# Django 1.8.18.dev20170123152445 documentation

# Source code for django.db.models.options

    from __future__ import unicode_literals

    import warnings
    from bisect import bisect
    from collections import OrderedDict, defaultdict
    from itertools import chain

    from django.apps import apps
    from django.conf import settings
    from django.core.exceptions import FieldDoesNotExist
    from django.db.models.fields import AutoField
    from django.db.models.fields.proxy import OrderWrt
    from django.db.models.fields.related import ManyToManyField
    from django.utils import six
    from django.utils.datastructures import ImmutableList, OrderedSet
    from django.utils.deprecation import RemovedInDjango110Warning
    from django.utils.encoding import (
        force_text, python_2_unicode_compatible, smart_text,
    )
    from django.utils.functional import cached_property
    from django.utils.lru_cache import lru_cache
    from django.utils.text import camel_case_to_spaces
    from django.utils.translation import override, string_concat

    PROXY_PARENTS = object()

    EMPTY_RELATION_TREE = tuple()

    IMMUTABLE_WARNING = (
        "The return type of '%s' should never be mutated. If you want to manipulate this list "
        "for your own use, make a copy first."
    )

    DEFAULT_NAMES = ('verbose_name', 'verbose_name_plural', 'db_table', 'ordering',
                     'unique_together', 'permissions', 'get_latest_by',
                     'order_with_respect_to', 'app_label', 'db_tablespace',
                     'abstract', 'managed', 'proxy', 'swappable', 'auto_created',
                     'index_together', 'apps', 'default_permissions',
                     'select_on_save', 'default_related_name')

    class raise_deprecation(object):
        def __init__(self, suggested_alternative):
            self.suggested_alternative = suggested_alternative

        def __call__(self, fn):
            def wrapper(*args, **kwargs):
                warnings.warn(
                    "'%s is an unofficial API that has been deprecated. "
                    "You may be able to replace it with '%s'" % (
                        fn.__name__,
                        self.suggested_alternative,
                    ),
                    RemovedInDjango110Warning, stacklevel=2
                )
                return fn(*args, **kwargs)
            return wrapper

    def normalize_together(option_together):
        """
        option_together can be either a tuple of tuples, or a single
        tuple of two strings. Normalize it to a tuple of tuples, so that
        calling code can uniformly expect that.
        """
        try:
            if not option_together:
                return ()
            if not isinstance(option_together, (tuple, list)):
                raise TypeError
            first_element = next(iter(option_together))
            if not isinstance(first_element, (tuple, list)):
                option_together = (option_together,)
            # Normalize everything to tuples
            return tuple(tuple(ot) for ot in option_together)
        except TypeError:
            # If the value of option_together isn't valid, return it
            # verbatim; this will be picked up by the check framework later.
            return option_together

    def make_immutable_fields_list(name, data):
        return ImmutableList(data, warning=IMMUTABLE_WARNING % name)

    @python_2_unicode_compatible
    class Options(object):
        FORWARD_PROPERTIES = ('fields', 'many_to_many', 'concrete_fields',
                              'local_concrete_fields', '_forward_fields_map')
        REVERSE_PROPERTIES = ('related_objects', 'fields_map', '_relation_tree')

        def __init__(self, meta, app_label=None):
            self._get_fields_cache = {}
            self.proxied_children = []
            self.local_fields = []
            self.local_many_to_many = []
            self.virtual_fields = []
            self.model_name = None
            self.verbose_name = None
            self.verbose_name_plural = None
            self.db_table = ''
            self.ordering = []
            self.unique_together = []
            self.index_together = []
            self.select_on_save = False
            self.default_permissions = ('add', 'change', 'delete')
            self.permissions = []
            self.object_name = None
            self.app_label = app_label
            self.get_latest_by = None
            self.order_with_respect_to = None
            self.db_tablespace = settings.DEFAULT_TABLESPACE
            self.meta = meta
            self.pk = None
            self.has_auto_field = False
            self.auto_field = None
            self.abstract = False
            self.managed = True
            self.proxy = False
            # For any class that is a proxy (including automatically created
            # classes for deferred object loading), proxy_for_model tells us
            # which class this model is proxying. Note that proxy_for_model
            # can create a chain of proxy models. For non-proxy models, the
            # variable is always None.
            self.proxy_for_model = None
            # For any non-abstract class, the concrete class is the model
            # in the end of the proxy_for_model chain. In particular, for
            # concrete models, the concrete_model is always the class itself.
            self.concrete_model = None
            self.swappable = None
            self.parents = OrderedDict()
            self.auto_created = False

            # To handle various inheritance situations, we need to track where
            # managers came from (concrete or abstract base classes). `managers`
            # keeps a list of 3-tuples of the form:
            # (creation_counter, instance, abstract(=True))
            self.managers = []

            # List of all lookups defined in ForeignKey 'limit_choices_to' options
            # from *other* models. Needed for some admin checks. Internal use only.
            self.related_fkey_lookups = []

            # A custom app registry to use, if you're making a separate model set.
            self.apps = apps

            self.default_related_name = None

        @lru_cache(maxsize=None)
        def _map_model(self, link):
            # This helper function is used to allow backwards compatibility with
            # the previous API. No future methods should use this function.
            # It maps a field to (field, model or related_model,) depending on the
            # field type.
            model = link.model._meta.concrete_model
            if model is self.model:
                model = None
            return link, model

        @lru_cache(maxsize=None)
        def _map_model_details(self, link):
            # This helper function is used to allow backwards compatibility with
            # the previous API. No future methods should use this function.
            # This function maps a field to a tuple of:
            #  (field, model or related_model, direct, is_m2m) depending on the
            # field type.
            direct = not link.auto_created or link.concrete
            model = link.model._meta.concrete_model
            if model is self.model:
                model = None
            m2m = link.is_relation and link.many_to_many
            return link, model, direct, m2m

        @property
        def app_config(self):
            # Don't go through get_app_config to avoid triggering imports.
            return self.apps.app_configs.get(self.app_label)

        @property
        def installed(self):
            return self.app_config is not None

        @property
        def abstract_managers(self):
            return [
                (counter, instance.name, instance) for counter, instance, abstract
                in self.managers if abstract
            ]

        @property
        def concrete_managers(self):
            return [
                (counter, instance.name, instance) for counter, instance, abstract
                in self.managers if not abstract
            ]

        def contribute_to_class(self, cls, name):
            from django.db import connection
            from django.db.backends.utils import truncate_name

            cls._meta = self
            self.model = cls
            # First, construct the default values for these options.
            self.object_name = cls.__name__
            self.model_name = self.object_name.lower()
            self.verbose_name = camel_case_to_spaces(self.object_name)

            # Store the original user-defined values for each option,
            # for use when serializing the model definition
            self.original_attrs = {}

            # Next, apply any overridden values from 'class Meta'.
            if self.meta:
                meta_attrs = self.meta.__dict__.copy()
                for name in self.meta.__dict__:
                    # Ignore any private attributes that Django doesn't care about.
                    # NOTE: We can't modify a dictionary's contents while looping
                    # over it, so we loop over the *original* dictionary instead.
                    if name.startswith('_'):
                        del meta_attrs[name]
                for attr_name in DEFAULT_NAMES:
                    if attr_name in meta_attrs:
                        setattr(self, attr_name, meta_attrs.pop(attr_name))
                        self.original_attrs[attr_name] = getattr(self, attr_name)
                    elif hasattr(self.meta, attr_name):
                        setattr(self, attr_name, getattr(self.meta, attr_name))
                        self.original_attrs[attr_name] = getattr(self, attr_name)

                ut = meta_attrs.pop('unique_together', self.unique_together)
                self.unique_together = normalize_together(ut)

                it = meta_attrs.pop('index_together', self.index_together)
                self.index_together = normalize_together(it)

                # verbose_name_plural is a special case because it uses a 's'
                # by default.
                if self.verbose_name_plural is None:
                    self.verbose_name_plural = string_concat(self.verbose_name, 's')

                # Any leftover attributes must be invalid.
                if meta_attrs != {}:
                    raise TypeError("'class Meta' got invalid attribute(s): %s" % ','.join(meta_attrs.keys()))
            else:
                self.verbose_name_plural = string_concat(self.verbose_name, 's')
            del self.meta

            # If the db_table wasn't provided, use the app_label + model_name.
            if not self.db_table:
                self.db_table = "%s_%s" % (self.app_label, self.model_name)
                self.db_table = truncate_name(self.db_table, connection.ops.max_name_length())

        def _prepare(self, model):
            if self.order_with_respect_to:
                # The app registry will not be ready at this point, so we cannot
                # use get_field().
                query = self.order_with_respect_to
                try:
                    self.order_with_respect_to = next(
                        f for f in self._get_fields(reverse=False)
                        if f.name == query or f.attname == query
                    )
                except StopIteration:
                    raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, query))

                self.ordering = ('_order',)
                if not any(isinstance(field, OrderWrt) for field in model._meta.local_fields):
                    model.add_to_class('_order', OrderWrt())
            else:
                self.order_with_respect_to = None

            if self.pk is None:
                if self.parents:
                    # Promote the first parent link in lieu of adding yet another
                    # field.
                    field = next(six.itervalues(self.parents))
                    # Look for a local field with the same name as the
                    # first parent link. If a local field has already been
                    # created, use it instead of promoting the parent
                    already_created = [fld for fld in self.local_fields if fld.name == field.name]
                    if already_created:
                        field = already_created[0]
                    field.primary_key = True
                    self.setup_pk(field)
                else:
                    auto = AutoField(verbose_name='ID', primary_key=True,
                            auto_created=True)
                    model.add_to_class('id', auto)

        def add_field(self, field, virtual=False):
            # Insert the given field in the order in which it was created, using
            # the "creation_counter" attribute of the field.
            # Move many-to-many related fields from self.fields into
            # self.many_to_many.
            if virtual:
                self.virtual_fields.append(field)
            elif field.is_relation and field.many_to_many:
                self.local_many_to_many.insert(bisect(self.local_many_to_many, field), field)
            else:
                self.local_fields.insert(bisect(self.local_fields, field), field)
                self.setup_pk(field)

            # If the field being added is a relation to another known field,
            # expire the cache on this field and the forward cache on the field
            # being referenced, because there will be new relationships in the
            # cache. Otherwise, expire the cache of references *to* this field.
            # The mechanism for getting at the related model is slightly odd -
            # ideally, we'd just ask for field.related_model. However, related_model
            # is a cached property, and all the models haven't been loaded yet, so
            # we need to make sure we don't cache a string reference.
            if field.is_relation and hasattr(field.rel, 'to') and field.rel.to:
                try:
                    field.rel.to._meta._expire_cache(forward=False)
                except AttributeError:
                    pass
                self._expire_cache()
            else:
                self._expire_cache(reverse=False)

        def setup_pk(self, field):
            if not self.pk and field.primary_key:
                self.pk = field
                field.serialize = False

        def setup_proxy(self, target):
            """
            Does the internal setup so that the current model is a proxy for
            "target".
            """
            self.pk = target._meta.pk
            self.proxy_for_model = target
            self.db_table = target._meta.db_table

        def __repr__(self):
            return '<Options for %s>' % self.object_name

        def __str__(self):
            return "%s.%s" % (smart_text(self.app_label), smart_text(self.model_name))

        @property
        def verbose_name_raw(self):
            """
            There are a few places where the untranslated verbose name is needed
            (so that we get the same value regardless of currently active
            locale).
            """
            with override(None):
                return force_text(self.verbose_name)

        @property
        def swapped(self):
            """
            Has this model been swapped out for another? If so, return the model
            name of the replacement; otherwise, return None.

            For historical reasons, model name lookups using get_model() are
            case insensitive, so we make sure we are case insensitive here.
            """
            if self.swappable:
                model_label = '%s.%s' % (self.app_label, self.model_name)
                swapped_for = getattr(settings, self.swappable, None)
                if swapped_for:
                    try:
                        swapped_label, swapped_object = swapped_for.split('.')
                    except ValueError:
                        # setting not in the format app_label.model_name
                        # raising ImproperlyConfigured here causes problems with
                        # test cleanup code - instead it is raised in get_user_model
                        # or as part of validation.
                        return swapped_for

                    if '%s.%s' % (swapped_label, swapped_object.lower()) not in (None, model_label):
                        return swapped_for
            return None

        @cached_property
        def fields(self):
            """
            Returns a list of all forward fields on the model and its parents,
            excluding ManyToManyFields.

            Private API intended only to be used by Django itself; get_fields()
            combined with filtering of field properties is the public API for
            obtaining this field list.
            """
            # For legacy reasons, the fields property should only contain forward
            # fields that are not virtual or with a m2m cardinality. Therefore we
            # pass these three filters as filters to the generator.
            # The third lambda is a longwinded way of checking f.related_model - we don't
            # use that property directly because related_model is a cached property,
            # and all the models may not have been loaded yet; we don't want to cache
            # the string reference to the related_model.
            is_not_an_m2m_field = lambda f: not (f.is_relation and f.many_to_many)
            is_not_a_generic_relation = lambda f: not (f.is_relation and f.one_to_many)
            is_not_a_generic_foreign_key = lambda f: not (
                f.is_relation and f.many_to_one and not (hasattr(f.rel, 'to') and f.rel.to)
            )
            return make_immutable_fields_list(
                "fields",
                (f for f in self._get_fields(reverse=False) if
                is_not_an_m2m_field(f) and is_not_a_generic_relation(f)
                and is_not_a_generic_foreign_key(f))
            )

        @cached_property
        def concrete_fields(self):
            """
            Returns a list of all concrete fields on the model and its parents.

            Private API intended only to be used by Django itself; get_fields()
            combined with filtering of field properties is the public API for
            obtaining this field list.
            """
            return make_immutable_fields_list(
                "concrete_fields", (f for f in self.fields if f.concrete)
            )

        @cached_property
        def local_concrete_fields(self):
            """
            Returns a list of all concrete fields on the model.

            Private API intended only to be used by Django itself; get_fields()
            combined with filtering of field properties is the public API for
            obtaining this field list.
            """
            return make_immutable_fields_list(
                "local_concrete_fields", (f for f in self.local_fields if f.concrete)
            )

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_fields_with_model(self):
            return [self._map_model(f) for f in self.get_fields()]

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_concrete_fields_with_model(self):
            return [self._map_model(f) for f in self.concrete_fields]

        @cached_property
        def many_to_many(self):
            """
            Returns a list of all many to many fields on the model and its parents.

            Private API intended only to be used by Django itself; get_fields()
            combined with filtering of field properties is the public API for
            obtaining this list.
            """
            return make_immutable_fields_list(
                "many_to_many",
                (f for f in self._get_fields(reverse=False)
                if f.is_relation and f.many_to_many)
            )

        @cached_property
        def related_objects(self):
            """
            Returns all related objects pointing to the current model. The related
            objects can come from a one-to-one, one-to-many, or many-to-many field
            relation type.

            Private API intended only to be used by Django itself; get_fields()
            combined with filtering of field properties is the public API for
            obtaining this field list.
            """
            all_related_fields = self._get_fields(forward=False, reverse=True, include_hidden=True)
            return make_immutable_fields_list(
                "related_objects",
                (obj for obj in all_related_fields
                if not obj.hidden or obj.field.many_to_many)
            )

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_m2m_with_model(self):
            return [self._map_model(f) for f in self.many_to_many]

        @cached_property
        def _forward_fields_map(self):
            res = {}
            fields = self._get_fields(reverse=False)
            for field in fields:
                res[field.name] = field
                # Due to the way Django's internals work, get_field() should also
                # be able to fetch a field by attname. In the case of a concrete
                # field with relation, includes the *_id name too
                try:
                    res[field.attname] = field
                except AttributeError:
                    pass
            return res

        @cached_property
        def fields_map(self):
            res = {}
            fields = self._get_fields(forward=False, include_hidden=True)
            for field in fields:
                res[field.name] = field
                # Due to the way Django's internals work, get_field() should also
                # be able to fetch a field by attname. In the case of a concrete
                # field with relation, includes the *_id name too
                try:
                    res[field.attname] = field
                except AttributeError:
                    pass
            return res

        def get_field(self, field_name, many_to_many=None):
            """
            Returns a field instance given a field name. The field can be either a
            forward or reverse field, unless many_to_many is specified; if it is,
            only forward fields will be returned.

            The many_to_many argument exists for backwards compatibility reasons;
            it has been deprecated and will be removed in Django 1.10.
            """
            m2m_in_kwargs = many_to_many is not None
            if m2m_in_kwargs:
                # Always throw a warning if many_to_many is used regardless of
                # whether it alters the return type or not.
                warnings.warn(
                    "The 'many_to_many' argument on get_field() is deprecated; "
                    "use a filter on field.many_to_many instead.",
                    RemovedInDjango110Warning
                )

            try:
                # In order to avoid premature loading of the relation tree
                # (expensive) we prefer checking if the field is a forward field.
                field = self._forward_fields_map[field_name]

                if many_to_many is False and field.many_to_many:
                    raise FieldDoesNotExist(
                        '%s has no field named %r' % (self.object_name, field_name)
                    )

                return field
            except KeyError:
                # If the app registry is not ready, reverse fields are
                # unavailable, therefore we throw a FieldDoesNotExist exception.
                if not self.apps.models_ready:
                    raise FieldDoesNotExist(
                        "%s has no field named %r. The app cache isn't ready yet, "
                        "so if this is an auto-created related field, it won't "
                        "be available yet." % (self.object_name, field_name)
                    )

            try:
                if m2m_in_kwargs:
                    # Previous API does not allow searching reverse fields.
                    raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))

                # Retrieve field instance by name from cached or just-computed
                # field map.
                return self.fields_map[field_name]
            except KeyError:
                raise FieldDoesNotExist('%s has no field named %r' % (self.object_name, field_name))

        @raise_deprecation(suggested_alternative="get_field()")
        def get_field_by_name(self, name):
            return self._map_model_details(self.get_field(name))

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_all_field_names(self):
            names = set()
            fields = self.get_fields()
            for field in fields:
                # For backwards compatibility GenericForeignKey should not be
                # included in the results.
                if field.is_relation and field.many_to_one and field.related_model is None:
                    continue
                # Relations to child proxy models should not be included.
                if (field.model != self.model and
                        field.model._meta.concrete_model == self.concrete_model):
                    continue

                names.add(field.name)
                if hasattr(field, 'attname'):
                    names.add(field.attname)
            return list(names)

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_all_related_objects(self, local_only=False, include_hidden=False,
                                    include_proxy_eq=False):

            include_parents = True if local_only is False else PROXY_PARENTS
            fields = self._get_fields(
                forward=False, reverse=True,
                include_parents=include_parents,
                include_hidden=include_hidden,
            )
            fields = (obj for obj in fields if not isinstance(obj.field, ManyToManyField))
            if include_proxy_eq:
                children = chain.from_iterable(c._relation_tree
                                               for c in self.concrete_model._meta.proxied_children
                                               if c is not self)
                relations = (f.rel for f in children
                             if include_hidden or not f.rel.field.rel.is_hidden())
                fields = chain(fields, relations)
            return list(fields)

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_all_related_objects_with_model(self, local_only=False, include_hidden=False,
                                               include_proxy_eq=False):
            return [
                self._map_model(f) for f in self.get_all_related_objects(
                    local_only=local_only,
                    include_hidden=include_hidden,
                    include_proxy_eq=include_proxy_eq,
                )
            ]

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_all_related_many_to_many_objects(self, local_only=False):
            include_parents = True if local_only is not True else PROXY_PARENTS
            fields = self._get_fields(
                forward=False, reverse=True,
                include_parents=include_parents, include_hidden=True
            )
            return [obj for obj in fields if isinstance(obj.field, ManyToManyField)]

        @raise_deprecation(suggested_alternative="get_fields()")
        def get_all_related_m2m_objects_with_model(self):
            fields = self._get_fields(forward=False, reverse=True, include_hidden=True)
            return [self._map_model(obj) for obj in fields if isinstance(obj.field, ManyToManyField)]

        def get_base_chain(self, model):
            """
            Returns a list of parent classes leading to 'model' (order from closet
            to most distant ancestor). This has to handle the case were 'model' is
            a grandparent or even more distant relation.
            """
            if not self.parents:
                return None
            if model in self.parents:
                return [model]
            for parent in self.parents:
                res = parent._meta.get_base_chain(model)
                if res:
                    res.insert(0, parent)
                    return res
            return None

        def get_parent_list(self):
            """
            Returns all the ancestors of this model as a list ordered by MRO.
            Useful for determining if something is an ancestor, regardless of lineage.
            """
            result = OrderedSet(self.parents)
            for parent in self.parents:
                for ancestor in parent._meta.get_parent_list():
                    result.add(ancestor)
            return list(result)

        def get_ancestor_link(self, ancestor):
            """
            Returns the field on the current model which points to the given
            "ancestor". This is possible an indirect link (a pointer to a parent
            model, which points, eventually, to the ancestor). Used when
            constructing table joins for model inheritance.

            Returns None if the model isn't an ancestor of this one.
            """
            if ancestor in self.parents:
                return self.parents[ancestor]
            for parent in self.parents:
                # Tries to get a link field from the immediate parent
                parent_link = parent._meta.get_ancestor_link(ancestor)
                if parent_link:
                    # In case of a proxied model, the first link
                    # of the chain to the ancestor is that parent
                    # links
                    return self.parents[parent] or parent_link

        def _populate_directed_relation_graph(self):
            """
            This method is used by each model to find its reverse objects. As this
            method is very expensive and is accessed frequently (it looks up every
            field in a model, in every app), it is computed on first access and then
            is set as a property on every model.
            """
            related_objects_graph = defaultdict(list)

            all_models = self.apps.get_models(include_auto_created=True)
            for model in all_models:
                # Abstract model's fields are copied to child models, hence we will
                # see the fields from the child models.
                if model._meta.abstract:
                    continue
                fields_with_relations = (
                    f for f in model._meta._get_fields(reverse=False, include_parents=False)
                    if f.is_relation and f.related_model is not None
                )
                for f in fields_with_relations:
                    if not isinstance(f.rel.to, six.string_types):
                        related_objects_graph[f.rel.to._meta].append(f)

            for model in all_models:
                # Set the relation_tree using the internal __dict__. In this way
                # we avoid calling the cached property. In attribute lookup,
                # __dict__ takes precedence over a data descriptor (such as
                # @cached_property). This means that the _meta._relation_tree is
                # only called if related_objects is not in __dict__.
                related_objects = related_objects_graph[model._meta]
                model._meta.__dict__['_relation_tree'] = related_objects
            # It seems it is possible that self is not in all_models, so guard
            # against that with default for get().
            return self.__dict__.get('_relation_tree', EMPTY_RELATION_TREE)

        @cached_property
        def _relation_tree(self):
            return self._populate_directed_relation_graph()

        def _expire_cache(self, forward=True, reverse=True):
            # This method is usually called by apps.cache_clear(), when the
            # registry is finalized, or when a new field is added.
            properties_to_expire = []
            if forward:
                properties_to_expire.extend(self.FORWARD_PROPERTIES)
            if reverse and not self.abstract:
                properties_to_expire.extend(self.REVERSE_PROPERTIES)

            for cache_key in properties_to_expire:
                try:
                    delattr(self, cache_key)
                except AttributeError:
                    pass

            self._get_fields_cache = {}

        def get_fields(self, include_parents=True, include_hidden=False):
            """
            Returns a list of fields associated to the model. By default, includes
            forward and reverse fields, fields derived from inheritance, but not
            hidden fields. The returned fields can be changed using the parameters:

            - include_parents: include fields derived from inheritance
            - include_hidden:  include fields that have a related_name that
                               starts with a "+"
            """
            if include_parents is False:
                include_parents = PROXY_PARENTS
            return self._get_fields(include_parents=include_parents, include_hidden=include_hidden)

        def _get_fields(self, forward=True, reverse=True, include_parents=True, include_hidden=False,
                        seen_models=None):
            """
            Internal helper function to return fields of the model.
            * If forward=True, then fields defined on this model are returned.
            * If reverse=True, then relations pointing to this model are returned.
            * If include_hidden=True, then fields with is_hidden=True are returned.
            * The include_parents argument toggles if fields from parent models
              should be included. It has three values: True, False, and
              PROXY_PARENTS. When set to PROXY_PARENTS, the call will return all
              fields defined for the current model or any of its parents in the
              parent chain to the model's concrete model.
            """
            if include_parents not in (True, False, PROXY_PARENTS):
                raise TypeError("Invalid argument for include_parents: %s" % (include_parents,))
            # This helper function is used to allow recursion in ``get_fields()``
            # implementation and to provide a fast way for Django's internals to
            # access specific subsets of fields.

            # We must keep track of which models we have already seen. Otherwise we
            # could include the same field multiple times from different models.
            topmost_call = False
            if seen_models is None:
                seen_models = set()
                topmost_call = True
            seen_models.add(self.model)

            # Creates a cache key composed of all arguments
            cache_key = (forward, reverse, include_parents, include_hidden, topmost_call)

            try:
                # In order to avoid list manipulation. Always return a shallow copy
                # of the results.
                return self._get_fields_cache[cache_key]
            except KeyError:
                pass

            fields = []
            # Recursively call _get_fields() on each parent, with the same
            # options provided in this call.
            if include_parents is not False:
                for parent in self.parents:
                    # In diamond inheritance it is possible that we see the same
                    # model from two different routes. In that case, avoid adding
                    # fields from the same parent again.
                    if parent in seen_models:
                        continue
                    if (parent._meta.concrete_model != self.concrete_model and
                            include_parents == PROXY_PARENTS):
                        continue
                    for obj in parent._meta._get_fields(
                            forward=forward, reverse=reverse, include_parents=include_parents,
                            include_hidden=include_hidden, seen_models=seen_models):
                        if hasattr(obj, 'parent_link') and obj.parent_link:
                            continue
                        fields.append(obj)
            if reverse:
                # Tree is computed once and cached until the app cache is expired.
                # It is composed of a list of fields pointing to the current model
                # from other models.
                all_fields = self._relation_tree
                for field in all_fields:
                    # If hidden fields should be included or the relation is not
                    # intentionally hidden, add to the fields dict.
                    if include_hidden or not field.rel.hidden:
                        fields.append(field.rel)

            if forward:
                fields.extend(
                    field for field in chain(self.local_fields, self.local_many_to_many)
                )
                # Virtual fields are recopied to each child model, and they get a
                # different model as field.model in each child. Hence we have to
                # add the virtual fields separately from the topmost call. If we
                # did this recursively similar to local_fields, we would get field
                # instances with field.model != self.model.
                if topmost_call:
                    fields.extend(
                        f for f in self.virtual_fields
                    )

            # In order to avoid list manipulation. Always
            # return a shallow copy of the results
            fields = make_immutable_fields_list("get_fields()", fields)

            # Store result into cache for later access
            self._get_fields_cache[cache_key] = fields
            return fields