save(self, *args, **kwargs):
        if self.pk is None:
            if self.moda and not self.localField:
                self.localField = self.moda.wantedField
        super(modB, self).save(*args, **kwargs)

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