解决 Django 中在模型定义中过滤相关模型的问题

在 Django 中，有时需要在模型定义中过滤相关模型，例如在 Chore 模型中，需要过滤 Schedule 模型以查找与特定日期相关的 Chore。但是，在尝试编写自定义方法来过滤相关模型时，可能会遇到以下错误：

Attribute Error: 'ForeignRelatedObjectsDescriptor' object has no attribute all|filter

这是因为在模型定义中直接访问相关模型时，Django 会返回一个 ForeignRelatedObjectsDescriptor 对象，该对象不具有 all() 或 filter() 等方法。

2、解决方案 为了解决这个问题，可以在模型定义中使用 self 来访问相关模型，这样就可以使用 all() 或 filter() 等方法来过滤相关模型。例如，在 Chore 模型中，可以将以下代码：

schedule_count = Chore.schedule_set.all().count()

更改为：

schedule_count = self.schedule_set.all().count()

这样就可以成功地过滤 Schedule 模型并返回相关 Chore 的数量。

以下是完整的代码示例：

from django.db import models
from datetime import date, timedelta

class ChoreManager(models.Manager):
    def by_day(self, week_day):
        if week_day == 'today':
            week_day = date.today().strftime("%A")

        chores = self.filter(week_day=week_day)

        if chores.count() > 0:
            return chores
        else:
            return False

    def today(self):
        return self.by_day(week_day='today')

class Chore(models.Model):
    chores = ChoreManager()
    name = models.CharField(max_length=50)
    notes = models.TextField(null=True)

    def scheduled(self, week_day=None):
        if week_day is None:
            schedule_count = self.schedule_set.all().count()
        else:
            if week_day == 'today':
                week_day = date.today().strftime("%A")

            schedule_count = self.schedule_set.filter(week_day=week_day).count()

        if schedule_count > 0:
            return True
        else:
            return False

    def times_by_day(self, week_day):
        if self.scheduled() == True:
            if week_day == 'today':
                week_day = date.today().strftime("%A")

            return Chore.schedule_set.filter(week_day=week_day).values('time_due')
        else:
            return False

class Schedule(models.Model):
    chore = models.ForeignKey('Chore')
    week_day = models.CharField(max_length=9)
    time_due = models.TimeField()

    def mark_complete(self):
        completed_event = Schedule.completedevent_set.create()
        completed_event.save()

    def completed_this_week(self):
        today = date.today()
        weekstart = today - timedelta(days=today.weekday())
        weekend = weekstart + timedelta(days=7, hours=23, minutes=59, seconds=59)

        if Schedule.completedevent_set.filter(datetime_completed__gte=weekstart, datetime_completed__lte=weekend).count() > 0:
            return True
        else:
            return False

class CompletedEvent(models.Model):
    schedule = models.ForeignKey('Schedule')
    datetime_completed = models.DateTimeField(auto_now_add=True)

通过在模型定义中使用 self 来访问相关模型，就可以成功地过滤相关模型并返回相关 Chore 的数量。