from django import forms
from django.contrib import admin
from django.contrib.admin import AdminSite
from django.contrib.auth.backends import ModelBackend
from django.contrib.auth.middleware import AuthenticationMiddleware
from django.contrib.contenttypes.admin import GenericStackedInline
from django.contrib.messages.middleware import MessageMiddleware
from django.contrib.sessions.middleware import SessionMiddleware
from django.core import checks
from django.test import SimpleTestCase, override_settings

from .models import (
    Album, Author, Book, City, Influence, Song, State, TwoAlbumFKAndAnE,
)


class SongForm(forms.ModelForm):
    pass


class ValidFields(admin.ModelAdmin):
    form = SongForm
    fields = ['title']


class ValidFormFieldsets(admin.ModelAdmin):
    def get_form(self, request, obj=None, **kwargs):
        class ExtraFieldForm(SongForm):
            name = forms.CharField(max_length=50)
        return ExtraFieldForm

    fieldsets = (
        (None, {
            'fields': ('name',),
        }),
    )


class MyAdmin(admin.ModelAdmin):
    def check(self, **kwargs):
        return ['error!']


class AuthenticationMiddlewareSubclass(AuthenticationMiddleware):
    pass


class MessageMiddlewareSubclass(MessageMiddleware):
    pass


class ModelBackendSubclass(ModelBackend):
    pass


class SessionMiddlewareSubclass(SessionMiddleware):
    pass


@override_settings(
    SILENCED_SYSTEM_CHECKS=['fields.W342'],  # ForeignKey(unique=True)
    INSTALLED_APPS=[
        'django.contrib.admin',
        'django.contrib.auth',
        'django.contrib.contenttypes',
        'django.contrib.messages',
        'admin_checks',
    ],
)
class SystemChecksTestCase(SimpleTestCase):

    def test_checks_are_performed(self):
        admin.site.register(Song, MyAdmin)
        try:
            errors = checks.run_checks()
            expected = ['error!']
            self.assertEqual(errors, expected)
        finally:
            admin.site.unregister(Song)

    @override_settings(INSTALLED_APPS=['django.contrib.admin'])
    def test_apps_dependencies(self):
        errors = admin.checks.check_dependencies()
        expected = [
            checks.Error(
                "'django.contrib.contenttypes' must be in "
                "INSTALLED_APPS in order to use the admin application.",
                id="admin.E401",
            ),
            checks.Error(
                "'django.contrib.auth' must be in INSTALLED_APPS in order "
                "to use the admin application.",
                id='admin.E405',
            ),
            checks.Error(
                "'django.contrib.messages' must be in INSTALLED_APPS in order "
                "to use the admin application.",
                id='admin.E406',
            ),
        ]
        self.assertEqual(errors, expected)

    @override_settings(TEMPLATES=[])
    def test_no_template_engines(self):
        self.assertEqual(admin.checks.check_dependencies(), [
            checks.Error(
                "A 'django.template.backends.django.DjangoTemplates' "
                "instance must be configured in TEMPLATES in order to use "
                "the admin application.",
                id='admin.E403',
            )
        ])

    @override_settings(
        TEMPLATES=[{
            'BACKEND': 'django.template.backends.django.DjangoTemplates',
            'DIRS': [],
            'APP_DIRS': True,
            'OPTIONS': {
                'context_processors': [],
            },
        }],
    )
    def test_context_processor_dependencies(self):
        expected = [
            checks.Error(
                "'django.contrib.auth.context_processors.auth' must be "
                "enabled in DjangoTemplates (TEMPLATES) if using the default "
                "auth backend in order to use the admin application.",
                id='admin.E402',
            ),
            checks.Error(
                "'django.contrib.messages.context_processors.messages' must "
                "be enabled in DjangoTemplates (TEMPLATES) in order to use "
                "the admin application.",
                id='admin.E404',
            )
        ]
        self.assertEqual(admin.checks.check_dependencies(), expected)
        # The first error doesn't happen if
        # 'django.contrib.auth.backends.ModelBackend' isn't in
        # AUTHENTICATION_BACKENDS.
        with self.settings(AUTHENTICATION_BACKENDS=[]):
            self.assertEqual(admin.checks.check_dependencies(), expected[1:])

    @override_settings(
        AUTHENTICATION_BACKENDS=['admin_checks.tests.ModelBackendSubclass'],
        TEMPLATES=[{
            'BACKEND': 'django.template.backends.django.DjangoTemplates',
            'DIRS': [],
            'APP_DIRS': True,
            'OPTIONS': {
                'context_processors': ['django.contrib.messages.context_processors.messages'],
            },
        }],
    )
    def test_context_processor_dependencies_model_backend_subclass(self):
        self.assertEqual(admin.checks.check_dependencies(), [
            checks.Error(
                "'django.contrib.auth.context_processors.auth' must be "
                "enabled in DjangoTemplates (TEMPLATES) if using the default "
                "auth backend in order to use the admin application.",
                id='admin.E402',
            ),
        ])

    @override_settings(
        TEMPLATES=[
            {
                'BACKEND': 'django.template.backends.dummy.TemplateStrings',
                'DIRS': [],
                'APP_DIRS': True,
            },
            {
                'BACKEND': 'django.template.backends.django.DjangoTemplates',
                'DIRS': [],
                'APP_DIRS': True,
                'OPTIONS': {
                    'context_processors': [
                        'django.contrib.auth.context_processors.auth',
                        'django.contrib.messages.context_processors.messages',
                    ],
                },
            },
        ],
    )
    def test_several_templates_backends(self):
        self.assertEqual(admin.checks.check_dependencies(), [])

    @override_settings(MIDDLEWARE=[])
    def test_middleware_dependencies(self):
        errors = admin.checks.check_dependencies()
        expected = [
            checks.Error(
                "'django.contrib.auth.middleware.AuthenticationMiddleware' "
                "must be in MIDDLEWARE in order to use the admin application.",
                id='admin.E408',
            ),
            checks.Error(
                "'django.contrib.messages.middleware.MessageMiddleware' "
                "must be in MIDDLEWARE in order to use the admin application.",
                id='admin.E409',
            ),
            checks.Error(
                "'django.contrib.sessions.middleware.SessionMiddleware' "
                "must be in MIDDLEWARE in order to use the admin application.",
                id='admin.E410',
            ),
        ]
        self.assertEqual(errors, expected)

    @override_settings(MIDDLEWARE=[
        'admin_checks.tests.AuthenticationMiddlewareSubclass',
        'admin_checks.tests.MessageMiddlewareSubclass',
        'admin_checks.tests.SessionMiddlewareSubclass',
    ])
    def test_middleware_subclasses(self):
        self.assertEqual(admin.checks.check_dependencies(), [])

    @override_settings(MIDDLEWARE=[
        'django.contrib.does.not.Exist',
        'django.contrib.auth.middleware.AuthenticationMiddleware',
        'django.contrib.messages.middleware.MessageMiddleware',
        'django.contrib.sessions.middleware.SessionMiddleware',
    ])
    def test_admin_check_ignores_import_error_in_middleware(self):
        self.assertEqual(admin.checks.check_dependencies(), [])

    def test_custom_adminsite(self):
        class CustomAdminSite(admin.AdminSite):
            pass

        custom_site = CustomAdminSite()
        custom_site.register(Song, MyAdmin)
        try:
            errors = checks.run_checks()
            expected = ['error!']
            self.assertEqual(errors, expected)
        finally:
            custom_site.unregister(Song)

    def test_allows_checks_relying_on_other_modeladmins(self):
        class MyBookAdmin(admin.ModelAdmin):
            def check(self, **kwargs):
                errors = super().check(**kwargs)
                author_admin = self.admin_site._registry.get(Author)
                if author_admin is None:
                    errors.append('AuthorAdmin missing!')
                return errors

        class MyAuthorAdmin(admin.ModelAdmin):
            pass

        admin.site.register(Book, MyBookAdmin)
        admin.site.register(Author, MyAuthorAdmin)
        try:
            self.assertEqual(admin.site.check(None), [])
        finally:
            admin.site.unregister(Book)
            admin.site.unregister(Author)

    def test_field_name_not_in_list_display(self):
        class SongAdmin(admin.ModelAdmin):
            list_editable = ["original_release"]

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'list_editable[0]' refers to 'original_release', "
                "which is not contained in 'list_display'.",
                obj=SongAdmin,
                id='admin.E122',
            )
        ]
        self.assertEqual(errors, expected)

    def test_list_editable_not_a_list_or_tuple(self):
        class SongAdmin(admin.ModelAdmin):
            list_editable = 'test'

        self.assertEqual(SongAdmin(Song, AdminSite()).check(), [
            checks.Error(
                "The value of 'list_editable' must be a list or tuple.",
                obj=SongAdmin,
                id='admin.E120',
            )
        ])

    def test_list_editable_missing_field(self):
        class SongAdmin(admin.ModelAdmin):
            list_editable = ('test',)

        self.assertEqual(SongAdmin(Song, AdminSite()).check(), [
            checks.Error(
                "The value of 'list_editable[0]' refers to 'test', which is "
                "not an attribute of 'admin_checks.Song'.",
                obj=SongAdmin,
                id='admin.E121',
            )
        ])

    def test_readonly_and_editable(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = ["original_release"]
            list_display = ["pk", "original_release"]
            list_editable = ["original_release"]
            fieldsets = [
                (None, {
                    "fields": ["title", "original_release"],
                }),
            ]
        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'list_editable[0]' refers to 'original_release', "
                "which is not editable through the admin.",
                obj=SongAdmin,
                id='admin.E125',
            )
        ]
        self.assertEqual(errors, expected)

    def test_editable(self):
        class SongAdmin(admin.ModelAdmin):
            list_display = ["pk", "title"]
            list_editable = ["title"]
            fieldsets = [
                (None, {
                    "fields": ["title", "original_release"],
                }),
            ]

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_custom_modelforms_with_fields_fieldsets(self):
        """
        # Regression test for #8027: custom ModelForms with fields/fieldsets
        """
        errors = ValidFields(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_custom_get_form_with_fieldsets(self):
        """
        The fieldsets checks are skipped when the ModelAdmin.get_form() method
        is overridden.
        """
        errors = ValidFormFieldsets(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_fieldsets_fields_non_tuple(self):
        """
        The first fieldset's fields must be a list/tuple.
        """
        class NotATupleAdmin(admin.ModelAdmin):
            list_display = ["pk", "title"]
            list_editable = ["title"]
            fieldsets = [
                (None, {
                    "fields": "title"  # not a tuple
                }),
            ]

        errors = NotATupleAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'fieldsets[0][1]['fields']' must be a list or tuple.",
                obj=NotATupleAdmin,
                id='admin.E008',
            )
        ]
        self.assertEqual(errors, expected)

    def test_nonfirst_fieldset(self):
        """
        The second fieldset's fields must be a list/tuple.
        """
        class NotATupleAdmin(admin.ModelAdmin):
            fieldsets = [
                (None, {
                    "fields": ("title",)
                }),
                ('foo', {
                    "fields": "author"  # not a tuple
                }),
            ]

        errors = NotATupleAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'fieldsets[1][1]['fields']' must be a list or tuple.",
                obj=NotATupleAdmin,
                id='admin.E008',
            )
        ]
        self.assertEqual(errors, expected)

    def test_exclude_values(self):
        """
        Tests for basic system checks of 'exclude' option values (#12689)
        """
        class ExcludedFields1(admin.ModelAdmin):
            exclude = 'foo'

        errors = ExcludedFields1(Book, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'exclude' must be a list or tuple.",
                obj=ExcludedFields1,
                id='admin.E014',
            )
        ]
        self.assertEqual(errors, expected)

    def test_exclude_duplicate_values(self):
        class ExcludedFields2(admin.ModelAdmin):
            exclude = ('name', 'name')

        errors = ExcludedFields2(Book, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'exclude' contains duplicate field(s).",
                obj=ExcludedFields2,
                id='admin.E015',
            )
        ]
        self.assertEqual(errors, expected)

    def test_exclude_in_inline(self):
        class ExcludedFieldsInline(admin.TabularInline):
            model = Song
            exclude = 'foo'

        class ExcludedFieldsAlbumAdmin(admin.ModelAdmin):
            model = Album
            inlines = [ExcludedFieldsInline]

        errors = ExcludedFieldsAlbumAdmin(Album, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'exclude' must be a list or tuple.",
                obj=ExcludedFieldsInline,
                id='admin.E014',
            )
        ]
        self.assertEqual(errors, expected)

    def test_exclude_inline_model_admin(self):
        """
        Regression test for #9932 - exclude in InlineModelAdmin should not
        contain the ForeignKey field used in ModelAdmin.model
        """
        class SongInline(admin.StackedInline):
            model = Song
            exclude = ['album']

        class AlbumAdmin(admin.ModelAdmin):
            model = Album
            inlines = [SongInline]

        errors = AlbumAdmin(Album, AdminSite()).check()
        expected = [
            checks.Error(
                "Cannot exclude the field 'album', because it is the foreign key "
                "to the parent model 'admin_checks.Album'.",
                obj=SongInline,
                id='admin.E201',
            )
        ]
        self.assertEqual(errors, expected)

    def test_valid_generic_inline_model_admin(self):
        """
        Regression test for #22034 - check that generic inlines don't look for
        normal ForeignKey relations.
        """
        class InfluenceInline(GenericStackedInline):
            model = Influence

        class SongAdmin(admin.ModelAdmin):
            inlines = [InfluenceInline]

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_generic_inline_model_admin_non_generic_model(self):
        """
        A model without a GenericForeignKey raises problems if it's included
        in a GenericInlineModelAdmin definition.
        """
        class BookInline(GenericStackedInline):
            model = Book

        class SongAdmin(admin.ModelAdmin):
            inlines = [BookInline]

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "'admin_checks.Book' has no GenericForeignKey.",
                obj=BookInline,
                id='admin.E301',
            )
        ]
        self.assertEqual(errors, expected)

    def test_generic_inline_model_admin_bad_ct_field(self):
        """
        A GenericInlineModelAdmin errors if the ct_field points to a
        nonexistent field.
        """
        class InfluenceInline(GenericStackedInline):
            model = Influence
            ct_field = 'nonexistent'

        class SongAdmin(admin.ModelAdmin):
            inlines = [InfluenceInline]

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "'ct_field' references 'nonexistent', which is not a field on 'admin_checks.Influence'.",
                obj=InfluenceInline,
                id='admin.E302',
            )
        ]
        self.assertEqual(errors, expected)

    def test_generic_inline_model_admin_bad_fk_field(self):
        """
        A GenericInlineModelAdmin errors if the ct_fk_field points to a
        nonexistent field.
        """
        class InfluenceInline(GenericStackedInline):
            model = Influence
            ct_fk_field = 'nonexistent'

        class SongAdmin(admin.ModelAdmin):
            inlines = [InfluenceInline]

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "'ct_fk_field' references 'nonexistent', which is not a field on 'admin_checks.Influence'.",
                obj=InfluenceInline,
                id='admin.E303',
            )
        ]
        self.assertEqual(errors, expected)

    def test_generic_inline_model_admin_non_gfk_ct_field(self):
        """
        A GenericInlineModelAdmin raises problems if the ct_field points to a
        field that isn't part of a GenericForeignKey.
        """
        class InfluenceInline(GenericStackedInline):
            model = Influence
            ct_field = 'name'

        class SongAdmin(admin.ModelAdmin):
            inlines = [InfluenceInline]

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "'admin_checks.Influence' has no GenericForeignKey using "
                "content type field 'name' and object ID field 'object_id'.",
                obj=InfluenceInline,
                id='admin.E304',
            )
        ]
        self.assertEqual(errors, expected)

    def test_generic_inline_model_admin_non_gfk_fk_field(self):
        """
        A GenericInlineModelAdmin raises problems if the ct_fk_field points to
        a field that isn't part of a GenericForeignKey.
        """
        class InfluenceInline(GenericStackedInline):
            model = Influence
            ct_fk_field = 'name'

        class SongAdmin(admin.ModelAdmin):
            inlines = [InfluenceInline]

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "'admin_checks.Influence' has no GenericForeignKey using "
                "content type field 'content_type' and object ID field 'name'.",
                obj=InfluenceInline,
                id='admin.E304',
            )
        ]
        self.assertEqual(errors, expected)

    def test_app_label_in_admin_checks(self):
        class RawIdNonexistentAdmin(admin.ModelAdmin):
            raw_id_fields = ('nonexistent',)

        errors = RawIdNonexistentAdmin(Album, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'raw_id_fields[0]' refers to 'nonexistent', "
                "which is not an attribute of 'admin_checks.Album'.",
                obj=RawIdNonexistentAdmin,
                id='admin.E002',
            )
        ]
        self.assertEqual(errors, expected)

    def test_fk_exclusion(self):
        """
        Regression test for #11709 - when testing for fk excluding (when exclude is
        given) make sure fk_name is honored or things blow up when there is more
        than one fk to the parent model.
        """
        class TwoAlbumFKAndAnEInline(admin.TabularInline):
            model = TwoAlbumFKAndAnE
            exclude = ("e",)
            fk_name = "album1"

        class MyAdmin(admin.ModelAdmin):
            inlines = [TwoAlbumFKAndAnEInline]

        errors = MyAdmin(Album, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_inline_self_check(self):
        class TwoAlbumFKAndAnEInline(admin.TabularInline):
            model = TwoAlbumFKAndAnE

        class MyAdmin(admin.ModelAdmin):
            inlines = [TwoAlbumFKAndAnEInline]

        errors = MyAdmin(Album, AdminSite()).check()
        expected = [
            checks.Error(
                "'admin_checks.TwoAlbumFKAndAnE' has more than one ForeignKey "
                "to 'admin_checks.Album'. You must specify a 'fk_name' "
                "attribute.",
                obj=TwoAlbumFKAndAnEInline,
                id='admin.E202',
            )
        ]
        self.assertEqual(errors, expected)

    def test_inline_with_specified(self):
        class TwoAlbumFKAndAnEInline(admin.TabularInline):
            model = TwoAlbumFKAndAnE
            fk_name = "album1"

        class MyAdmin(admin.ModelAdmin):
            inlines = [TwoAlbumFKAndAnEInline]

        errors = MyAdmin(Album, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_inlines_property(self):
        class CitiesInline(admin.TabularInline):
            model = City

        class StateAdmin(admin.ModelAdmin):
            @property
            def inlines(self):
                return [CitiesInline]

        errors = StateAdmin(State, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_readonly(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = ("title",)

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_readonly_on_method(self):
        def my_function(obj):
            pass

        class SongAdmin(admin.ModelAdmin):
            readonly_fields = (my_function,)

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_readonly_on_modeladmin(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = ("readonly_method_on_modeladmin",)

            def readonly_method_on_modeladmin(self, obj):
                pass

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_readonly_dynamic_attribute_on_modeladmin(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = ("dynamic_method",)

            def __getattr__(self, item):
                if item == "dynamic_method":
                    def method(obj):
                        pass
                    return method
                raise AttributeError

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_readonly_method_on_model(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = ("readonly_method_on_model",)

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_nonexistent_field(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = ("title", "nonexistent")

        errors = SongAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'readonly_fields[1]' is not a callable, an attribute "
                "of 'SongAdmin', or an attribute of 'admin_checks.Song'.",
                obj=SongAdmin,
                id='admin.E035',
            )
        ]
        self.assertEqual(errors, expected)

    def test_nonexistent_field_on_inline(self):
        class CityInline(admin.TabularInline):
            model = City
            readonly_fields = ['i_dont_exist']  # Missing attribute

        errors = CityInline(State, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'readonly_fields[0]' is not a callable, an attribute "
                "of 'CityInline', or an attribute of 'admin_checks.City'.",
                obj=CityInline,
                id='admin.E035',
            )
        ]
        self.assertEqual(errors, expected)

    def test_readonly_fields_not_list_or_tuple(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = 'test'

        self.assertEqual(SongAdmin(Song, AdminSite()).check(), [
            checks.Error(
                "The value of 'readonly_fields' must be a list or tuple.",
                obj=SongAdmin,
                id='admin.E034',
            )
        ])

    def test_extra(self):
        class SongAdmin(admin.ModelAdmin):
            def awesome_song(self, instance):
                if instance.title == "Born to Run":
                    return "Best Ever!"
                return "Status unknown."

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_readonly_lambda(self):
        class SongAdmin(admin.ModelAdmin):
            readonly_fields = (lambda obj: "test",)

        errors = SongAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_graceful_m2m_fail(self):
        """
        Regression test for #12203/#12237 - Fail more gracefully when a M2M field that
        specifies the 'through' option is included in the 'fields' or the 'fieldsets'
        ModelAdmin options.
        """
        class BookAdmin(admin.ModelAdmin):
            fields = ['authors']

        errors = BookAdmin(Book, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'fields' cannot include the ManyToManyField 'authors', "
                "because that field manually specifies a relationship model.",
                obj=BookAdmin,
                id='admin.E013',
            )
        ]
        self.assertEqual(errors, expected)

    def test_cannot_include_through(self):
        class FieldsetBookAdmin(admin.ModelAdmin):
            fieldsets = (
                ('Header 1', {'fields': ('name',)}),
                ('Header 2', {'fields': ('authors',)}),
            )

        errors = FieldsetBookAdmin(Book, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'fieldsets[1][1][\"fields\"]' cannot include the ManyToManyField "
                "'authors', because that field manually specifies a relationship model.",
                obj=FieldsetBookAdmin,
                id='admin.E013',
            )
        ]
        self.assertEqual(errors, expected)

    def test_nested_fields(self):
        class NestedFieldsAdmin(admin.ModelAdmin):
            fields = ('price', ('name', 'subtitle'))

        errors = NestedFieldsAdmin(Book, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_nested_fieldsets(self):
        class NestedFieldsetAdmin(admin.ModelAdmin):
            fieldsets = (
                ('Main', {'fields': ('price', ('name', 'subtitle'))}),
            )

        errors = NestedFieldsetAdmin(Book, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_explicit_through_override(self):
        """
        Regression test for #12209 -- If the explicitly provided through model
        is specified as a string, the admin should still be able use
        Model.m2m_field.through
        """
        class AuthorsInline(admin.TabularInline):
            model = Book.authors.through

        class BookAdmin(admin.ModelAdmin):
            inlines = [AuthorsInline]

        errors = BookAdmin(Book, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_non_model_fields(self):
        """
        Regression for ensuring ModelAdmin.fields can contain non-model fields
        that broke with r11737
        """
        class SongForm(forms.ModelForm):
            extra_data = forms.CharField()

        class FieldsOnFormOnlyAdmin(admin.ModelAdmin):
            form = SongForm
            fields = ['title', 'extra_data']

        errors = FieldsOnFormOnlyAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_non_model_first_field(self):
        """
        Regression for ensuring ModelAdmin.field can handle first elem being a
        non-model field (test fix for UnboundLocalError introduced with r16225).
        """
        class SongForm(forms.ModelForm):
            extra_data = forms.CharField()

            class Meta:
                model = Song
                fields = '__all__'

        class FieldsOnFormOnlyAdmin(admin.ModelAdmin):
            form = SongForm
            fields = ['extra_data', 'title']

        errors = FieldsOnFormOnlyAdmin(Song, AdminSite()).check()
        self.assertEqual(errors, [])

    def test_check_sublists_for_duplicates(self):
        class MyModelAdmin(admin.ModelAdmin):
            fields = ['state', ['state']]

        errors = MyModelAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "The value of 'fields' contains duplicate field(s).",
                obj=MyModelAdmin,
                id='admin.E006'
            )
        ]
        self.assertEqual(errors, expected)

    def test_check_fieldset_sublists_for_duplicates(self):
        class MyModelAdmin(admin.ModelAdmin):
            fieldsets = [
                (None, {
                    'fields': ['title', 'album', ('title', 'album')]
                }),
            ]

        errors = MyModelAdmin(Song, AdminSite()).check()
        expected = [
            checks.Error(
                "There are duplicate field(s) in 'fieldsets[0][1]'.",
                obj=MyModelAdmin,
                id='admin.E012'
            )
        ]
        self.assertEqual(errors, expected)

    def test_list_filter_works_on_through_field_even_when_apps_not_ready(self):
        """
        Ensure list_filter can access reverse fields even when the app registry
        is not ready; refs #24146.
        """
        class BookAdminWithListFilter(admin.ModelAdmin):
            list_filter = ['authorsbooks__featured']

        # Temporarily pretending apps are not ready yet. This issue can happen
        # if the value of 'list_filter' refers to a 'through__field'.
        Book._meta.apps.ready = False
        try:
            errors = BookAdminWithListFilter(Book, AdminSite()).check()
            self.assertEqual(errors, [])
        finally:
            Book._meta.apps.ready = True