Django常见问题集

django2.0发行文档说，django2.0最后一个支持的是python3.4+...

所以，还在用python2.7的同学只能手动指定版本下载了

pip2 install django==1.8.17 或 pip2 install django==1.10

摘自：https://docs.djangoproject.com/en/2.0/releases/2.0/

Python2.7.x 下载django的报错

'''
在新创建的Django项目中执行makemigrations时，遇到：
ValueError: The field admin.LogEntry.user was declared with a lazy reference to 'fault_reporting.userinfo', but app 'fault_reporting' isn't installed.
这类app并没有安装的错误
而仔细观察，这个报错的app名字是我们之前项目中的app名字，但现在却在我们当前的项目中报错了。究其原因，则是因为之前的项目中的app缓存被存在了Django的模块内的"某些地方"
而我们当在新的项目中用到"某些地方"的功能时，执行了之前的缓存内容，从而引发错误
而普通的解决办法，就是删除当前项目中的.idea文件和__pycache__文件，然后就是删除migrations文件下的00开头的py文件都解决不了。甚至是卸载Django重新安装————都没用！
因为我们并没有删除Django模块中"某些地方"的缓存文件(pip uninstall Django都清不了这些缓存文件)。so，我们要手动去删除这这个"某个地方"的缓存文件
打开你的解释器下的Lib\site-packages\django\contrib\admin\migrations
删除除了__init__.py外的其他的文件
然后再次运行makemigrations即可
see also：https://stackoverflow.com/questions/40222268/valueerror-in-django-when-running-the-python-manage-py-migrate-command
'''

makemigrations时遇到ValueError:Related mode 'app01.Userinfo' cannot be resolved

from django.db import models

class UserInfo(models.Model):
    user = models.CharField(max_length=32)
    pwd = models.CharField(max_length=32)

class Token(models.Model):
    user = models.OneToOneField(UserInfo,on_delete=None)
    token = models.CharField(max_length=64)

# 关键字on_delete=None
# 2.0版本在源码中取消了on_delete=None，需要手动传递，

class OneToOneField(ForeignKey):

    def __init__(self, to, on_delete, to_field=None, **kwargs): # 源码中取消默认值
        kwargs['unique'] = True
        super().__init__(to, on_delete, to_field=to_field, **kwargs)

# 而在django1.11.5和1.8.17版本中，都是有默值的
def __init__(self, to, on_delete=None, to_field=None, **kwargs):

django2.x版本关于model在绑定外键的时候，遇到on_delete报错

 启动报错：SyntaxError: Generator expression must be parenthesized，有时候，在使用命令python manager.py runserver时，会发先启动失败，报错日志如下：
 [root@cs es_demo]# python37 manage.py runserver
 Unhandled exception in thread started by <function check_errors.<locals>.wrapper at 0x7f220f0edae8>
 Traceback (most recent call last):
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/utils/autoreload.py", line 227, in wrapper
     fn(*args, **kwargs)
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/core/management/commands/runserver.py", line 117, in inner_run
     autoreload.raise_last_exception()
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/utils/autoreload.py", line 250, in raise_last_exception
     six.reraise(*_exception)
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/utils/six.py", line 685, in reraise
     raise value.with_traceback(tb)
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/utils/autoreload.py", line 227, in wrapper
     fn(*args, **kwargs)
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/__init__.py", line 27, in setup
     apps.populate(settings.INSTALLED_APPS)
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/apps/registry.py", line 85, in populate
     app_config = AppConfig.create(entry)
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/apps/config.py", line 94, in create
     module = import_module(entry)
   File "/usr/local/python/python37/lib/python3.7/importlib/__init__.py", line 127, in import_module
     return _bootstrap._gcd_import(name[level:], package, level)
   File "<frozen importlib._bootstrap>", line 1006, in _gcd_import
   File "<frozen importlib._bootstrap>", line 983, in _find_and_load
   File "<frozen importlib._bootstrap>", line 967, in _find_and_load_unlocked
   File "<frozen importlib._bootstrap>", line 677, in _load_unlocked
   File "<frozen importlib._bootstrap_external>", line 728, in exec_module
   File "<frozen importlib._bootstrap>", line 219, in _call_with_frames_removed
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/contrib/admin/__init__.py", line 4, in <module>
     from django.contrib.admin.filters import (
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/contrib/admin/filters.py", line 10, in <module>
     from django.contrib.admin.options import IncorrectLookupParameters
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/contrib/admin/options.py", line 12, in <module>
     from django.contrib.admin import helpers, widgets
   File "/usr/local/python/python37/lib/python3.7/site-packages/django/contrib/admin/widgets.py", line 151
     '%s=%s' % (k, v) for k, v in params.items(),
     ^
 SyntaxError: Generator expression must be parenthesized

 可以发现最后一行提示说是语法错误，说是生成器表达式有问题，那具体的报错行内容是'%s=%s' % (k, v) for k, v in params.items(),，可以看到，最后一个标点符号很有意思，我们找到源码文件对应的行，把这个标点符号去掉即可。
 ps：如果是pycharm中修改的话，由于这个文件是源码，还要注意提示框中选择I want to edit this file anyway。

Django之启动报错：Generator expression must be parenthesized

# Django1.11 + Python3.6 + win10
"""
可能的原因
在ROM与ajax交互时，难免会遇到传递queryset对象，但是你会发现使用`JsonResponse`并不好用
"""
from django.shortcuts import render, HttpResponse
from django.http import JsonResponse
def ui_index(request):
    if request.method == 'POST':
        pk = request.POST.get('pk')
        result = ui_models.UiCase.objects.filter(case_vest_project=pk)
        # return HttpResponse({"case": result})
        return JsonResponse({"case": result})  # 依然不好使

# 就算使用`JsonResponse({"case": result})`依然不好使：
```
TypeError: Object of type 'QuerySet' is not JSON serializable
```

**可能的解决办法**

```python
from django.core import serializers
def ui_index(request):
    if request.method == 'POST':
        pk = request.POST.get('pk')
        result = ui_models.UiCase.objects.filter(case_vest_project=pk)
        return JsonResponse({"case": serializers.serialize('json', result)})
```

这个时候，可以使用`serializers`来解决。然后前端的ajax正常接收即可，无需反序列化：

```html
<script>
    $(".selected_project").on('click', 'li', function () {
        var pk = $(this).attr('pk');
        $.ajax({
            url: "{% url 'ui_index' %}",
            type: "POST",
            data: {'pk': pk},
            success: function (data) {
                console.log(typeof(data), data);
                if (data) {
                    $.each(data, function (index, item) {
                        console.log(item.case_name, item.pk, )
                    })
                }
            }
        })
    })
</script>
```

see also：[【已解决】Object of type 'QuerySet' is not JSON serializable](<http://www.voidcn.com/article/p-dwqjezai-bpk.html>)

Django1.11：SyntaxError: Generator expression must be parenthesized

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欢迎斧正，that's all