# Τεκμηρίωση του Django - 1.10.7

# Πηγαίος κώδικας για το django.db.models.options

    from __future__ import unicode_literals

    import copy
    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 import connections
    from django.db.models import Manager
    from django.db.models.fields import AutoField
    from django.db.models.fields.proxy import OrderWrt
    from django.utils import six
    from django.utils.datastructures import ImmutableList, OrderedSet
    from django.utils.deprecation import (
        RemovedInDjango20Warning, warn_about_renamed_method,
    )
    from django.utils.encoding import (
        force_text, python_2_unicode_compatible, smart_text,
    )
    from django.utils.functional import cached_property
    from django.utils.text import camel_case_to_spaces
    from django.utils.translation import override, string_concat

    NOT_PROVIDED = object()

    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', 'required_db_features',
        'required_db_vendor', 'base_manager_name', 'default_manager_name',
        'manager_inheritance_from_future',
    )

    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', 'managers', 'managers_map', 'base_manager',
            'default_manager',
        }
        REVERSE_PROPERTIES = {'related_objects', 'fields_map', '_relation_tree'}

        default_apps = apps

        def __init__(self, meta, app_label=None):
            self._get_fields_cache = {}
            self.local_fields = []
            self.local_many_to_many = []
            self.private_fields = []
            self.manager_inheritance_from_future = False
            self.local_managers = []
            self.base_manager_name = None
            self.default_manager_name = None
            self.model_name = None
            self.verbose_name = None
            self.verbose_name_plural = None
            self.db_table = ''
            self.ordering = []
            self._ordering_clash = False
            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.required_db_features = []
            self.required_db_vendor = None
            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

            # 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 = self.default_apps

            self.default_related_name = None

        @property
        def label(self):
            return '%s.%s' % (self.app_label, self.object_name)

        @property
        def label_lower(self):
            return '%s.%s' % (self.app_label, self.model_name)

        @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

        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)

                self.unique_together = normalize_together(self.unique_together)
                self.index_together = normalize_together(self.index_together)

                # 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')

                # order_with_respect_and ordering are mutually exclusive.
                self._ordering_clash = bool(self.ordering and self.order_with_respect_to)

                # 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)
                    if not field.remote_field.parent_link:
                        warnings.warn(
                            'Add parent_link=True to %s as an implicit link is '
                            'deprecated.' % field, RemovedInDjango20Warning
                        )
                else:
                    auto = AutoField(verbose_name='ID', primary_key=True, auto_created=True)
                    model.add_to_class('id', auto)

        def add_manager(self, manager):
            self.local_managers.append(manager)
            self._expire_cache()

        def add_field(self, field, private=False, virtual=NOT_PROVIDED):
            if virtual is not NOT_PROVIDED:
                warnings.warn(
                    "The `virtual` argument of Options.add_field() has been renamed to `private`.",
                    RemovedInDjango20Warning, stacklevel=2
                )
                private = virtual
            # 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 private:
                self.private_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.remote_field, 'model') and field.remote_field.model:
                try:
                    field.remote_field.model._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))

        def can_migrate(self, connection):
            """
            Return True if the model can/should be migrated on the `connection`.
            `connection` can be either a real connection or a connection alias.
            """
            if self.proxy or self.swapped or not self.managed:
                return False
            if isinstance(connection, six.string_types):
                connection = connections[connection]
            if self.required_db_vendor:
                return self.required_db_vendor == connection.vendor
            if self.required_db_features:
                return all(getattr(connection.features, feat, False)
                           for feat in self.required_db_features)
            return True

        @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:
                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()) != self.label_lower:
                        return swapped_for
            return None

        @cached_property
        def managers(self):
            managers = []
            seen_managers = set()
            bases = (b for b in self.model.mro() if hasattr(b, '_meta'))
            for depth, base in enumerate(bases):
                for manager in base._meta.local_managers:
                    if manager.name in seen_managers:
                        continue

                    manager = copy.copy(manager)
                    manager.model = self.model
                    seen_managers.add(manager.name)
                    managers.append((depth, manager.creation_counter, manager))

                    # Used for deprecation of legacy manager inheritance,
                    # remove afterwards. (RemovedInDjango20Warning)
                    manager._originating_model = base

            return make_immutable_fields_list(
                "managers",
                (m[2] for m in sorted(managers)),
            )

        @cached_property
        def managers_map(self):
            return {manager.name: manager for manager in self.managers}

        @cached_property
        def base_manager(self):
            base_manager_name = self.base_manager_name
            if not base_manager_name:
                # Get the first parent's base_manager_name if there's one.
                for parent in self.model.mro()[1:]:
                    if hasattr(parent, '_meta'):
                        if parent._base_manager.name != '_base_manager':
                            base_manager_name = parent._base_manager.name
                        break

            if base_manager_name:
                try:
                    return self.managers_map[base_manager_name]
                except KeyError:
                    raise ValueError(
                        "%s has no manager named %r" % (
                            self.object_name,
                            base_manager_name,
                        )
                    )

            # Deprecation shim for `use_for_related_fields`.
            for i, base_manager_class in enumerate(self.default_manager.__class__.mro()):
                if getattr(base_manager_class, 'use_for_related_fields', False):
                    if not getattr(base_manager_class, 'silence_use_for_related_fields_deprecation', False):
                        warnings.warn(
                            "use_for_related_fields is deprecated, instead "
                            "set Meta.base_manager_name on '{}'.".format(self.model._meta.label),
                            RemovedInDjango20Warning, 2
                        )

                    if i == 0:
                        manager = self.default_manager
                    else:
                        manager = base_manager_class()
                        manager.name = '_base_manager'
                        manager.model = self.model

                    return manager

            manager = Manager()
            manager.name = '_base_manager'
            manager.model = self.model
            manager.auto_created = True
            return manager

        @cached_property
        def default_manager(self):
            default_manager_name = self.default_manager_name
            if not default_manager_name and not self.local_managers:
                # Get the first parent's default_manager_name if there's one.
                for parent in self.model.mro()[1:]:
                    if hasattr(parent, '_meta'):
                        default_manager_name = parent._meta.default_manager_name
                        break

            if default_manager_name:
                try:
                    return self.managers_map[default_manager_name]
                except KeyError:
                    raise ValueError(
                        "%s has no manager named %r" % (
                            self.object_name,
                            default_manager_name,
                        )
                    )

            if self.managers:
                return self.managers[0]

        @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 private 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.
            def is_not_an_m2m_field(f):
                return not (f.is_relation and f.many_to_many)

            def is_not_a_generic_relation(f):
                return not (f.is_relation and f.one_to_many)

            def is_not_a_generic_foreign_key(f):
                return not (
                    f.is_relation and f.many_to_one and not (hasattr(f.remote_field, 'model') and f.remote_field.model)
                )

            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)
            )

        @property
        @warn_about_renamed_method(
            'Options', 'virtual_fields', 'private_fields',
            RemovedInDjango20Warning
        )
        def virtual_fields(self):
            return self.private_fields

        @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)
            )

        @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)
            )

        @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):
            """
            Return a field instance given the name of a forward or reverse field.
            """
            try:
                # In order to avoid premature loading of the relation tree
                # (expensive) we prefer checking if the field is a forward field.
                return self._forward_fields_map[field_name]
            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:
                # 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))

        def get_base_chain(self, model):
            """
            Return a list of parent classes leading to `model` (ordered from
            closest to most distant ancestor). This has to handle the case where
            `model` is a grandparent or even more distant relation.
            """
            if not self.parents:
                return []
            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 []

        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:
                opts = model._meta
                # Abstract model's fields are copied to child models, hence we will
                # see the fields from the child models.
                if opts.abstract:
                    continue
                fields_with_relations = (
                    f for f in opts._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.remote_field.model, six.string_types):
                        related_objects_graph[f.remote_field.model._meta.concrete_model._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.concrete_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.
            if forward:
                for cache_key in self.FORWARD_PROPERTIES:
                    if cache_key in self.__dict__:
                        delattr(self, cache_key)
            if reverse and not self.abstract:
                for cache_key in self.REVERSE_PROPERTIES:
                    if cache_key in self.__dict__:
                        delattr(self, cache_key)
            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 getattr(obj, 'parent_link', False) and obj.model != self.concrete_model:
                            continue
                        fields.append(obj)
            if reverse and not self.proxy:
                # 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.remote_field.hidden:
                        fields.append(field.remote_field)

            if forward:
                fields.extend(
                    field for field in chain(self.local_fields, self.local_many_to_many)
                )
                # Private 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 private 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.private_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