Question

我想从另一个模型中获取外键值并订阅它。它在字段声明为ForeignKey时有效，但当字段声明为ManyToManyField时，它不起作用。我该怎么办？

请帮忙。

class Document(models.Model):
    project = models.ForeignKey(Project)
    text = models.ForeignKey(Text, null=True, blank=True)
    language = models.ManyToManyField(Language, null=True, blank=True)

    def save(self, *args, **kwargs):
        self.text = self.project.text                     #WORKS
        self.language  = self.project.language.all()      #NOT WORKS
        super(Document, self).save(*args, **kwargs)

Answer 1

这样的事情有用吗？

class Document(models.Model):
    project = models.ForeignKey(Project)
    text = models.ForeignKey(Text, null=True, blank=True)
    languages = models.ManyToManyField(Language)  # no need to null/blank + put the name on plural if using a many to many relation.

    def save(self, *args, **kwargs):
        self.text = self.project.text
        super(Document, self).save(*args, **kwargs)

        # super() must be called BEFORE feeding your many to many.
        project_languages = self.project.languages.all()
        self.languages.add(*project_languages)

使用多对多字段属性时，您需要等待在多对多属性中添加值之前创建实例。使用add执行此操作，与外键字段相反。

Answer 2

您需要使用.add()初始化ManyToManyField

更改此行

# use plural naming convention when its ManyToMany relation `language` should be `languages` in your Document model
self.language  = self.project.language.all()

到

# call save method of super class before adding values to many-to-many 
super(Document, self).save(*args, **kwargs)
p_language = self.project.language.all()
self.language.add(*p_language)

因此，在添加多对多关系之前，您必须先调用save。由于add会立即影响数据库，因此您无需事后保存。

在ManyToManyField的情况下从另一个模型获取值

2 个答案: