python - 保存模型前如何检查ManytoMany是否有值？

假设我有两个类/模型：Hand和Finger。想象一下，指纹是一个TextField，而一个手在models.py中的手指数是未知的：

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通过admin.py添加，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']}]

并且保存Hand时，我们需要检查初始化时与Hand相关的手指数。还可以访问Finger模型中的is_a_thumb字段，以从Hand填充/关联has_a_thumb。


我如何从每个is_a_thumb访问Finger，以便如果有一个手指在is_a_thumb位置，它会在保存之前更改Hand的has_a_thumb值
我如何访问号码。分配给Finger的Hand，以便在保存之前可以更新num_fingers？

最佳答案

我不确定是否可以保存您的对象。而且我认为这不是正确的方法。我相信您需要做的是监听before信号并更新您的手。

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在这里是否正确。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？如果这是对的，那么您需要将关系更改为一对多（手指要握住FK的手指）。

关于python - 保存模型前如何检查ManytoMany是否有值？，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/37583355/