您可以覆盖每个模型的save函数以检查之前的"is_global"项
项目模型

def save(self):
    if self.is_global:
        other_global = Project.objects.filter(is_global=True).exists()
        if other_global:
            #handle the error, eg, raise an exception or send a message
            return
    super.save()

计划模型

def save(self):
    if self.is_global:
        other_global = Plan.objects.filter(is_global=True).exists()
        if other_global:
            #handle the error, eg, raise an exception or send a message
            return
        if not self.project.is_global:
            #handle the error, eg, raise an exception or send a message
            return 
    super.save()