Django测试中外键关联失败，疑与related_name='+'有关

问题场景 #

定义了如下模型：

class Model1(BaseModel):
    hidden = models.ForeignKey(
        Model2, 
        default=10,
        related_name='+',
    )

class Model2(models.Model):
    # 模型内容

在Django测试过程中，保存Model1实例时触发外键约束错误，报错信息如下：

File "/app/src/save.py", line 317, in save_app
    model1.object.save()
  File "/app/venv/lib/python3.7/site-packages/django/db/models/base.py", line 740, in save
    force_update=force_update, update_fields=update_fields)
  File "/app/venv/lib/python3.7/site-packages/django/db/models/base.py", line 778, in save_base
    force_update, using, update_fields,
  File "/app/venv/lib/python3.7/site-packages/django/db/models/base.py", line 881, in _save_table
    results = self._do_insert(cls._base_manager, using, fields, returning_fields, raw)
  File "/app/venv/lib/python3.7/site-packages/django/db/models/base.py", line 921, in _do_insert
    using=using, raw=raw,
  File "/app/venv/lib/python3.7/site-packages/django/db/models/manager.py", line 85, in manager_method
    return getattr(self.get_queryset(), name)(*args, **kwargs)
  File "/app/venv/lib/python3.7/site-packages/django/db/models/query.py", line 1270, in _insert
    return query.get_compiler(using=using).execute_sql(returning_fields)
  File "/app/venv/lib/python3.7/site-packages/django/db/models/sql/compiler.py", line 1416, in execute_sql
    cursor.execute(sql, params)
  File "/app/venv/lib/python3.7/site-packages/django/db/backends/utils.py", line 66, in execute
    return self._execute_with_wrappers(sql, params, many=False, executor=self._execute)
  File "/app/venv/lib/python3.7/site-packages/django/db/backends/utils.py", line 75, in _execute_with_wrappers
    return executor(sql, params, many, context)
  File "/app/venv/lib/python3.7/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)
  File "/app/venv/lib/python3.7/site-packages/django/db/utils.py", line 90, in __exit__
    raise dj_exc_value.with_traceback(traceback) from exc_value
  File "/app/venv/lib/python3.7/site-packages/django/db/backends/utils.py", line 84, in _execute
    return self.cursor.execute(sql, params)

django.db.utils.IntegrityError: 对表"Model1"的插入或更新违反外键约束"Model1_hidden_1ba57f04_fk_Model2"
DETAIL: 键(Model1_hidden_id)=(10)不存在于表"Model2"中。

问题根源 #

和related_name='+'无关，这个参数仅用于禁用Model2到Model1的反向关联管理器，不会影响外键约束的有效性。真正的问题是：测试数据库是全新初始化的，没有提前创建id=10的Model2实例，而Model1的外键默认值硬编码为10，插入时找不到对应记录，触发了数据库的完整性约束检查。

无需修改related_name的解决办法 #

1. 测试用例初始化时创建对应Model2实例 #

在测试类的setUp方法中，提前创建id为10的Model2记录，确保测试环境中有对应数据：

from django.test import TestCase
from .models import Model1, Model2

class YourModelTestCase(TestCase):
    def setUp(self):
        # 提前创建外键默认值对应的Model2实例
        Model2.objects.create(id=10)
    
    def test_model1_save(self):
        # 此时创建并保存Model1实例就不会触发约束错误
        model1 = Model1()
        model1.save()

2. 用可调用对象作为外键默认值 #

将默认值从固定的10改为一个函数，动态获取或创建对应的Model2实例，确保任何环境下都有可用的关联记录：

def get_default_model2():
    # get_or_create会自动创建不存在的记录
    obj, _ = Model2.objects.get_or_create(id=10)
    return obj.id

class Model1(BaseModel):
    hidden = models.ForeignKey(
        Model2, 
        default=get_default_model2,
        related_name='+',
    )

3. 使用Fixture预加载测试数据 #

创建包含id=10的Model2记录的fixture文件，测试时自动加载到数据库：

• 首先在app目录下创建fixtures/model2_default.json文件：

[
    {
        "model": "your_app_name.model2",
        "pk": 10,
        "fields": {}
    }
]

• 然后在测试用例中指定加载该fixture：

from django.test import TestCase

class YourModelTestCase(TestCase):
    fixtures = ['model2_default.json']
    
    def test_model1_save(self):
        model1 = Model1()
        model1.save()  # 正常执行

内容的提问来源于stack exchange，提问作者Andy Li