Django 查询选择具有非零子级的父级
我的模型本身带有外键，如下所示：
class Concept(models.Model):
name = models.CharField(max_length=200)
category = models.ForeignKey('self')
但我无法弄清楚如何选择具有非零子值的所有概念。这可以通过 django QuerySet API 实现吗？或者我必须编写自定义 SQL？
I have model with a Foreign Key to itself like this:
class Concept(models.Model):
name = models.CharField(max_length=200)
category = models.ForeignKey('self')
But I can't figure out how I can select all concepts that have nonzero children value. Is this possible with django QuerySet API or I must write custom SQL?
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如果我理解正确的话，每个
Concept可能有另一个Concept作为父级，并且它被设置到category字段中。换句话说，至少有一个子概念的
Concept将在category字段中至少被引用一次。一般来说，这在 Django 中并不容易实现；但是，如果您没有太多类别，您可以考虑类似
SELECT * FROM CONCEPTS WHERE CONCEPTS.ID IN (SELECT CATEGORY FROM CONCEPTS);的查询 - 这是您可以做的事情使用 Django 轻松映射：请注意，如 Django 文档，该查询在某些数据库上可能存在性能问题；因此，您应该将其作为列表：
但请注意，这可能会遇到某些数据库限制 - 例如，Oracle 允许此类列表中最多包含 1000 个元素。
If I understand it correctly, each
Conceptmay have anotherConceptas parent, and this is set into thecategoryfield.In other words, a
Conceptwith at least a child will be referenced at least once in thecategoryfield.Generally speaking, this is not really easy to get in Django; however if you do not have too many categories, you can think for a query of the like of
SELECT * FROM CONCEPTS WHERE CONCEPTS.ID IN (SELECT CATEGORY FROM CONCEPTS);- and this is something you can map easily with Django:Note that, as stated on Django documentation, this query may have performance issues on certain databases; therefore you should instead put it as a list:
But please be aware that this could hit some database limitation -- for instance, Oracle allows up to 1000 elements in such lists.
像这样的事情怎么样：
How about something like this:
您在那里编写的方式将需要
category的值。修复该问题后（在字段构造函数中使用null=True），请使用以下命令：The way you have it written there will require a value for
category. Once you have fixed that (withnull=Truein the field constructor), use this: