Django模型设可为空ForeignKey，为何管理后台仍要求必填？

问题场景 #

我使用Django与Python为ContentProgress模型定制管理后台，该模型的custom_user_content字段是设置了null=True的ForeignKey。但在Django管理后台中，若不为该字段赋值就保存或编辑ContentProgress实例，会触发“Custom user Content此字段为必填项”的错误。相关代码如下：

Admin类代码 #

class ContentProgressAdmin(admin.ModelAdmin, ExportCsvMixin):
    list_display = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'created_at', 'updated_at']
    fields = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'real_seconds_listened', 'custom_user_content']
    search_fields = ['user__username', 'lesson__name', 'user__email']
    list_filter = ['done', ('custom_user_content', RelatedDropdownFilter), ('user__company', RelatedDropdownFilter)]
    autocomplete_fields = ['user', 'lesson', 'custom_user_content']
    actions = ['export_as_csv']

admin.site.register(ContentProgress, ContentProgressAdmin)

Model代码 #

class ContentProgress(Progress):
    lesson = models.ForeignKey(MediaFile, on_delete=models.CASCADE)
    custom_user_content = models.ForeignKey(CustomUserContent, null=True, on_delete=models.CASCADE)

    def __str__(self):
        return f'{self.lesson.name} - {int(self.in_percent * 100)}%'

原因分析 #

Django中null=True仅控制数据库层面是否允许字段存储NULL值，而表单（包括后台管理表单）的必填验证由blank=True参数决定。同时，当字段被加入autocomplete_fields后，后台默认会将该字段的表单控件设为必填状态，即使模型中设置了null=True。

解决方法 #

方法一：修改模型字段（推荐） #

在模型的custom_user_content字段上同时添加blank=True，让表单层面允许空值：

class ContentProgress(Progress):
    lesson = models.ForeignKey(MediaFile, on_delete=models.CASCADE)
    # 同时设置null=True和blank=True
    custom_user_content = models.ForeignKey(CustomUserContent, null=True, blank=True, on_delete=models.CASCADE)

    def __str__(self):
        return f'{self.lesson.name} - {int(self.in_percent * 100)}%'

方法二：在Admin类中强制设置字段非必填（无需修改模型） #

如果不想修改模型，可以重写Admin类的formfield_for_foreignkey方法，将该字段设为非必填：

class ContentProgressAdmin(admin.ModelAdmin, ExportCsvMixin):
    list_display = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'created_at', 'updated_at']
    fields = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'real_seconds_listened', 'custom_user_content']
    search_fields = ['user__username', 'lesson__name', 'user__email']
    list_filter = ['done', ('custom_user_content', RelatedDropdownFilter), ('user__company', RelatedDropdownFilter)]
    autocomplete_fields = ['user', 'lesson', 'custom_user_content']
    actions = ['export_as_csv']

    def formfield_for_foreignkey(self, db_field, request, **kwargs):
        if db_field.name == 'custom_user_content':
            kwargs['required'] = False
        return super().formfield_for_foreignkey(db_field, request, **kwargs)

admin.site.register(ContentProgress, ContentProgressAdmin)

方法三：自定义表单类 #

创建自定义表单，在表单中设置custom_user_content字段为非必填，然后在Admin类中指定该表单：

from django import forms
from .models import ContentProgress

class ContentProgressForm(forms.ModelForm):
    class Meta:
        model = ContentProgress
        fields = '__all__'
    
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.fields['custom_user_content'].required = False

class ContentProgressAdmin(admin.ModelAdmin, ExportCsvMixin):
    form = ContentProgressForm
    list_display = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'created_at', 'updated_at']
    fields = ['user', 'lesson', 'in_percent', 'in_seconds', 'done', 'real_seconds_listened', 'custom_user_content']
    search_fields = ['user__username', 'lesson__name', 'user__email']
    list_filter = ['done', ('custom_user_content', RelatedDropdownFilter), ('user__company', RelatedDropdownFilter)]
    autocomplete_fields = ['user', 'lesson', 'custom_user_content']
    actions = ['export_as_csv']

admin.site.register(ContentProgress, ContentProgressAdmin)

内容的提问来源于stack exchange，提问作者Chau Loi