英文:
How to unite second level ManyToManyFields?
问题
以下是翻译的代码部分：
class Tag(models.Model):name = models.CharField(max_length=50)class Element(models.Model):tags = models.ManyToManyField('Tag')class Category(models.Model):elements = models.ManyToManyField('Element')@propertydef tags(self):... # how can I do this?
希望这对你有所帮助。如果有任何其他疑问，请随时提出。
英文:
I have models as follows:
class Tag(models.Model):name = models.CharField(max_length=50)class Element(models.Model):tags = models.ManyToManyField('Tag')class Category(models.Model):elements = models.ManyToManyField('Element')@propertydef tags(self):... # how can I do this?
How can I get the union of all the tags that appear in elements of a given category?
I could do something like this:
def tags(self):all_tags = Tag.objects.none()for element in self.elements.all():all_tags = all_tags | element.tags.all()return all_tags.distinct()
But is there a way to do this directly at database level?
答案1
得分: 3
使用双下划线符号来遍历关系：
def tags(self):return Tag.objects.filter(element__category=self)
英文:
Use the double underscore notation to traverse relations:
def tags(self):return Tag.objects.filter(element__category=self)
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