Django:CharField有固定长度,怎么样？

Question

我希望在我的模型中有一个固定长度的CharField.换句话说,我希望只有指定的长度才有效.

我试着做点什么

volumenumber = models.CharField('Volume Number', max_length=4, min_length=4)

但它给了我一个错误(似乎我可以同时使用max_length和min_length).

还有另一种快捷方式吗？

谢谢

编辑:

根据一些人的建议,我会更具体一些:

我的模型是这样的:

class Volume(models.Model):
    vid = models.AutoField(primary_key=True)
    jid = models.ForeignKey(Journals, db_column='jid', null=True, verbose_name = "Journal")
    volumenumber = models.CharField('Volume Number')
    date_publication = models.CharField('Date of Publication', max_length=6, blank=True)
    class Meta:
        db_table = u'volume'
        verbose_name = "Volume"
        ordering = ['jid', 'volumenumber']
        unique_together = ('jid', 'volumenumber')
    def __unicode__(self):
        return (str(self.jid) + ' - ' + str(self.volumenumber))

我想要的是volumenumber必须是4个字符.

IE如果有人插入'4b'django会出错,因为它需要一个4个字符的字符串.

所以我尝试了

volumenumber = models.CharField('Volume Number', max_length=4, min_length=4)

但它给了我这个错误:

Validating models...
Unhandled exception in thread started by <function inner_run at 0x70feb0>
Traceback (most recent call last):
  File "/Library/Python/2.5/site-packages/django/core/management/commands/runserver.py", line 48, in inner_run
    self.validate(display_num_errors=True)
  File "/Library/Python/2.5/site-packages/django/core/management/base.py", line 249, in validate
    num_errors = get_validation_errors(s, app)
  File "/Library/Python/2.5/site-packages/django/core/management/validation.py", line 28, in get_validation_errors
    for (app_name, error) in get_app_errors().items():
  File "/Library/Python/2.5/site-packages/django/db/models/loading.py", line 131, in get_app_errors
    self._populate()
  File "/Library/Python/2.5/site-packages/django/db/models/loading.py", line 58, in _populate
    self.load_app(app_name, True)
  File "/Library/Python/2.5/site-packages/django/db/models/loading.py", line 74, in load_app
    models = import_module('.models', app_name)
  File "/Library/Python/2.5/site-packages/django/utils/importlib.py", line 35, in import_module
    __import__(name)
  File "/Users/Giovanni/src/djangoTestSite/../djangoTestSite/journaldb/models.py", line 120, in <module>
    class Volume(models.Model):
  File "/Users/Giovanni/src/djangoTestSite/../djangoTestSite/journaldb/models.py", line 123, in Volume
    volumenumber = models.CharField('Volume Number', max_length=4, min_length=4)
TypeError: __init__() got an unexpected keyword argument 'min_length'

如果我只使用"max_length"或"min_length",那显然不会出现.

我阅读了django网站上的文档,似乎我是对的(我不能同时使用它们)所以我问是否有另一种方法可以解决问题.

再次感谢

Answer 1

你甚至不必写自定义的.只需使用RegexValidatorDjango提供的.

from django.core.validators import RegexValidator

class MyModel(models.Model):
    myfield = models.CharField(validators=[RegexValidator(regex='^.{4}$', message='Length has to be 4', code='nomatch')])

来自Django Docs: class RegexValidator(\[regex=None, message=None, code=None\])

regex:要匹配的有效正则表达式.有关Python中正则表达式的更多信息,请查看此优秀的HowTo:http: //docs.python.org/howto/regex.html

message:如果失败,将返回给用户的消息.

code:ValidationError返回的错误代码.对于您的使用案例并不重要,您可以将其删除.

请注意,我建议的正则表达式将允许任何字符,包括空格.要仅允许使用字母数字字符,请替换"." 在正则表达式参数中使用'\ w'.对于其他要求,ReadTheDocs;).

Answer 2

有点像上面那样,但是对于它的价值,你也可以继续使用django提供的MinLengthValidator.为我工作.代码看起来像这样:

from django.core.validators import MinLengthValidator
...
class Volume(models.Model):
volumenumber = models.CharField('Volume Number', max_length=4, validators=[MinLengthValidator(4)])
...

Answer 3

CharField数据库模型字段实例只有一个max_length参数,如文档中所示.这可能是因为SQL中只有最大字符长度约束.

另一方面,Form Field CharField对象确实有一个min_length参数.因此,您必须为此特定模型编写自定义ModelForm,并使用自定义模型覆盖默认管理模型表单.

像这样的东西:

# admin.py

from django import forms

...

class VolumeForm(forms.ModelForm):
    volumenumber = forms.CharField(max_length=4, min_length=4)

    class Meta:
        model = Volume


class VolumeAdmin(admin.ModelAdmin):
    form = VolumeForm

...

admin.site.register(Volume, VolumeAdmin)

Answer 4

您可以按@Ben的建议编写自定义验证程序.截至本答复发布之日,可在https://docs.djangoproject.com/en/dev/ref/validators/找到执行此操作的说明.

代码将是这样的(从链接复制):

from django.core.exceptions import ValidationError

def validate_length(value,length=6):
    if len(str(value))!=length:
        raise ValidationError(u'%s is not the correct length' % value)

from django.db import models

class MyModel(models.Model):
    constraint_length_charField = models.CharField(validators=[validate_length])