class ModelSerializer

    def _get_model_fields(self, field_names, declared_fields, extra_kwargs):
        """
        Returns all the model fields that are being mapped to by fields
        on the serializer class.
        Returned as a dict of 'model field name' -> 'model field'.
        Used internally by `get_uniqueness_field_options`.
        """
        model = getattr(self.Meta, 'model')
        model_fields = {}

        for field_name in field_names:
            if field_name in declared_fields:
                # If the field is declared on the serializer
                field = declared_fields[field_name]
                source = field.source or field_name
            else:
                try:
                    source = extra_kwargs[field_name]['source']
                except KeyError:
                    source = field_name

            if '.' in source or source == '*':
                # Model fields will always have a simple source mapping,
                # they can't be nested attribute lookups.
                continue

            try:
                field = model._meta.get_field(source)
                if isinstance(field, DjangoModelField):
                    model_fields[source] = field
            except FieldDoesNotExist:
                pass

        return model_fields

    def _read_only_defaults(self):
        fields = [
            field for field in self.fields.values()
            if (field.read_only) and (field.default != empty) and (field.source != '*') and ('.' not in field.source)
        ]

        defaults = OrderedDict()
        for field in fields:
            try:
                default = field.get_default()
            except SkipField:
                continue
            defaults[field.field_name] = default

        return defaults

    def bind(self, field_name, parent):
        """
        Initializes the field name and parent for the field instance.
        Called when a field is added to the parent serializer instance.
        """

        # In order to enforce a consistent style, we error if a redundant
        # 'source' argument has been used. For example:
        # my_field = serializer.CharField(source='my_field')
        assert self.source != field_name, (
            "It is redundant to specify `source='%s'` on field '%s' in "
            "serializer '%s', because it is the same as the field name. "
            "Remove the `source` keyword argument." %
            (field_name, self.__class__.__name__, parent.__class__.__name__)
        )

        self.field_name = field_name
        self.parent = parent

        # `self.label` should default to being based on the field name.
        if self.label is None:
            self.label = field_name.replace('_', ' ').capitalize()

        # self.source should default to being the same as the field name.
        if self.source is None:
            self.source = field_name

        # self.source_attrs is a list of attributes that need to be looked up
        # when serializing the instance, or populating the validated data.
        if self.source == '*':
            self.source_attrs = []
        else:
            self.source_attrs = self.source.split('.')

    def build_field(self, field_name, info, model_class, nested_depth):
        """
        Return a two tuple of (cls, kwargs) to build a serializer field with.
        """
        if field_name in info.fields_and_pk:
            model_field = info.fields_and_pk[field_name]
            return self.build_standard_field(field_name, model_field)

        elif field_name in info.relations:
            relation_info = info.relations[field_name]
            if not nested_depth:
                return self.build_relational_field(field_name, relation_info)
            else:
                return self.build_nested_field(field_name, relation_info, nested_depth)

        elif hasattr(model_class, field_name):
            return self.build_property_field(field_name, model_class)

        elif field_name == self.url_field_name:
            return self.build_url_field(field_name, model_class)

        return self.build_unknown_field(field_name, model_class)

    def build_nested_field(self, field_name, relation_info, nested_depth):
        """
        Create nested fields for forward and reverse relationships.
        """
        class NestedSerializer(ModelSerializer):
            class Meta:
                model = relation_info.related_model
                depth = nested_depth - 1
                fields = '__all__'

        field_class = NestedSerializer
        field_kwargs = get_nested_relation_kwargs(relation_info)

        return field_class, field_kwargs

    def build_property_field(self, field_name, model_class):
        """
        Create a read only field for model methods and properties.
        """
        field_class = ReadOnlyField
        field_kwargs = {}

        return field_class, field_kwargs

    def build_relational_field(self, field_name, relation_info):
        """
        Create fields for forward and reverse relationships.
        """
        field_class = self.serializer_related_field
        field_kwargs = get_relation_kwargs(field_name, relation_info)

        to_field = field_kwargs.pop('to_field', None)
        if to_field and not relation_info.reverse and not relation_info.related_model._meta.get_field(to_field).primary_key:
            field_kwargs['slug_field'] = to_field
            field_class = self.serializer_related_to_field

        # `view_name` is only valid for hyperlinked relationships.
        if not issubclass(field_class, HyperlinkedRelatedField):
            field_kwargs.pop('view_name', None)

        return field_class, field_kwargs

    def build_standard_field(self, field_name, model_field):
        """
        Create regular model fields.
        """
        field_mapping = ClassLookupDict(self.serializer_field_mapping)

        field_class = field_mapping[model_field]
        field_kwargs = get_field_kwargs(field_name, model_field)

        # Special case to handle when a OneToOneField is also the primary key
        if model_field.one_to_one and model_field.primary_key:
            field_class = self.serializer_related_field
            field_kwargs['queryset'] = model_field.related_model.objects

        if 'choices' in field_kwargs:
            # Fields with choices get coerced into `ChoiceField`
            # instead of using their regular typed field.
            field_class = self.serializer_choice_field
            # Some model fields may introduce kwargs that would not be valid
            # for the choice field. We need to strip these out.
            # Eg. models.DecimalField(max_digits=3, decimal_places=1, choices=DECIMAL_CHOICES)
            valid_kwargs = {
                'read_only', 'write_only',
                'required', 'default', 'initial', 'source',
                'label', 'help_text', 'style',
                'error_messages', 'validators', 'allow_null', 'allow_blank',
                'choices'
            }
            for key in list(field_kwargs):
                if key not in valid_kwargs:
                    field_kwargs.pop(key)

        if not issubclass(field_class, ModelField):
            # `model_field` is only valid for the fallback case of
            # `ModelField`, which is used when no other typed field
            # matched to the model field.
            field_kwargs.pop('model_field', None)

        if not issubclass(field_class, CharField) and not issubclass(field_class, ChoiceField):
            # `allow_blank` is only valid for textual fields.
            field_kwargs.pop('allow_blank', None)

        if postgres_fields and isinstance(model_field, postgres_fields.ArrayField):
            # Populate the `child` argument on `ListField` instances generated
            # for the PostgreSQL specific `ArrayField`.
            child_model_field = model_field.base_field
            child_field_class, child_field_kwargs = self.build_standard_field(
                'child', child_model_field
            )
            field_kwargs['child'] = child_field_class(**child_field_kwargs)

        return field_class, field_kwargs

    def build_unknown_field(self, field_name, model_class):
        """
        Raise an error on any unknown fields.
        """
        raise ImproperlyConfigured(
            'Field name `%s` is not valid for model `%s`.' %
            (field_name, model_class.__name__)
        )

    def build_url_field(self, field_name, model_class):
        """
        Create a field representing the object's own URL.
        """
        field_class = self.serializer_url_field
        field_kwargs = get_url_kwargs(model_class)

        return field_class, field_kwargs

    def fail(self, key, **kwargs):
        """
        A helper method that simply raises a validation error.
        """
        try:
            msg = self.error_messages[key]
        except KeyError:
            class_name = self.__class__.__name__
            msg = MISSING_ERROR_MESSAGE.format(class_name=class_name, key=key)
            raise AssertionError(msg)
        message_string = msg.format(**kwargs)
        raise ValidationError(message_string, code=key)

    def get_attribute(self, instance):
        """
        Given the *outgoing* object instance, return the primitive value
        that should be used for this field.
        """
        try:
            return get_attribute(instance, self.source_attrs)
        except (KeyError, AttributeError) as exc:
            if self.default is not empty:
                return self.get_default()
            if self.allow_null:
                return None
            if not self.required:
                raise SkipField()
            msg = (
                'Got {exc_type} when attempting to get a value for field '
                '`{field}` on serializer `{serializer}`.\nThe serializer '
                'field might be named incorrectly and not match '
                'any attribute or key on the `{instance}` instance.\n'
                'Original exception text was: {exc}.'.format(
                    exc_type=type(exc).__name__,
                    field=self.field_name,
                    serializer=self.parent.__class__.__name__,
                    instance=instance.__class__.__name__,
                    exc=exc
                )
            )
            raise type(exc)(msg)

    def get_default(self):
        """
        Return the default value to use when validating data if no input
        is provided for this field.

        If a default has not been set for this field then this will simply
        raise `SkipField`, indicating that no value should be set in the
        validated data for this field.
        """
        if self.default is empty or getattr(self.root, 'partial', False):
            # No default, or this is a partial update.
            raise SkipField()
        if callable(self.default):
            if hasattr(self.default, 'set_context'):
                self.default.set_context(self)
            return self.default()
        return self.default

    def get_default_field_names(self, declared_fields, model_info):
        """
        Return the default list of field names that will be used if the
        `Meta.fields` option is not specified.
        """
        return (
            [model_info.pk.name] +
            list(declared_fields) +
            list(model_info.fields) +
            list(model_info.forward_relations)
        )

    def get_extra_kwargs(self):
        """
        Return a dictionary mapping field names to a dictionary of
        additional keyword arguments.
        """
        extra_kwargs = copy.deepcopy(getattr(self.Meta, 'extra_kwargs', {}))

        read_only_fields = getattr(self.Meta, 'read_only_fields', None)
        if read_only_fields is not None:
            if not isinstance(read_only_fields, (list, tuple)):
                raise TypeError(
                    'The `read_only_fields` option must be a list or tuple. '
                    'Got %s.' % type(read_only_fields).__name__
                )
            for field_name in read_only_fields:
                kwargs = extra_kwargs.get(field_name, {})
                kwargs['read_only'] = True
                extra_kwargs[field_name] = kwargs

        else:
            # Guard against the possible misspelling `readonly_fields` (used
            # by the Django admin and others).
            assert not hasattr(self.Meta, 'readonly_fields'), (
                'Serializer `%s.%s` has field `readonly_fields`; '
                'the correct spelling for the option is `read_only_fields`.' %
                (self.__class__.__module__, self.__class__.__name__)
            )

        return extra_kwargs

    def get_field_names(self, declared_fields, info):
        """
        Returns the list of all field names that should be created when
        instantiating this serializer class. This is based on the default
        set of fields, but also takes into account the `Meta.fields` or
        `Meta.exclude` options if they have been specified.
        """
        fields = getattr(self.Meta, 'fields', None)
        exclude = getattr(self.Meta, 'exclude', None)

        if fields and fields != ALL_FIELDS and not isinstance(fields, (list, tuple)):
            raise TypeError(
                'The `fields` option must be a list or tuple or "__all__". '
                'Got %s.' % type(fields).__name__
            )

        if exclude and not isinstance(exclude, (list, tuple)):
            raise TypeError(
                'The `exclude` option must be a list or tuple. Got %s.' %
                type(exclude).__name__
            )

        assert not (fields and exclude), (
            "Cannot set both 'fields' and 'exclude' options on "
            "serializer {serializer_class}.".format(
                serializer_class=self.__class__.__name__
            )
        )

        assert not (fields is None and exclude is None), (
            "Creating a ModelSerializer without either the 'fields' attribute "
            "or the 'exclude' attribute has been deprecated since 3.3.0, "
            "and is now disallowed. Add an explicit fields = '__all__' to the "
            "{serializer_class} serializer.".format(
                serializer_class=self.__class__.__name__
            ),
        )

        if fields == ALL_FIELDS:
            fields = None

        if fields is not None:
            # Ensure that all declared fields have also been included in the
            # `Meta.fields` option.

            # Do not require any fields that are declared in a parent class,
            # in order to allow serializer subclasses to only include
            # a subset of fields.
            required_field_names = set(declared_fields)
            for cls in self.__class__.__bases__:
                required_field_names -= set(getattr(cls, '_declared_fields', []))

            for field_name in required_field_names:
                assert field_name in fields, (
                    "The field '{field_name}' was declared on serializer "
                    "{serializer_class}, but has not been included in the "
                    "'fields' option.".format(
                        field_name=field_name,
                        serializer_class=self.__class__.__name__
                    )
                )
            return fields

        # Use the default set of field names if `Meta.fields` is not specified.
        fields = self.get_default_field_names(declared_fields, info)

        if exclude is not None:
            # If `Meta.exclude` is included, then remove those fields.
            for field_name in exclude:
                assert field_name not in self._declared_fields, (
                    "Cannot both declare the field '{field_name}' and include "
                    "it in the {serializer_class} 'exclude' option. Remove the "
                    "field or, if inherited from a parent serializer, disable "
                    "with `{field_name} = None`."
                    .format(
                        field_name=field_name,
                        serializer_class=self.__class__.__name__
                    )
                )

                assert field_name in fields, (
                    "The field '{field_name}' was included on serializer "
                    "{serializer_class} in the 'exclude' option, but does "
                    "not match any model field.".format(
                        field_name=field_name,
                        serializer_class=self.__class__.__name__
                    )
                )
                fields.remove(field_name)

        return fields

    def get_unique_for_date_validators(self):
        """
        Determine a default set of validators for the following constraints:

        * unique_for_date
        * unique_for_month
        * unique_for_year
        """
        info = model_meta.get_field_info(self.Meta.model)
        default_manager = self.Meta.model._default_manager
        field_names = [field.source for field in self.fields.values()]

        validators = []

        for field_name, field in info.fields_and_pk.items():
            if field.unique_for_date and field_name in field_names:
                validator = UniqueForDateValidator(
                    queryset=default_manager,
                    field=field_name,
                    date_field=field.unique_for_date
                )
                validators.append(validator)

            if field.unique_for_month and field_name in field_names:
                validator = UniqueForMonthValidator(
                    queryset=default_manager,
                    field=field_name,
                    date_field=field.unique_for_month
                )
                validators.append(validator)

            if field.unique_for_year and field_name in field_names:
                validator = UniqueForYearValidator(
                    queryset=default_manager,
                    field=field_name,
                    date_field=field.unique_for_year
                )
                validators.append(validator)

        return validators

    def get_unique_together_validators(self):
        """
        Determine a default set of validators for any unique_together constraints.
        """
        model_class_inheritance_tree = (
            [self.Meta.model] +
            list(self.Meta.model._meta.parents)
        )

        # The field names we're passing though here only include fields
        # which may map onto a model field. Any dotted field name lookups
        # cannot map to a field, and must be a traversal, so we're not
        # including those.
        field_names = {
            field.source for field in self._writable_fields
            if (field.source != '*') and ('.' not in field.source)
        }

        # Special Case: Add read_only fields with defaults.
        field_names |= {
            field.source for field in self.fields.values()
            if (field.read_only) and (field.default != empty) and (field.source != '*') and ('.' not in field.source)
        }

        # Note that we make sure to check `unique_together` both on the
        # base model class, but also on any parent classes.
        validators = []
        for parent_class in model_class_inheritance_tree:
            for unique_together in parent_class._meta.unique_together:
                if field_names.issuperset(set(unique_together)):
                    validator = UniqueTogetherValidator(
                        queryset=parent_class._default_manager,
                        fields=unique_together
                    )
                    validators.append(validator)
        return validators

    def get_uniqueness_extra_kwargs(self, field_names, declared_fields, extra_kwargs):
        """
        Return any additional field options that need to be included as a
        result of uniqueness constraints on the model. This is returned as
        a two-tuple of:

        ('dict of updated extra kwargs', 'mapping of hidden fields')
        """
        if getattr(self.Meta, 'validators', None) is not None:
            return (extra_kwargs, {})

        model = getattr(self.Meta, 'model')
        model_fields = self._get_model_fields(
            field_names, declared_fields, extra_kwargs
        )

        # Determine if we need any additional `HiddenField` or extra keyword
        # arguments to deal with `unique_for` dates that are required to
        # be in the input data in order to validate it.
        unique_constraint_names = set()

        for model_field in model_fields.values():
            # Include each of the `unique_for_*` field names.
            unique_constraint_names |= {model_field.unique_for_date, model_field.unique_for_month,
                                        model_field.unique_for_year}

        unique_constraint_names -= {None}

        # Include each of the `unique_together` field names,
        # so long as all the field names are included on the serializer.
        for parent_class in [model] + list(model._meta.parents):
            for unique_together_list in parent_class._meta.unique_together:
                if set(field_names).issuperset(set(unique_together_list)):
                    unique_constraint_names |= set(unique_together_list)

        # Now we have all the field names that have uniqueness constraints
        # applied, we can add the extra 'required=...' or 'default=...'
        # arguments that are appropriate to these fields, or add a `HiddenField` for it.
        hidden_fields = {}
        uniqueness_extra_kwargs = {}

        for unique_constraint_name in unique_constraint_names:
            # Get the model field that is referred too.
            unique_constraint_field = model._meta.get_field(unique_constraint_name)

            if getattr(unique_constraint_field, 'auto_now_add', None):
                default = CreateOnlyDefault(timezone.now)
            elif getattr(unique_constraint_field, 'auto_now', None):
                default = timezone.now
            elif unique_constraint_field.has_default():
                default = unique_constraint_field.default
            else:
                default = empty

            if unique_constraint_name in model_fields:
                # The corresponding field is present in the serializer
                if default is empty:
                    uniqueness_extra_kwargs[unique_constraint_name] = {'required': True}
                else:
                    uniqueness_extra_kwargs[unique_constraint_name] = {'default': default}
            elif default is not empty:
                # The corresponding field is not present in the
                # serializer. We have a default to use for it, so
                # add in a hidden field that populates it.
                hidden_fields[unique_constraint_name] = HiddenField(default=default)

        # Update `extra_kwargs` with any new options.
        for key, value in uniqueness_extra_kwargs.items():
            if key in extra_kwargs:
                value.update(extra_kwargs[key])
            extra_kwargs[key] = value

        return extra_kwargs, hidden_fields

    def include_extra_kwargs(self, kwargs, extra_kwargs):
        """
        Include any 'extra_kwargs' that have been included for this field,
        possibly removing any incompatible existing keyword arguments.
        """
        if extra_kwargs.get('read_only', False):
            for attr in [
                'required', 'default', 'allow_blank', 'allow_null',
                'min_length', 'max_length', 'min_value', 'max_value',
                'validators', 'queryset'
            ]:
                kwargs.pop(attr, None)

        if extra_kwargs.get('default') and kwargs.get('required') is False:
            kwargs.pop('required')

        if extra_kwargs.get('read_only', kwargs.get('read_only', False)):
            extra_kwargs.pop('required', None)  # Read only fields should always omit the 'required' argument.

        kwargs.update(extra_kwargs)

        return kwargs

    def is_valid(self, raise_exception=False):
        assert not hasattr(self, 'restore_object'), (
            'Serializer `%s.%s` has old-style version 2 `.restore_object()` '
            'that is no longer compatible with REST framework 3. '
            'Use the new-style `.create()` and `.update()` methods instead.' %
            (self.__class__.__module__, self.__class__.__name__)
        )

        assert hasattr(self, 'initial_data'), (
            'Cannot call `.is_valid()` as no `data=` keyword argument was '
            'passed when instantiating the serializer instance.'
        )

        if not hasattr(self, '_validated_data'):
            try:
                self._validated_data = self.run_validation(self.initial_data)
            except ValidationError as exc:
                self._validated_data = {}
                self._errors = exc.detail
            else:
                self._errors = {}

        if self._errors and raise_exception:
            raise ValidationError(self.errors)

        return not bool(self._errors)

    @classmethod
    def many_init(cls, *args, **kwargs):
        """
        This method implements the creation of a `ListSerializer` parent
        class when `many=True` is used. You can customize it if you need to
        control which keyword arguments are passed to the parent, and
        which are passed to the child.

        Note that we're over-cautious in passing most arguments to both parent
        and child classes in order to try to cover the general case. If you're
        overriding this method you'll probably want something much simpler, eg:

        @classmethod
        def many_init(cls, *args, **kwargs):
            kwargs['child'] = cls()
            return CustomListSerializer(*args, **kwargs)
        """
        allow_empty = kwargs.pop('allow_empty', None)
        child_serializer = cls(*args, **kwargs)
        list_kwargs = {
            'child': child_serializer,
        }
        if allow_empty is not None:
            list_kwargs['allow_empty'] = allow_empty
        list_kwargs.update({
            key: value for key, value in kwargs.items()
            if key in LIST_SERIALIZER_KWARGS
        })
        meta = getattr(cls, 'Meta', None)
        list_serializer_class = getattr(meta, 'list_serializer_class', ListSerializer)
        return list_serializer_class(*args, **list_kwargs)

    def save(self, **kwargs):
        assert not hasattr(self, 'save_object'), (
            'Serializer `%s.%s` has old-style version 2 `.save_object()` '
            'that is no longer compatible with REST framework 3. '
            'Use the new-style `.create()` and `.update()` methods instead.' %
            (self.__class__.__module__, self.__class__.__name__)
        )

        assert hasattr(self, '_errors'), (
            'You must call `.is_valid()` before calling `.save()`.'
        )

        assert not self.errors, (
            'You cannot call `.save()` on a serializer with invalid data.'
        )

        # Guard against incorrect use of `serializer.save(commit=False)`
        assert 'commit' not in kwargs, (
            "'commit' is not a valid keyword argument to the 'save()' method. "
            "If you need to access data before committing to the database then "
            "inspect 'serializer.validated_data' instead. "
            "You can also pass additional keyword arguments to 'save()' if you "
            "need to set extra attributes on the saved model instance. "
            "For example: 'serializer.save(owner=request.user)'.'"
        )

        assert not hasattr(self, '_data'), (
            "You cannot call `.save()` after accessing `serializer.data`."
            "If you need to access data before committing to the database then "
            "inspect 'serializer.validated_data' instead. "
        )

        validated_data = dict(
            list(self.validated_data.items()) +
            list(kwargs.items())
        )

        if self.instance is not None:
            self.instance = self.update(self.instance, validated_data)
            assert self.instance is not None, (
                '`update()` did not return an object instance.'
            )
        else:
            self.instance = self.create(validated_data)
            assert self.instance is not None, (
                '`create()` did not return an object instance.'
            )

        return self.instance

    def validate(self, attrs):
        return attrs

    def validate_empty_values(self, data):
        """
        Validate empty values, and either:

        * Raise `ValidationError`, indicating invalid data.
        * Raise `SkipField`, indicating that the field should be ignored.
        * Return (True, data), indicating an empty value that should be
          returned without any further validation being applied.
        * Return (False, data), indicating a non-empty value, that should
          have validation applied as normal.
        """
        if self.read_only:
            return (True, self.get_default())

        if data is empty:
            if getattr(self.root, 'partial', False):
                raise SkipField()
            if self.required:
                self.fail('required')
            return (True, self.get_default())

        if data is None:
            if not self.allow_null:
                self.fail('null')
            return (True, None)

        return (False, data)