python - 如何在保存模型之前检查ManytoMany是否有值？

Question

假设我有两个类/模型:Hand 和 Finger。假设指纹是一个 TextField，而一只手有未知数量的手指，在 models.py 中:

from django.db import models

class Hand(models.Model):
    smoothness = models.IntegerField(default=0,
        validators=[
            MaxValueValidator(5),
            MinValueValidator(0)
        ])
    fingers = models.ManyToManyField('Finger')
    num_fingers = models.IntegerField(default=0)
    has_thumb = models.BooleanField(default=False)

    def save(self, *args, **kwargs):
        self.num_fingers = check_num_finger() # How do I do that?
        if check_has_thumb(): # How do I do that?
            self.has_thumb = True 
        super(Hand, self).save(*args, **kwargs)


class Finger(model.Model):
    is_a_thumb = models.BooleanField(default=False)
    fingerprint = models.TextField()

新的 Hand 是通过 admin.py 添加的，如下所示:

from django.contrib import admin
from .models import Hand, Finger

@admin.register(Finger)
class FingerAdmin(admin.ModelAdmin):
    fieldsets = [('Finger', {'fields': ['is_a_thumb','fingerprint']})]
    list_display = ('is_a_thumb', 'fingerprint',)

@admin.register(Hand)
class HandAdmin(admin.ModelAdmin):
    fieldsets = [('Hand', {'fields': ['smoothness']},
                 ('Fingers', {'fields': ['fingers']}]

并且在保存Hand时，我们需要检查初始化时有多少个手指与Hand相关。还可以访问手指模型内的 is_a_thumb 字段，以从手部填充/关联 has_a_thumb。

• 如何从每个手指访问is_a_thumb，以便如果有一个手指is_a_thumb，它会更改保存之前 Hand 的 has_a_thumb 值

• 如何访问号码。分配给手的手指，以便我可以在保存之前更新num_fingers？

Answer 1

我不确定你是否能做到before你的对象被保存。我认为这不是正确的方法。我相信你需要做的就是听 m2m_changed 发出信号并更新你的手牌。

from django.db.models.signals import m2m_changed

def fingers_changed(sender, **kwargs):
    #sender     Hand.fingers.through (the intermediate m2m class)
    #instance   hand (the Hand instance being modified)
    #action     "pre_add" (followed by a separate signal with "post_add")
    #reverse    False (Hand contains the ManyToManyField, so this call modifies the forward relation)
    #model      Finger (the class of the objects added to the Hand)
    #pk_set     finger_ids beign added (when post_add its a set of all,  not the new ones only, but all)
    #using      "default" (since the default router sends writes here)
    if kwargs['action'] == 'post_add':
        hand = kwargs['instance']
        hand.num_fingers = hand.fingers.count() #or len(kwargs['pk_set'])
        hand.has_thumb = hand.fingers.filter(is_a_thumb=True).exists()
        hand.save()

m2m_changed.connect(fingers_changed, sender=Hand.fingers.through)

注意:顺便说一句，我不确定 M2M 是否是正确的关系。是的，一只手可以有很多手指，但是一个手指只能属于一只手吗？如果这是 True 那么 mb 您需要将关系更改为一对多(手指有 FK 到手)。