django/docs/ref/checks.txt

======================
System check framework
======================

The system check framework is a set of static checks for validating Django
projects. It detects common problems and provides hints for how to fix them.
The framework is extensible so you can easily add your own checks.

For details on how to add your own checks and integrate them with Django's
system checks, see the :doc:`System check topic guide </topics/checks>`.

Builtin tags
------------

Django's system checks are organized using the following tags:

* ``models``: Checks governing model, field and manager definitions.
* ``signals``: Checks on signal declarations and handler registrations.
* ``admin``: Checks of any admin site declarations.
* ``compatibility``: Flagging potential problems with version upgrades.
* ``security``: Checks security related configuration.

Some checks may be registered with multiple tags.

Core system checks
------------------

Models
~~~~~~

* **models.E001**: ``<swappable>`` is not of the form ``app_label.app_name``.
* **models.E002**: ``<SETTING>`` references ``<model>``, which has not been
  installed, or is abstract.
* **models.E003**: The model has two many-to-many relations through the
  intermediate model ``<app_label>.<model>``.
* **models.E004**: ``id`` can only be used as a field name if the field also
  sets ``primary_key=True``.
* **models.E005**: The field ``<field name>`` from parent model ``<model>``
  clashes with the field ``<field name>`` from parent model ``<model>``.
* **models.E006**: The field clashes with the field ``<field name>`` from model
  ``<model>``.
* **models.E007**: Field ``<field name>`` has column name ``<column name>``
  that is used by another field.
* **models.E008**: ``index_together`` must be a list or tuple.
* **models.E009**: All ``index_together`` elements must be lists or tuples.
* **models.E010**: ``unique_together`` must be a list or tuple.
* **models.E011**: All ``unique_together`` elements must be lists or tuples.
* **models.E012**: ``index_together/unique_together`` refers to the
  non-existent field ``<field name>``.
* **models.E013**: ``index_together/unique_together`` refers to a
  ``ManyToManyField`` ``<field name>``, but ``ManyToManyField``\s are not
  supported for that option.
* **models.E014**: ``ordering`` must be a tuple or list (even if you want to
  order by only one field).
* **models.E015**: ``ordering`` refers to the non-existent field
  ``<field name>``.
* **models.E016**: ``index_together/unique_together`` refers to field
  ``<field_name>`` which is not local to model ``<model>``.
* **models.E017**: Proxy model ``<model>`` contains model fields.
* **models.E018**: Autogenerated column name too long for field ``<field>``.
  Maximum length is ``<maximum length>`` for database ``<alias>``.
* **models.E019**: Autogenerated column name too long for M2M field
  ``<M2M field>``. Maximum length is ``<maximum length>`` for database
  ``<alias>``.
* **models.E020**: The ``<model>.check()`` class method is currently overridden.

Fields
~~~~~~

* **fields.E001**: Field names must not end with an underscore.
* **fields.E002**: Field names must not contain ``"__"``.
* **fields.E003**: ``pk`` is a reserved word that cannot be used as a field
  name.
* **fields.E004**: ``choices`` must be an iterable (e.g., a list or tuple).
* **fields.E005**: ``choices`` must be an iterable returning ``(actual value,
  human readable name)`` tuples.
* **fields.E006**: ``db_index`` must be ``None``, ``True`` or ``False``.
* **fields.E007**: Primary keys must not have ``null=True``.
* **fields.E100**: ``AutoField``\s must set primary_key=True.
* **fields.E110**: ``BooleanField``\s do not accept null values.
* **fields.E120**: ``CharField``\s must define a ``max_length`` attribute.
* **fields.E121**: ``max_length`` must be a positive integer.
* **fields.W122**: ``max_length`` is ignored when used with ``IntegerField``.
* **fields.E130**: ``DecimalField``\s must define a ``decimal_places`` attribute.
* **fields.E131**: ``decimal_places`` must be a non-negative integer.
* **fields.E132**: ``DecimalField``\s must define a ``max_digits`` attribute.
* **fields.E133**: ``max_digits`` must be a non-negative integer.
* **fields.E134**: ``max_digits`` must be greater or equal to ``decimal_places``.
* **fields.E140**: ``FilePathField``\s must have either ``allow_files`` or
  ``allow_folders`` set to True.
* **fields.E150**: ``GenericIPAddressField``\s cannot accept blank values if
  null values are not allowed, as blank values are stored as nulls.
* **fields.E160**: The options ``auto_now``, ``auto_now_add``, and ``default``
  are mutually exclusive. Only one of these options may be present.
* **fields.W161**: Fixed default value provided.
* **fields.E900**: ``IPAddressField`` has been removed except for support in
  historical migrations.
* **fields.W900**: ``IPAddressField`` has been deprecated. Support for it
  (except in historical migrations) will be removed in Django 1.9. *This check
  appeared in Django 1.7 and 1.8*.

File Fields
~~~~~~~~~~~

* **fields.E200**: ``unique`` is not a valid argument for a ``FileField``.
* **fields.E201**: ``primary_key`` is not a valid argument for a ``FileField``.
* **fields.E210**: Cannot use ``ImageField`` because Pillow is not installed.

Related Fields
~~~~~~~~~~~~~~

* **fields.E300**: Field defines a relation with model ``<model>``, which is
  either not installed, or is abstract.
* **fields.E301**: Field defines a relation with the model ``<model>`` which
  has been swapped out.
* **fields.E302**: Accessor for field ``<field name>`` clashes with field
  ``<field name>``.
* **fields.E303**: Reverse query name for field ``<field name>`` clashes with
  field ``<field name>``.
* **fields.E304**: Field name ``<field name>`` clashes with accessor for
  ``<field name>``.
* **fields.E305**: Field name ``<field name>`` clashes with reverse query name
  for ``<field name>``.
* **fields.E306**: Related name must be a valid Python identifier or end with
  a ``'+'``.
* **fields.E310**: None of the fields ``<field1>``, ``<field2>``, ... on model
  ``<model>`` have a ``unique=True`` constraint.
* **fields.E311**: ``<model>`` must set ``unique=True`` because it is
  referenced by a ``ForeignKey``.
* **fields.E320**: Field specifies ``on_delete=SET_NULL``, but cannot be null.
* **fields.E321**: The field specifies ``on_delete=SET_DEFAULT``, but has no
  default value.
* **fields.E330**: ``ManyToManyField``\s cannot be unique.
* **fields.E331**: Field specifies a many-to-many relation through model
  ``<model>``, which has not been installed.
* **fields.E332**: Many-to-many fields with intermediate tables must not be
  symmetrical.
* **fields.E333**: The model is used as an intermediate model by ``<model>``,
  but it has more than two foreign keys to ``<model>``, which is ambiguous.
  You must specify which two foreign keys Django should use via the
  ``through_fields`` keyword argument.
* **fields.E334**: The model is used as an intermediate model by ``<model>``,
  but it has more than one foreign key from ``<model>``, which is ambiguous.
  You must specify which foreign key Django should use via the
  ``through_fields`` keyword argument.
* **fields.E335**: The model is used as an intermediate model by ``<model>``,
  but it has more than one foreign key to ``<model>``, which is ambiguous.
  You must specify which foreign key Django should use via the
  ``through_fields`` keyword argument.
* **fields.E336**: The model is used as an intermediary model by ``<model>``,
  but it does not have foreign key to ``<model>`` or ``<model>``.
* **fields.E337**: Field specifies ``through_fields`` but does not provide the
  names of the two link fields that should be used for the relation through
  ``<model>``.
* **fields.E338**: The intermediary model ``<through model>`` has no field
  ``<field name>``.
* **fields.E339**: ``<model>.<field name>`` is not a foreign key to ``<model>``.
* **fields.W340**: ``null`` has no effect on ``ManyToManyField``.
* **fields.W341**: ``ManyToManyField`` does not support ``validators``.
* **fields.W342**: Setting ``unique=True`` on a ``ForeignKey`` has the same
  effect as using a ``OneToOneField``.

Signals
~~~~~~~

* **signals.E001**: ``<handler>`` was connected to the ``<signal>`` signal with
  a lazy reference to the ``<model>`` sender, which has not been installed.

Backwards Compatibility
~~~~~~~~~~~~~~~~~~~~~~~

The following checks are performed to warn the user of any potential problems
that might occur as a result of a version upgrade.

* **1_6.W001**: Some project unit tests may not execute as expected. *This
  check was removed in Django 1.8 due to false positives*.
* **1_6.W002**: ``BooleanField`` does not have a default value. *This
  check was removed in Django 1.8 due to false positives*.
* **1_7.W001**:  Django 1.7 changed the global defaults for the
  ``MIDDLEWARE_CLASSES.``
  ``django.contrib.sessions.middleware.SessionMiddleware``,
  ``django.contrib.auth.middleware.AuthenticationMiddleware``, and
  ``django.contrib.messages.middleware.MessageMiddleware`` were removed from
  the defaults. If your project needs these middleware then you should
  configure this setting. *This check was removed in Django 1.9*.

Admin
-----

Admin checks are all performed as part of the ``admin`` tag.

The following checks are performed on any
:class:`~django.contrib.admin.ModelAdmin` (or subclass) that is registered
with the admin site:

* **admin.E001**: The value of ``raw_id_fields`` must be a list or tuple.
* **admin.E002**: The value of ``raw_id_fields[n]`` refers to ``<field name>``,
  which is not an attribute of ``<model>``.
* **admin.E003**: The value of ``raw_id_fields[n]`` must be a ``ForeignKey`` or
  ``ManyToManyField``.
* **admin.E004**: The value of ``fields`` must be a list or tuple.
* **admin.E005**: Both ``fieldsets`` and ``fields`` are specified.
* **admin.E006**: The value of ``fields`` contains duplicate field(s).
* **admin.E007**: The value of ``fieldsets`` must be a list or tuple.
* **admin.E008**: The value of ``fieldsets[n]`` must be a list or tuple.
* **admin.E009**: The value of ``fieldsets[n]`` must be of length 2.
* **admin.E010**: The value of ``fieldsets[n][1]`` must be a dictionary.
* **admin.E011**: The value of ``fieldsets[n][1]`` must contain the key
  ``fields``.
* **admin.E012**: There are duplicate field(s) in ``fieldsets[n][1]``.
* **admin.E013**: ``fields[n]/fieldsets[n][m]`` cannot include the
  ``ManyToManyField`` ``<field name>``, because that field manually specifies a
  relationship model.
* **admin.E014**: The value of ``exclude`` must be a list or tuple.
* **admin.E015**: The value of ``exclude`` contains duplicate field(s).
* **admin.E016**: The value of ``form`` must inherit from ``BaseModelForm``.
* **admin.E017**: The value of ``filter_vertical`` must be a list or tuple.
* **admin.E018**: The value of ``filter_horizontal`` must be a list or tuple.
* **admin.E019**: The value of ``filter_vertical[n]/filter_vertical[n]`` refers
  to ``<field name>``, which is not an attribute of ``<model>``.
* **admin.E020**: The value of ``filter_vertical[n]/filter_vertical[n]`` must
  be a ``ManyToManyField``.
* **admin.E021**: The value of ``radio_fields`` must be a dictionary.
* **admin.E022**: The value of ``radio_fields`` refers to ``<field name>``,
  which is not an attribute of ``<model>``.
* **admin.E023**: The value of ``radio_fields`` refers to ``<field name>``,
  which is not a ``ForeignKey``, and does not have a ``choices`` definition.
* **admin.E024**: The value of ``radio_fields[<field name>]`` must be either
  ``admin.HORIZONTAL`` or ``admin.VERTICAL``.
* **admin.E025**: The value of ``view_on_site`` must be either a callable or a
  boolean value.
* **admin.E026**: The value of ``prepopulated_fields`` must be a dictionary.
* **admin.E027**: The value of ``prepopulated_fields`` refers to
  ``<field name>``, which is not an attribute of ``<model>``.
* **admin.E028**: The value of ``prepopulated_fields`` refers to
  ``<field name>``, which must not be a ``DateTimeField``, ``ForeignKey`` or
  ``ManyToManyField``.
* **admin.E029**: The value of ``prepopulated_fields[<field name>]`` must be a
  list or tuple.
* **admin.E030**: The value of ``prepopulated_fields`` refers to
  ``<field name>``, which is not an attribute of ``<model>``.
* **admin.E031**: The value of ``ordering`` must be a list or tuple.
* **admin.E032**: The value of ``ordering`` has the random ordering marker
  ``?``, but contains other fields as well.
* **admin.E033**: The value of ``ordering`` refers to ``<field name>``, which
  is not an attribute of ``<model>``.
* **admin.E034**: The value of ``readonly_fields`` must be a list or tuple.
* **admin.E035**: The value of ``readonly_fields[n]`` is not a callable, an
  attribute of ``<ModelAdmin class>``, or an attribute of ``<model>``.

ModelAdmin
~~~~~~~~~~

The following checks are performed on any
:class:`~django.contrib.admin.ModelAdmin` that is registered
with the admin site:

* **admin.E101**: The value of ``save_as`` must be a boolean.
* **admin.E102**: The value of ``save_on_top`` must be a boolean.
* **admin.E103**: The value of ``inlines`` must be a list or tuple.
* **admin.E104**: ``<InlineModelAdmin class>`` must inherit from
  ``BaseModelAdmin``.
* **admin.E105**: ``<InlineModelAdmin class>`` must have a ``model`` attribute.
* **admin.E106**: The value of ``<InlineModelAdmin class>.model`` must be a
  ``Model``.
* **admin.E107**: The value of ``list_display`` must be a list or tuple.
* **admin.E108**: The value of ``list_display[n]`` refers to ``<label>``,
  which is not a callable, an attribute of ``<ModelAdmin class>``, or an
  attribute or method on ``<model>``.
* **admin.E109**: The value of ``list_display[n]`` must not be a
  ``ManyToManyField``.
* **admin.E110**: The value of ``list_display_links`` must be a list, a tuple,
  or ``None``.
* **admin.E111**: The value of ``list_display_links[n]`` refers to ``<label>``,
  which is not defined in ``list_display``.
* **admin.E112**: The value of ``list_filter`` must be a list or tuple.
* **admin.E113**: The value of ``list_filter[n]`` must inherit from
  ``ListFilter``.
* **admin.E114**: The value of ``list_filter[n]`` must not inherit from
  ``FieldListFilter``.
* **admin.E115**: The value of ``list_filter[n][1]`` must inherit from
  ``FieldListFilter``.
* **admin.E116**: The value of ``list_filter[n]`` refers to ``<label>``,
  which does not refer to a Field.
* **admin.E117**: The value of ``list_select_related`` must be a boolean,
  tuple or list.
* **admin.E118**: The value of ``list_per_page`` must be an integer.
* **admin.E119**: The value of ``list_max_show_all`` must be an integer.
* **admin.E120**: The value of ``list_editable`` must be a list or tuple.
* **admin.E121**: The value of ``list_editable[n]`` refers to ``<label>``,
  which is not an attribute of ``<model>``.
* **admin.E122**: The value of ``list_editable[n]`` refers to ``<label>``,
  which is not contained in ``list_display``.
* **admin.E123**: The value of ``list_editable[n]`` cannot be in both
  ``list_editable`` and ``list_display_links``.
* **admin.E124**: The value of ``list_editable[n]`` refers to the first field
  in ``list_display`` (``<label>``), which cannot be used unless
  ``list_display_links`` is set.
* **admin.E125**: The value of ``list_editable[n]`` refers to ``<field name>``,
  which is not editable through the admin.
* **admin.E126**: The value of ``search_fields`` must be a list or tuple.
* **admin.E127**: The value of ``date_hierarchy`` refers to ``<field name>``,
  which is not an attribute of ``<model>``.
* **admin.E128**: The value of ``date_hierarchy`` must be a ``DateField`` or
  ``DateTimeField``.

InlineModelAdmin
~~~~~~~~~~~~~~~~

The following checks are performed on any
:class:`~django.contrib.admin.InlineModelAdmin` that is registered as an
inline on a :class:`~django.contrib.admin.ModelAdmin`.

* **admin.E201**: Cannot exclude the field ``<field name>``, because it is the
  foreign key to the parent model ``<app_label>.<model>``.
* **admin.E202**: ``<model>`` has no ``ForeignKey`` to ``<parent model>``./
  ``<model>`` has more than one ``ForeignKey`` to ``<parent model>``.
* **admin.E203**: The value of ``extra`` must be an integer.
* **admin.E204**: The value of ``max_num`` must be an integer.
* **admin.E205**: The value of ``min_num`` must be an integer.
* **admin.E206**: The value of ``formset`` must inherit from
  ``BaseModelFormSet``.

GenericInlineModelAdmin
~~~~~~~~~~~~~~~~~~~~~~~

The following checks are performed on any
:class:`~django.contrib.contenttypes.admin.GenericInlineModelAdmin` that is
registered as an inline on a :class:`~django.contrib.admin.ModelAdmin`.

* **admin.E301**: ``'ct_field'`` references ``<label>``, which is not a field
  on ``<model>``.
* **admin.E302**: ``'ct_fk_field'`` references ``<label>``, which is not a
  field on ``<model>``.
* **admin.E303**: ``<model>`` has no ``GenericForeignKey``.
* **admin.E304**: ``<model>`` has no ``GenericForeignKey`` using content type
  field ``<field name>`` and object ID field ``<field name>``.


Auth
----

* **auth.E001**: ``REQUIRED_FIELDS`` must be a list or tuple.
* **auth.E002**: The field named as the ``USERNAME_FIELD`` for a custom user
  model must not be included in ``REQUIRED_FIELDS``.
* **auth.E003**: ``<field>`` must be unique because it is named as the
  ``USERNAME_FIELD``.
* **auth.W004**: ``<field>`` is named as the ``USERNAME_FIELD``, but it is not
  unique.


Content Types
-------------

The following checks are performed when a model contains a
:class:`~django.contrib.contenttypes.fields.GenericForeignKey` or
:class:`~django.contrib.contenttypes.fields.GenericRelation`:

* **contenttypes.E001**: The ``GenericForeignKey`` object ID references the
  non-existent field ``<field>``.
* **contenttypes.E002**: The ``GenericForeignKey`` content type references the
  non-existent field ``<field>``.
* **contenttypes.E003**: ``<field>`` is not a ``ForeignKey``.
* **contenttypes.E004**: ``<field>`` is not a ``ForeignKey`` to
  ``contenttypes.ContentType``.

Security
--------

The security checks do not make your site secure. They do not audit code, do
intrusion detection, or do anything particularly complex. Rather, they help
perform an automated, low-hanging-fruit checklist. They help you remember the
simple things that improve your site's security.

Some of these checks may not be appropriate for your particular deployment
configuration. For instance, if you do your HTTP to HTTPS redirection in a load
balancer, it'd be irritating to be constantly warned about not having enabled
:setting:`SECURE_SSL_REDIRECT`. Use :setting:`SILENCED_SYSTEM_CHECKS` to
silence unneeded checks.

The following checks will be run if you use the :djadminopt:`--deploy` option
of the :djadmin:`check` command:

* **security.W001**: You do not have
  :class:`django.middleware.security.SecurityMiddleware` in your
  :setting:`MIDDLEWARE_CLASSES` so the :setting:`SECURE_HSTS_SECONDS`,
  :setting:`SECURE_CONTENT_TYPE_NOSNIFF`, :setting:`SECURE_BROWSER_XSS_FILTER`,
  and :setting:`SECURE_SSL_REDIRECT` settings will have no effect.
* **security.W002**: You do not have
  :class:`django.middleware.clickjacking.XFrameOptionsMiddleware` in your
  :setting:`MIDDLEWARE_CLASSES`, so your pages will not be served with an
  ``'x-frame-options'`` header. Unless there is a good reason for your
  site to be served in a frame, you should consider enabling this
  header to help prevent clickjacking attacks.
* **security.W003**: You don't appear to be using Django's built-in cross-site
  request forgery protection via the middleware
  (:class:`django.middleware.csrf.CsrfViewMiddleware` is not in your
  :setting:`MIDDLEWARE_CLASSES`). Enabling the middleware is the safest
  approach to ensure you don't leave any holes.
* **security.W004**: You have not set a value for the
  :setting:`SECURE_HSTS_SECONDS` setting. If your entire site is served only
  over SSL, you may want to consider setting a value and enabling :ref:`HTTP
  Strict Transport Security <http-strict-transport-security>`. Be sure to read
  the documentation first; enabling HSTS carelessly can cause serious,
  irreversible problems.
* **security.W005**: You have not set the
  :setting:`SECURE_HSTS_INCLUDE_SUBDOMAINS` setting to ``True``. Without this,
  your site is potentially vulnerable to attack via an insecure connection to a
  subdomain. Only set this to ``True`` if you are certain that all subdomains of
  your domain should be served exclusively via SSL.
* **security.W006**: Your :setting:`SECURE_CONTENT_TYPE_NOSNIFF` setting is not
  set to ``True``, so your pages will not be served with an
  ``'x-content-type-options: nosniff'`` header. You should consider enabling
  this header to prevent the browser from identifying content types incorrectly.
* **security.W007**: Your :setting:`SECURE_BROWSER_XSS_FILTER` setting is not
  set to ``True``, so your pages will not be served with an
  ``'x-xss-protection: 1; mode=block'`` header. You should consider enabling
  this header to activate the browser's XSS filtering and help prevent XSS
  attacks.
* **security.W008**: Your :setting:`SECURE_SSL_REDIRECT` setting is not set to
  ``True``. Unless your site should be available over both SSL and non-SSL
  connections, you may want to either set this setting to ``True`` or configure
  a load balancer or reverse-proxy server  to redirect all connections to HTTPS.
* **security.W009**: Your :setting:`SECRET_KEY` has less than 50 characters or
  less than 5 unique characters. Please generate a long and random
  ``SECRET_KEY``, otherwise many of Django's security-critical features will be
  vulnerable to attack.
* **security.W010**: You have :mod:`django.contrib.sessions` in your
  :setting:`INSTALLED_APPS` but you have not set
  :setting:`SESSION_COOKIE_SECURE` to ``True``. Using a secure-only session
  cookie makes it more difficult for network traffic sniffers to hijack user
  sessions.
* **security.W011**: You have
  :class:`django.contrib.sessions.middleware.SessionMiddleware` in your
  :setting:`MIDDLEWARE_CLASSES`, but you have not set
  :setting:`SESSION_COOKIE_SECURE` to ``True``. Using a secure-only session
  cookie makes it more difficult for network traffic sniffers to hijack user
  sessions.
* **security.W012**: :setting:`SESSION_COOKIE_SECURE` is not set to ``True``.
  Using a secure-only session cookie makes it more difficult for network traffic
  sniffers to hijack user sessions.
* **security.W013**: You have :mod:`django.contrib.sessions` in your
  :setting:`INSTALLED_APPS`, but you have not set
  :setting:`SESSION_COOKIE_HTTPONLY` to ``True``. Using an ``HttpOnly`` session
  cookie makes it more difficult for cross-site scripting attacks to hijack user
  sessions.
* **security.W014**: You have
  :class:`django.contrib.sessions.middleware.SessionMiddleware` in your
  :setting:`MIDDLEWARE_CLASSES`, but you have not set
  :setting:`SESSION_COOKIE_HTTPONLY` to ``True``. Using an ``HttpOnly`` session
  cookie makes it more difficult for cross-site scripting attacks to hijack user
  sessions.
* **security.W015**: :setting:`SESSION_COOKIE_HTTPONLY` is not set to ``True``.
  Using an ``HttpOnly`` session cookie makes it more difficult for cross-site
  scripting attacks to hijack user sessions.
* **security.W016**: :setting:`CSRF_COOKIE_SECURE` is not set to ``True``.
  Using a secure-only CSRF cookie makes it more difficult for network traffic
  sniffers to steal the CSRF token.
* **security.W017**: :setting:`CSRF_COOKIE_HTTPONLY` is not set to ``True``.
  Using an ``HttpOnly`` CSRF cookie makes it more difficult for cross-site
  scripting attacks to steal the CSRF token.
* **security.W018**: You should not have :setting:`DEBUG` set to ``True`` in
  deployment.
* **security.W019**: You have
  :class:`django.middleware.clickjacking.XFrameOptionsMiddleware` in your
  :setting:`MIDDLEWARE_CLASSES`, but :setting:`X_FRAME_OPTIONS` is not set to
  ``'DENY'``. The default is ``'SAMEORIGIN'``, but unless there is a good reason
  for your site to serve other parts of itself in a frame, you should change
  it to ``'DENY'``.

Sites
-----

The following checks are performed on any model using a
:class:`~django.contrib.sites.managers.CurrentSiteManager`:

* **sites.E001**: ``CurrentSiteManager`` could not find a field named
  ``<field name>``.
* **sites.E002**: ``CurrentSiteManager`` cannot use ``<field>`` as it is not a
  ``ForeignKey`` or ``ManyToManyField``.

Database
--------

MySQL
~~~~~

If you're using MySQL, the following checks will be performed:

* **mysql.E001**: MySQL does not allow unique ``CharField``\s to have a
  ``max_length`` > 255.