"""
PostHyperlinkedModelSerializer inherits from HyperlinkedModelSerializer
This serializer allows us to add fields that do not belong to the model.

The `process_postonly_fields` method should be overriden.
Important: This method is called before the `pre_save` method.

Example of usage:

    class SomeModelSerializer(PostHyperlinkedModelSerializer):
        field_only_for_post = serializers.CharField()

        class Meta:
            model = SomeModel
            fields = ('field1', 'field2', ..., 'field_only_for_post')
            postonly_fields = ('field_only_for_post',)

        def process_postonly_fields(self, obj, post_attrs):
            post_only_for_post = post_attrs['post_only_for_post']
            # Process field ...

"""

from rest_framework.serializers import (HyperlinkedModelSerializerOptions,
                                        HyperlinkedModelSerializer)


class PostHyperlinkedModelSerializerOptions(HyperlinkedModelSerializerOptions):
    """
    Options for PostHyperlinkedModelSerializer
    """

    def __init__(self, meta):
        super(PostHyperlinkedModelSerializerOptions, self).__init__(meta)
        self.postonly_fields = getattr(meta, 'postonly_fields', ())


class PostHyperlinkedModelSerializer(HyperlinkedModelSerializer):
    _options_class = PostHyperlinkedModelSerializerOptions

    def to_native(self, obj):
        """
        Serialize objects -> primitives.
        """
        ret = self._dict_class()
        ret.fields = {}

        for field_name, field in self.fields.items():
            # Ignore all postonly_fields fron serialization
            if field_name in self.opts.postonly_fields:
                continue
            field.initialize(parent=self, field_name=field_name)
            key = self.get_field_key(field_name)
            value = field.field_to_native(obj, field_name)
            ret[key] = value
            ret.fields[key] = field
        return ret

    def restore_object(self, attrs, instance=None):
        model_attrs, post_attrs = {}, {}
        for attr, value in attrs.iteritems():
            if attr in self.opts.postonly_fields:
                post_attrs[attr] = value
            else:
                model_attrs[attr] = value
        obj = super(PostHyperlinkedModelSerializer,
                    self).restore_object(model_attrs, instance)
        # Method to process ignored postonly_fields
        self.process_postonly_fields(obj, post_attrs)
        return obj

    def process_postonly_fields(self, obj, post_attrs):
        """
        Placeholder method for processing data sent in POST.
        """
        pass