19.16 The admin’s checks: check_dependencies and ModelAdminChecks

When they run

SimpleAdminConfig.ready registers both functions, and AdminConfig.ready calls it before autodiscover. They run after every application is ready: under AdminConfig, after every admin module has registered its model admins.

The admin’s dependencies: check_dependencies

check_dependencies looks at the settings. It returns nothing when the admin is not installed, and takes no notice of the applications it is given.

The context processors are looked for in the first DjangoTemplates engine only, by the dotted paths the setting spells. The admin’s base template reads "user" and "messages", which the auth and messages processors set, and the sidebar includes "admin/app_list.html", which reads the request’s path. The warning needs a site in all_sites with AdminSite.enable_nav_sidebar true, its default.

The middleware are looked for by class: _contains_subclass imports each path in MIDDLEWARE and is satisfied by the class required or a subclass of it. It asks whether each is there, not in what order: the hint of "admin.E410" says to put SessionMiddleware before AuthenticationMiddleware, yet harbour’s settings with AuthenticationMiddleware moved first passed every check. With MessageMiddleware taken out:

# the check command, harbour's MIDDLEWARE without MessageMiddleware
SystemCheckError: System check identified some issues:

ERRORS:
?: (admin.E409) 'django.contrib.messages.middleware.MessageMiddleware' must be in MIDDLEWARE in order to use the admin application.

System check identified 1 issue (0 silenced).

Every site and its model admins: check_admin_app

check_admin_app calls AdminSite.check on each site in all_sites, a weak set that AdminSite.__init__ adds every site to, so a site is checked whether or not its URLs are in the URLconf, as long as something holds it. AdminSite.check passes over a model admin whose class is ModelAdmin itself, one its docstring calls not customized at all, and checks each other one whose model belongs to an application the run was given, or to any when it was given none. In the harbour office that is GroupAdmin, UserAdmin, PortAdmin and ShipAdmin, and not Voyage’s plain ModelAdmin.

BaseModelAdmin.check makes an instance of the class in BaseModelAdmin.checks_class and hands it the model admin. ModelAdmin names ModelAdminChecks there and InlineModelAdmin names InlineModelAdminChecks, both subclasses of BaseModelAdminChecks, which checks the options the two share; a project adds checks of its own by naming a subclass in checks_class. An inline is checked through its parent: for each class in inlines that inherits from InlineModelAdmin and names a model, ModelAdminChecks makes an instance with the parent’s model and site and calls its check.

What the options are checked against

The checks read the options as attributes and call none of the hooks, which take a request; the check of list_display_links against list_display is skipped when the class overrides ModelAdmin.get_list_display.

• The type. Many options must be a list or a tuple, others a dictionary, a boolean or an integer; form must inherit from BaseModelForm, an inline’s formset from BaseModelFormSet.
• Fields of the model. Each name in raw_id_fields, autocomplete_fields, filter_horizontal, filter_vertical, radio_fields, prepopulated_fields, list_editable and ordering must be a field, though ordering lets "?" alone, "pk", a path with "__" and an expression other than an F through. list_display and readonly_fields take more: a callable, or an attribute of the model admin or of the model, and list_display a path across relations, though no item of it may be a many-to-many field or a reverse foreign key. date_hierarchy must lead to a date or date-and-time field, and list_filter must take one of its three forms (Filters and search: list_filter, search_fields and date_hierarchy).
• Relations. raw_id_fields and autocomplete_fields must name foreign keys or many-to-many fields, and the model an autocomplete field points to must be registered on the same site, with search_fields on its model admin. filter_horizontal and filter_vertical must name many-to-many fields with no through model of their own.
• list_editable. Each field must be in list_display, editable, not the primary key, and not in list_display_links; nor the first column while list_display_links is empty, as it is by default, which links the first column. None there, or another column, frees it.
• Actions. Only the actions whose locations include ActionLocation.CHANGE_LIST are looked at: each permission in an action’s "allowed_permissions" needs a method named after it, as BaseModelAdmin.has_change_permission is after "change", and no two of them may share a name.
• An inline’s parent. InlineModelAdminChecks asks _get_foreign_key for the inline’s foreign key to the parent, by fk_name if set, reports its error as "admin.E202", and refuses an exclude naming that key.

A name in fields or fieldsets that is not a field passes, since, a comment says, it could be an extra field of the form, and search_fields is checked only for its type. A ShipAdmin with “draught” in fields and “beam” in search_fields passed the checks, and then its change form and its search each raised FieldError.

The messages

Most messages take one of three shapes, made by must_be, must_inherit_from and refer_to_missing_field: the option, or the item at fault with its index, and what it must be, must inherit from, or names that the model lacks. Each gives the class checked as its object. Here ShipAdmin has three mistakes, and harbour/crew/admin.py registers Sailor under the plain ModelAdmin:

# harbour/fleet/admin.py
class ShipAdmin(admin.ModelAdmin):
    list_display = ["name", "tonnage", "home", "is_large", "draught"]
    list_editable = ["name"]
    autocomplete_fields = ["captain"]
    # the rest as before

# the check command, with ShipAdmin as above
SystemCheckError: System check identified some issues:

ERRORS:
<class 'harbour.fleet.admin.ShipAdmin'>: (admin.E040) ModelAdmin must define "search_fields", because it's referenced by ShipAdmin.autocomplete_fields.
<class 'harbour.fleet.admin.ShipAdmin'>: (admin.E108) The value of 'list_display[4]' refers to 'draught', which is not a callable or attribute of 'ShipAdmin', or an attribute, method, or field on 'fleet.Ship'.
<class 'harbour.fleet.admin.ShipAdmin'>: (admin.E124) The value of 'list_editable[0]' refers to the first field in 'list_display' ('name'), which cannot be used unless 'list_display_links' is set.

System check identified 3 issues (0 silenced).