Django：类'Project'没有定义'_add_'，因此'+'运算符不能在其实例上使用

在下面的代码中，出于某种原因，在使用外键作为项目名称之后，在 def __str__(self): 部分，在第二个部分，就像在 self.Project 部分的 return self.Project + ' (' + self.Situation_Type + ') ' + ' [' + self.Status + '] ' 处所示，它显示了一个错误，

class 'Project' does not define '_add_', so the '+' operator cannot be used on its instances

This inspection detects names that should resolve but dont. Due to dynamic dispatch and ducttyping, this is possible in a limited but useful number of cases. Top-level and class level items are supported better than instance items.

自从这个错误开始出现以来，我的项目页面一直没有在列表中显示任何内容，但它表示有1个项目 . 可能是什么问题？

from django.db import models

# Create your models here.
from django.contrib.auth.models import User
from django.db import models
from django.core.mail import EmailMessage

# Create your models here.

class Project(models.Model):
   STATUS_CHOICE = (
       ('Project Manager', 'Project Manager'),
       ('Technician', 'Technician'),
       ('Tester', 'Tester')
   )
   STATUS_CHOICE_1 = (
       ('Work Assigned', 'Work Assigned'),
       ('Work in Progress', 'Work in Progress'),
       ('Testing', 'Testing'),
       ('Completed', 'Completed')
   )
   Project_Name = models.CharField(max_length=100)
   Project_Description = models.CharField(max_length=100)
   Admin_Name = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Admin_Name_users+')
   Admin_Mail_ID = models.EmailField(max_length=50)
   Project_Manager_1 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Project_Manager_1_users+')
   Project_Manager_1_Mail_ID = models.EmailField(max_length=50)
   Project_Manager_2 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Project_Manager_2_users+', blank=True, null=True)
   Project_Manager_2_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Technician_1 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Technician_1_users+')
   Technician_1_Mail_ID = models.EmailField(max_length=50)
   Technician_2 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Technician_2_users+', blank=True, null=True)
   Technician_2_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Technician_3 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Technician_3_users+', blank=True, null=True)
   Technician_3_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Tester_1 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Tester_1_users+')
   Tester_1_Mail_ID = models.EmailField(max_length=50, default='Example@gmail.com')
   Additional_User_1 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Ad_1_users+', blank=True, null=True)
   Additional_User_1_Type = models.CharField(max_length=18, choices=STATUS_CHOICE, blank=True, null=True)
   Additional_User_1_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Additional_User_2 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Ad_1_users+', blank=True, null=True)
   Additional_User_2_Type = models.CharField(max_length=18, choices=STATUS_CHOICE, blank=True, null=True)
   Additional_User_2_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Additional_User_3 = models.ForeignKey(User, on_delete=models.CASCADE, related_name='User.Ad_1_users+', blank=True, null=True)
   Additional_User_3_Type = models.CharField(max_length=18, choices=STATUS_CHOICE, blank=True, null=True)
   Additional_User_3_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Status_of_the_project = models.CharField(max_length=18, choices=STATUS_CHOICE_1)
   Created = models.DateTimeField(auto_now_add=True, null=True, blank=True)
   Finish_Date = models.DateTimeField(null=True, blank=True)
   Supporting_Documents = models.FileField(null=True, blank=True)


   def __str__(self):
       return self.Project_Name

   class Meta:
       verbose_name_plural = "List Of Projects"

class Bug(models.Model):

   STATUS_CHOICE = (
       ('Unassigned', 'Unassigned'),
       ('Assigned', 'Assigned'),
       ('Testing', 'Testing'),
       ('Tested', 'tested'),
       ('Fixed', 'Fixed')
   )
   STATUS_CHOICE_1 = (
       ('Bug', 'Bug'),
       ('Issue', 'Issue'),
       ('Enhancement', 'Enhancement'),
       ('Not an issue or bug', 'Not an issue or bug'),
       ('Fixed', 'Fixed')
   )
   Project = models.ForeignKey(Project, on_delete=models.CASCADE)
   Issue_Title = models.CharField(max_length=50, blank=True, null=True)
   Situation_Type = models.CharField(max_length=25, choices=STATUS_CHOICE_1)
   Basic_Description = models.CharField(max_length=100)
   Detailed_Description = models.TextField(default='The Description, here.')
   Status = models.CharField(max_length=18, choices=STATUS_CHOICE)
   Assigned_to = models.ForeignKey(User, on_delete=models.CASCADE)
   Assigned_to_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Admin_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Reported_by = models.CharField(max_length=50, blank=True, null=True)
   Reporters_Mail_ID = models.EmailField(max_length=50, blank=True, null=True)
   Reported_Date = models.DateTimeField(null=True, blank=True)
   Created = models.DateTimeField(auto_now_add=True, null=True, blank=True)
   Updated = models.DateTimeField(auto_now=True, null=True, blank=True)
   Deadline_Date = models.DateTimeField(null=True, blank=True)
   Supporting_Documents_By_Reporter = models.FileField(null=True, blank=True)
   Project_Managers_Comment = models.TextField(default='The Description, here.')
   Supporting_Documents_by_Project_Manager = models.FileField(null=True, blank=True)
   Technicians_Comment = models.TextField(default='The Description, here.')
   Supporting_Documents_by_Technician = models.FileField(null=True, blank=True)
   Testers_Comment = models.TextField(default='The Description, here.')
   Supporting_Documents_by_Tester = models.FileField(null=True, blank=True)

   def __str__(self):
       return self.Project + ' (' + self.Situation_Type + ') ' + ' [' + self.Status + '] '

def send_mail（admin，ass）：email = EmailMessage（'对任务所做的更改'，'已对您的某个任务报告进行了更改，我们特此请求您最早查看它 . '，to = [ admin，ass]）email.send（）

def send_mail(admin,ass):
    email=EmailMessage('Changes made to Task','Changes have been made to one of your Task reports and we hereby request you to have a look at it at the earliest.', to=[admin,ass])
    email.send()

这是我 admin.py 的代码

from django.contrib import admin
from .models import Bug, Project
from django.contrib.admin.models import LogEntry
admin.site.register(LogEntry)


# Register your models here.


class BugDisplay(admin.ModelAdmin):
       list_display = ('Project', 'Status', 'Basic_Description', 'Assigned_to', 'Created', 'Updated')
       list_filter = ('Status', 'Assigned_to', 'Project')
       search_fields = ('Reporters_Mail_ID', 'Reported_by', 'Basic_Description',)



admin.site.register(Bug, BugDisplay)


# Register your models here.

@admin.register(Project)
class ProjectDisplay(admin.ModelAdmin):
       list_display = ('Project_Name','Admin_Name', 'Project_Manager_1', 'Status_of_the_project')
       list_filter = ('Admin_Name', 'Project_Manager_1', 'Status_of_the_project')
       search_fields = ('Project_Name', 'Project_Description', 'Admin_Name', 'Admin_Mail_ID', 'Project_Manager_1 '
                        'Project_Manager_1_Mail_ID', 'Project_Manager_2 ', 'Project_Manager_2_Mail_ID',
                        'Technician_1',
                        'Technician_1_Mail_ID', 'Technician_2', 'Technician_2_Mail_ID', 'Technician_3',
                        'Technician_3_Mail_ID', 'Tester_1', 'Tester_1_Mail_ID', 'Additional_User_1', 'Additional_User_1_Type',
                        'Additional_User_1_Mail_ID', 'Additional_User_2', 'Additional_User_2_Type', 'Additional_User_2_Mail_ID',
                        'Additional_User_3', 'Additional_User_3_Type', 'Additional_User_3_Mail_ID', 'Status_of_the_project', 'Created',
                        'Finish_Date', 'Supporting_Documents'

）