"""
Serializers and ModelSerializers are similar to Forms and ModelForms.
Unlike forms, they are not constrained to dealing with HTML output, and
form encoded input.

Serialization in REST framework is a two-phase process:

1. Serializers marshal between complex types like model instances, and
python primitives.
2. The process of marshalling between python primitives and request and
response content is handled by parsers and renderers.
"""
from django.core.exceptions import ImproperlyConfigured
from django.core.exceptions import ValidationError as DjangoValidationError
from django.db import models
from django.db.models.fields import FieldDoesNotExist
from django.utils import six
from django.utils.translation import ugettext_lazy as _
from rest_framework.compat import OrderedDict
from rest_framework.exceptions import ValidationError
from rest_framework.fields import empty, set_value, Field, SkipField
from rest_framework.settings import api_settings
from rest_framework.utils import html, model_meta, representation
from rest_framework.utils.field_mapping import (
    get_url_kwargs, get_field_kwargs,
    get_relation_kwargs, get_nested_relation_kwargs,
    ClassLookupDict
)
from rest_framework.utils.serializer_helpers import (
    ReturnDict, ReturnList, BoundField, NestedBoundField, BindingDict
)
from rest_framework.validators import (
    UniqueForDateValidator, UniqueForMonthValidator, UniqueForYearValidator,
    UniqueTogetherValidator
)
import copy
import inspect
import warnings

# Note: We do the following so that users of the framework can use this style:
#
#     example_field = serializers.CharField(...)
#
# This helps keep the separation between model fields, form fields, and
# serializer fields more explicit.

from rest_framework.relations import *  # NOQA
from rest_framework.fields import *  # NOQA


# We assume that 'validators' are intended for the child serializer,
# rather than the parent serializer.
LIST_SERIALIZER_KWARGS = (
    'read_only', 'write_only', 'required', 'default', 'initial', 'source',
    'label', 'help_text', 'style', 'error_messages',
    'instance', 'data', 'partial', 'context'
)


# BaseSerializer
# --------------

class BaseSerializer(Field):
    """
    The BaseSerializer class provides a minimal class which may be used
    for writing custom serializer implementations.
    """
    def __init__(self, instance=None, data=None, **kwargs):
        self.instance = instance
        self._initial_data = data
        self.partial = kwargs.pop('partial', False)
        self._context = kwargs.pop('context', {})
        kwargs.pop('many', None)
        super(BaseSerializer, self).__init__(**kwargs)

    def __new__(cls, *args, **kwargs):
        # We override this method in order to automagically create
        # `ListSerializer` classes instead when `many=True` is set.
        if kwargs.pop('many', False):
            return cls.many_init(*args, **kwargs)
        return super(BaseSerializer, cls).__new__(cls, *args, **kwargs)

    @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)
        """
        child_serializer = cls(*args, **kwargs)
        list_kwargs = {'child': child_serializer}
        list_kwargs.update(dict([
            (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 to_internal_value(self, data):
        raise NotImplementedError('`to_internal_value()` must be implemented.')

    def to_representation(self, instance):
        raise NotImplementedError('`to_representation()` must be implemented.')

    def update(self, instance, validated_data):
        raise NotImplementedError('`update()` must be implemented.')

    def create(self, validated_data):
        raise NotImplementedError('`create()` must be implemented.')

    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.'
        )

        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 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__)
        )

        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)

    @property
    def data(self):
        if not hasattr(self, '_data'):
            if self.instance is not None and not getattr(self, '_errors', None):
                self._data = self.to_representation(self.instance)
            elif hasattr(self, '_validated_data') and not getattr(self, '_errors', None):
                self._data = self.to_representation(self.validated_data)
            else:
                self._data = self.get_initial()
        return self._data

    @property
    def errors(self):
        if not hasattr(self, '_errors'):
            msg = 'You must call `.is_valid()` before accessing `.errors`.'
            raise AssertionError(msg)
        return self._errors

    @property
    def validated_data(self):
        if not hasattr(self, '_validated_data'):
            msg = 'You must call `.is_valid()` before accessing `.validated_data`.'
            raise AssertionError(msg)
        return self._validated_data


# Serializer & ListSerializer classes
# -----------------------------------

class SerializerMetaclass(type):
    """
    This metaclass sets a dictionary named `base_fields` on the class.

    Any instances of `Field` included as attributes on either the class
    or on any of its superclasses will be include in the
    `base_fields` dictionary.
    """

    @classmethod
    def _get_declared_fields(cls, bases, attrs):
        fields = [(field_name, attrs.pop(field_name))
                  for field_name, obj in list(attrs.items())
                  if isinstance(obj, Field)]
        fields.sort(key=lambda x: x[1]._creation_counter)

        # If this class is subclassing another Serializer, add that Serializer's
        # fields.  Note that we loop over the bases in *reverse*. This is necessary
        # in order to maintain the correct order of fields.
        for base in bases[::-1]:
            if hasattr(base, '_declared_fields'):
                fields = list(base._declared_fields.items()) + fields

        return OrderedDict(fields)

    def __new__(cls, name, bases, attrs):
        attrs['_declared_fields'] = cls._get_declared_fields(bases, attrs)
        return super(SerializerMetaclass, cls).__new__(cls, name, bases, attrs)


@six.add_metaclass(SerializerMetaclass)
class Serializer(BaseSerializer):
    default_error_messages = {
        'invalid': _('Invalid data. Expected a dictionary, but got {datatype}.')
    }

    @property
    def fields(self):
        """
        A dictionary of {field_name: field_instance}.
        """
        # `fields` is evalutated lazily. We do this to ensure that we don't
        # have issues importing modules that use ModelSerializers as fields,
        # even if Django's app-loading stage has not yet run.
        if not hasattr(self, '_fields'):
            self._fields = BindingDict(self)
            for key, value in self.get_fields().items():
                self._fields[key] = value
        return self._fields

    def get_fields(self):
        """
        Returns a dictionary of {field_name: field_instance}.
        """
        # Every new serializer is created with a clone of the field instances.
        # This allows users to dynamically modify the fields on a serializer
        # instance without affecting every other serializer class.
        return copy.deepcopy(self._declared_fields)

    def get_validators(self):
        """
        Returns a list of validator callables.
        """
        # Used by the lazily-evaluated `validators` property.
        return getattr(getattr(self, 'Meta', None), 'validators', [])

    def get_initial(self):
        if self._initial_data is not None:
            return OrderedDict([
                (field_name, field.get_value(self._initial_data))
                for field_name, field in self.fields.items()
                if field.get_value(self._initial_data) is not empty
                and not field.read_only
            ])

        return OrderedDict([
            (field.field_name, field.get_initial())
            for field in self.fields.values()
            if not field.read_only
        ])

    def get_value(self, dictionary):
        # We override the default field access in order to support
        # nested HTML forms.
        if html.is_html_input(dictionary):
            return html.parse_html_dict(dictionary, prefix=self.field_name)
        return dictionary.get(self.field_name, empty)

    def run_validation(self, data=empty):
        """
        We override the default `run_validation`, because the validation
        performed by validators and the `.validate()` method should
        be coerced into an error dictionary with a 'non_fields_error' key.
        """
        if data is empty:
            if getattr(self.root, 'partial', False):
                raise SkipField()
            if self.required:
                self.fail('required')
            return self.get_default()

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

        if not isinstance(data, dict):
            message = self.error_messages['invalid'].format(
                datatype=type(data).__name__
            )
            raise ValidationError({
                api_settings.NON_FIELD_ERRORS_KEY: [message]
            })

        value = self.to_internal_value(data)
        try:
            self.run_validators(value)
            value = self.validate(value)
            assert value is not None, '.validate() should return the validated data'
        except ValidationError as exc:
            if isinstance(exc.detail, dict):
                # .validate() errors may be a dict, in which case, use
                # standard {key: list of values} style.
                raise ValidationError(dict([
                    (key, value if isinstance(value, list) else [value])
                    for key, value in exc.detail.items()
                ]))
            elif isinstance(exc.detail, list):
                raise ValidationError({
                    api_settings.NON_FIELD_ERRORS_KEY: exc.detail
                })
            else:
                raise ValidationError({
                    api_settings.NON_FIELD_ERRORS_KEY: [exc.detail]
                })
        except DjangoValidationError as exc:
            # Normally you should raise `serializers.ValidationError`
            # inside your codebase, but we handle Django's validation
            # exception class as well for simpler compat.
            # Eg. Calling Model.clean() explictily inside Serializer.validate()
            raise ValidationError({
                api_settings.NON_FIELD_ERRORS_KEY: list(exc.messages)
            })

        return value

    def to_internal_value(self, data):
        """
        Dict of native values <- Dict of primitive datatypes.
        """
        ret = OrderedDict()
        errors = OrderedDict()
        fields = [
            field for field in self.fields.values()
            if (not field.read_only) or (field.default is not empty)
        ]

        for field in fields:
            validate_method = getattr(self, 'validate_' + field.field_name, None)
            primitive_value = field.get_value(data)
            try:
                validated_value = field.run_validation(primitive_value)
                if validate_method is not None:
                    validated_value = validate_method(validated_value)
            except ValidationError as exc:
                errors[field.field_name] = exc.detail
            except DjangoValidationError as exc:
                errors[field.field_name] = list(exc.messages)
            except SkipField:
                pass
            else:
                set_value(ret, field.source_attrs, validated_value)

        if errors:
            raise ValidationError(errors)

        return ret

    def to_representation(self, instance):
        """
        Object instance -> Dict of primitive datatypes.
        """
        ret = OrderedDict()
        fields = [field for field in self.fields.values() if not field.write_only]

        for field in fields:
            attribute = field.get_attribute(instance)
            if attribute is None:
                ret[field.field_name] = None
            else:
                ret[field.field_name] = field.to_representation(attribute)

        return ret

    def validate(self, attrs):
        return attrs

    def __repr__(self):
        return representation.serializer_repr(self, indent=1)

    # The following are used for accessing `BoundField` instances on the
    # serializer, for the purposes of presenting a form-like API onto the
    # field values and field errors.

    def __iter__(self):
        for field in self.fields.values():
            yield self[field.field_name]

    def __getitem__(self, key):
        field = self.fields[key]
        value = self.data.get(key)
        error = self.errors.get(key) if hasattr(self, '_errors') else None
        if isinstance(field, Serializer):
            return NestedBoundField(field, value, error)
        return BoundField(field, value, error)

    # Include a backlink to the serializer class on return objects.
    # Allows renderers such as HTMLFormRenderer to get the full field info.

    @property
    def data(self):
        ret = super(Serializer, self).data
        return ReturnDict(ret, serializer=self)

    @property
    def errors(self):
        ret = super(Serializer, self).errors
        return ReturnDict(ret, serializer=self)


# There's some replication of `ListField` here,
# but that's probably better than obfuscating the call hierarchy.

class ListSerializer(BaseSerializer):
    child = None
    many = True

    default_error_messages = {
        'not_a_list': _('Expected a list of items but got type `{input_type}`.')
    }

    def __init__(self, *args, **kwargs):
        self.child = kwargs.pop('child', copy.deepcopy(self.child))
        assert self.child is not None, '`child` is a required argument.'
        assert not inspect.isclass(self.child), '`child` has not been instantiated.'
        super(ListSerializer, self).__init__(*args, **kwargs)
        self.child.bind(field_name='', parent=self)

    def get_initial(self):
        if self._initial_data is not None:
            return self.to_representation(self._initial_data)
        return []

    def get_value(self, dictionary):
        """
        Given the input dictionary, return the field value.
        """
        # We override the default field access in order to support
        # lists in HTML forms.
        if html.is_html_input(dictionary):
            return html.parse_html_list(dictionary, prefix=self.field_name)
        return dictionary.get(self.field_name, empty)

    def to_internal_value(self, data):
        """
        List of dicts of native values <- List of dicts of primitive datatypes.
        """
        if html.is_html_input(data):
            data = html.parse_html_list(data)

        if not isinstance(data, list):
            message = self.error_messages['not_a_list'].format(
                input_type=type(data).__name__
            )
            raise ValidationError({
                api_settings.NON_FIELD_ERRORS_KEY: [message]
            })

        ret = []
        errors = []

        for item in data:
            try:
                validated = self.child.run_validation(item)
            except ValidationError as exc:
                errors.append(exc.detail)
            else:
                ret.append(validated)
                errors.append({})

        if any(errors):
            raise ValidationError(errors)

        return ret

    def to_representation(self, data):
        """
        List of object instances -> List of dicts of primitive datatypes.
        """
        iterable = data.all() if (hasattr(data, 'all')) else data
        return [
            self.child.to_representation(item) for item in iterable
        ]

    def update(self, instance, validated_data):
        raise NotImplementedError(
            "Serializers with many=True do not support multiple update by "
            "default, only multiple create. For updates it is unclear how to "
            "deal with insertions and deletions. If you need to support "
            "multiple update, use a `ListSerializer` class and override "
            "`.update()` so you can specify the behavior exactly."
        )

    def create(self, validated_data):
        return [
            self.child.create(attrs) for attrs in validated_data
        ]

    def save(self, **kwargs):
        """
        Save and return a list of object instances.
        """
        validated_data = [
            dict(list(attrs.items()) + list(kwargs.items()))
            for attrs in self.validated_data
        ]

        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 __repr__(self):
        return representation.list_repr(self, indent=1)

    # Include a backlink to the serializer class on return objects.
    # Allows renderers such as HTMLFormRenderer to get the full field info.

    @property
    def data(self):
        ret = super(ListSerializer, self).data
        return ReturnList(ret, serializer=self)

    @property
    def errors(self):
        ret = super(ListSerializer, self).errors
        if isinstance(ret, dict):
            return ReturnDict(ret, serializer=self)
        return ReturnList(ret, serializer=self)


# ModelSerializer & HyperlinkedModelSerializer
# --------------------------------------------

class ModelSerializer(Serializer):
    """
    A `ModelSerializer` is just a regular `Serializer`, except that:

    * A set of default fields are automatically populated.
    * A set of default validators are automatically populated.
    * Default `.create()` and `.update()` implementations are provided.

    The process of automatically determining a set of serializer fields
    based on the model fields is reasonably complex, but you almost certainly
    don't need to dig into the implemention.

    If the `ModelSerializer` class *doesn't* generate the set of fields that
    you need you should either declare the extra/differing fields explicitly on
    the serializer class, or simply use a `Serializer` class.
    """
    _field_mapping = ClassLookupDict({
        models.AutoField: IntegerField,
        models.BigIntegerField: IntegerField,
        models.BooleanField: BooleanField,
        models.CharField: CharField,
        models.CommaSeparatedIntegerField: CharField,
        models.DateField: DateField,
        models.DateTimeField: DateTimeField,
        models.DecimalField: DecimalField,
        models.EmailField: EmailField,
        models.Field: ModelField,
        models.FileField: FileField,
        models.FloatField: FloatField,
        models.ImageField: ImageField,
        models.IntegerField: IntegerField,
        models.NullBooleanField: NullBooleanField,
        models.PositiveIntegerField: IntegerField,
        models.PositiveSmallIntegerField: IntegerField,
        models.SlugField: SlugField,
        models.SmallIntegerField: IntegerField,
        models.TextField: CharField,
        models.TimeField: TimeField,
        models.URLField: URLField,
    })
    _related_class = PrimaryKeyRelatedField

    def create(self, validated_data):
        """
        We have a bit of extra checking around this in order to provide
        descriptive messages when something goes wrong, but this method is
        essentially just:

            return ExampleModel.objects.create(**validated_data)

        If there are many to many fields present on the instance then they
        cannot be set until the model is instantiated, in which case the
        implementation is like so:

            example_relationship = validated_data.pop('example_relationship')
            instance = ExampleModel.objects.create(**validated_data)
            instance.example_relationship = example_relationship
            return instance

        The default implementation also does not handle nested relationships.
        If you want to support writable nested relationships you'll need
        to write an explicit `.create()` method.
        """
        # Check that the user isn't trying to handle a writable nested field.
        # If we don't do this explicitly they'd likely get a confusing
        # error at the point of calling `Model.objects.create()`.
        assert not any(
            isinstance(field, BaseSerializer) and (key in validated_attrs)
            for key, field in self.fields.items()
        ), (
            'The `.create()` method does not suport nested writable fields '
            'by default. Write an explicit `.create()` method for serializer '
            '`%s.%s`, or set `read_only=True` on nested serializer fields.' %
            (self.__class__.__module__, self.__class__.__name__)
        )

        ModelClass = self.Meta.model

        # Remove many-to-many relationships from validated_data.
        # They are not valid arguments to the default `.create()` method,
        # as they require that the instance has already been saved.
        info = model_meta.get_field_info(ModelClass)
        many_to_many = {}
        for field_name, relation_info in info.relations.items():
            if relation_info.to_many and (field_name in validated_data):
                many_to_many[field_name] = validated_data.pop(field_name)

        try:
            instance = ModelClass.objects.create(**validated_data)
        except TypeError as exc:
            msg = (
                'Got a `TypeError` when calling `%s.objects.create()`. '
                'This may be because you have a writable field on the '
                'serializer class that is not a valid argument to '
                '`%s.objects.create()`. You may need to make the field '
                'read-only, or override the %s.create() method to handle '
                'this correctly.\nOriginal exception text was: %s.' %
                (
                    ModelClass.__name__,
                    ModelClass.__name__,
                    self.__class__.__name__,
                    exc
                )
            )
            raise TypeError(msg)

        # Save many-to-many relationships after the instance is created.
        if many_to_many:
            for field_name, value in many_to_many.items():
                setattr(instance, field_name, value)

        return instance

    def update(self, instance, validated_data):
        assert not any(
            isinstance(field, BaseSerializer) and (key in validated_attrs)
            for key, field in self.fields.items()
        ), (
            'The `.update()` method does not suport nested writable fields '
            'by default. Write an explicit `.update()` method for serializer '
            '`%s.%s`, or set `read_only=True` on nested serializer fields.' %
            (self.__class__.__module__, self.__class__.__name__)
        )

        for attr, value in validated_data.items():
            setattr(instance, attr, value)
        instance.save()
        return instance

    def get_validators(self):
        # If the validators have been declared explicitly then use that.
        validators = getattr(getattr(self, 'Meta', None), 'validators', None)
        if validators is not None:
            return validators

        # Determine the default set of validators.
        validators = []
        model_class = self.Meta.model
        field_names = set([
            field.source for field in self.fields.values()
            if (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.
        for parent_class in [model_class] + list(model_class._meta.parents.keys()):
            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)

        # Add any unique_for_date/unique_for_month/unique_for_year constraints.
        info = model_meta.get_field_info(model_class)
        for field_name, field in info.fields_and_pk.items():
            if field.unique_for_date and field_name in field_names:
                validator = UniqueForDateValidator(
                    queryset=model_class._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=model_class._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=model_class._default_manager,
                    field=field_name,
                    date_field=field.unique_for_year
                )
                validators.append(validator)

        return validators

    def get_fields(self):
        declared_fields = copy.deepcopy(self._declared_fields)

        ret = OrderedDict()
        model = getattr(self.Meta, 'model')
        fields = getattr(self.Meta, 'fields', None)
        exclude = getattr(self.Meta, 'exclude', None)
        depth = getattr(self.Meta, 'depth', 0)
        extra_kwargs = getattr(self.Meta, 'extra_kwargs', {})

        assert not (fields and exclude), "Cannot set both 'fields' and 'exclude'."

        extra_kwargs = self._include_additional_options(extra_kwargs)

        # Retrieve metadata about fields & relationships on the model class.
        info = model_meta.get_field_info(model)

        # Use the default set of field names if none is supplied explicitly.
        if fields is None:
            fields = self._get_default_field_names(declared_fields, info)
            exclude = getattr(self.Meta, 'exclude', None)
            if exclude is not None:
                for field_name in exclude:
                    fields.remove(field_name)

        # Determine the set of model fields, and the fields that they map to.
        # We actually only need this to deal with the slightly awkward case
        # of supporting `unique_for_date`/`unique_for_month`/`unique_for_year`.
        model_field_mapping = {}
        for field_name in fields:
            if field_name in declared_fields:
                field = declared_fields[field_name]
                source = field.source or field_name
            else:
                try:
                    source = extra_kwargs[field_name]['source']
                except KeyError:
                    source = field_name
            # Model fields will always have a simple source mapping,
            # they can't be nested attribute lookups.
            if '.' not in source and source != '*':
                model_field_mapping[source] = field_name

        # 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.
        hidden_fields = {}
        unique_constraint_names = set()

        for model_field_name, field_name in model_field_mapping.items():
            try:
                model_field = model._meta.get_field(model_field_name)
            except FieldDoesNotExist:
                continue

            # Include each of the `unique_for_*` field names.
            unique_constraint_names |= set([
                model_field.unique_for_date,
                model_field.unique_for_month,
                model_field.unique_for_year
            ])

        unique_constraint_names -= set([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.keys()):
            for unique_together_list in parent_class._meta.unique_together:
                if set(fields).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.
        for unique_constraint_name in unique_constraint_names:
            # Get the model field that is refered 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_field_mapping:
                # The corresponding field is present in the serializer
                if unique_constraint_name not in extra_kwargs:
                    extra_kwargs[unique_constraint_name] = {}
                if default is empty:
                    if 'required' not in extra_kwargs[unique_constraint_name]:
                        extra_kwargs[unique_constraint_name]['required'] = True
                else:
                    if 'default' not in extra_kwargs[unique_constraint_name]:
                        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)

        # Now determine the fields that should be included on the serializer.
        for field_name in fields:
            if field_name in declared_fields:
                # Field is explicitly declared on the class, use that.
                ret[field_name] = declared_fields[field_name]
                continue

            elif field_name in info.fields_and_pk:
                # Create regular model fields.
                model_field = info.fields_and_pk[field_name]
                field_cls = self._field_mapping[model_field]
                kwargs = get_field_kwargs(field_name, model_field)
                if 'choices' in kwargs:
                    # Fields with choices get coerced into `ChoiceField`
                    # instead of using their regular typed field.
                    field_cls = ChoiceField
                if not issubclass(field_cls, 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.
                    kwargs.pop('model_field', None)
                if not issubclass(field_cls, CharField):
                    # `allow_blank` is only valid for textual fields.
                    kwargs.pop('allow_blank', None)

            elif field_name in info.relations:
                # Create forward and reverse relationships.
                relation_info = info.relations[field_name]
                if depth:
                    field_cls = self._get_nested_class(depth, relation_info)
                    kwargs = get_nested_relation_kwargs(relation_info)
                else:
                    field_cls = self._related_class
                    kwargs = get_relation_kwargs(field_name, relation_info)
                    # `view_name` is only valid for hyperlinked relationships.
                    if not issubclass(field_cls, HyperlinkedRelatedField):
                        kwargs.pop('view_name', None)

            elif hasattr(model, field_name):
                # Create a read only field for model methods and properties.
                field_cls = ReadOnlyField
                kwargs = {}

            elif field_name == api_settings.URL_FIELD_NAME:
                # Create the URL field.
                field_cls = HyperlinkedIdentityField
                kwargs = get_url_kwargs(model)

            else:
                raise ImproperlyConfigured(
                    'Field name `%s` is not valid for model `%s`.' %
                    (field_name, model.__class__.__name__)
                )

            # Check that any fields declared on the class are
            # also explicity included in `Meta.fields`.
            missing_fields = set(declared_fields.keys()) - set(fields)
            if missing_fields:
                missing_field = list(missing_fields)[0]
                raise ImproperlyConfigured(
                    'Field `%s` has been declared on serializer `%s`, but '
                    'is missing from `Meta.fields`.' %
                    (missing_field, self.__class__.__name__)
                )

            # Populate any kwargs defined in `Meta.extra_kwargs`
            extras = extra_kwargs.get(field_name, {})
            if extras.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 extras.get('default') and kwargs.get('required') is False:
                kwargs.pop('required')

            kwargs.update(extras)

            # Create the serializer field.
            ret[field_name] = field_cls(**kwargs)

        for field_name, field in hidden_fields.items():
            ret[field_name] = field

        return ret

    def _include_additional_options(self, extra_kwargs):
        read_only_fields = getattr(self.Meta, 'read_only_fields', None)
        if read_only_fields is not None:
            for field_name in read_only_fields:
                kwargs = extra_kwargs.get(field_name, {})
                kwargs['read_only'] = True
                extra_kwargs[field_name] = kwargs

        # These are all pending deprecation.
        write_only_fields = getattr(self.Meta, 'write_only_fields', None)
        if write_only_fields is not None:
            warnings.warn(
                "The `Meta.write_only_fields` option is pending deprecation. "
                "Use `Meta.extra_kwargs={<field_name>: {'write_only': True}}` instead.",
                PendingDeprecationWarning,
                stacklevel=3
            )
            for field_name in write_only_fields:
                kwargs = extra_kwargs.get(field_name, {})
                kwargs['write_only'] = True
                extra_kwargs[field_name] = kwargs

        view_name = getattr(self.Meta, 'view_name', None)
        if view_name is not None:
            warnings.warn(
                "The `Meta.view_name` option is pending deprecation. "
                "Use `Meta.extra_kwargs={'url': {'view_name': ...}}` instead.",
                PendingDeprecationWarning,
                stacklevel=3
            )
            kwargs = extra_kwargs.get(api_settings.URL_FIELD_NAME, {})
            kwargs['view_name'] = view_name
            extra_kwargs[api_settings.URL_FIELD_NAME] = kwargs

        lookup_field = getattr(self.Meta, 'lookup_field', None)
        if lookup_field is not None:
            warnings.warn(
                "The `Meta.lookup_field` option is pending deprecation. "
                "Use `Meta.extra_kwargs={'url': {'lookup_field': ...}}` instead.",
                PendingDeprecationWarning,
                stacklevel=3
            )
            kwargs = extra_kwargs.get(api_settings.URL_FIELD_NAME, {})
            kwargs['lookup_field'] = lookup_field
            extra_kwargs[api_settings.URL_FIELD_NAME] = kwargs

        return extra_kwargs

    def _get_default_field_names(self, declared_fields, model_info):
        return (
            [model_info.pk.name] +
            list(declared_fields.keys()) +
            list(model_info.fields.keys()) +
            list(model_info.forward_relations.keys())
        )

    def _get_nested_class(self, nested_depth, relation_info):
        class NestedSerializer(ModelSerializer):
            class Meta:
                model = relation_info.related
                depth = nested_depth
        return NestedSerializer


class HyperlinkedModelSerializer(ModelSerializer):
    """
    A type of `ModelSerializer` that uses hyperlinked relationships instead
    of primary key relationships. Specifically:

    * A 'url' field is included instead of the 'id' field.
    * Relationships to other instances are hyperlinks, instead of primary keys.
    """
    _related_class = HyperlinkedRelatedField

    def _get_default_field_names(self, declared_fields, model_info):
        return (
            [api_settings.URL_FIELD_NAME] +
            list(declared_fields.keys()) +
            list(model_info.fields.keys()) +
            list(model_info.forward_relations.keys())
        )

    def _get_nested_class(self, nested_depth, relation_info):
        class NestedSerializer(HyperlinkedModelSerializer):
            class Meta:
                model = relation_info.related
                depth = nested_depth
        return NestedSerializer