Very simple fixed value field for DRF.

constant_field.py

from rest_framework import serializers

class ConstantField(serializers.ReadOnlyField):
    def __init__(self, value, *args, **kwargs):
        self._constant = value
        super(ConstantField, self).__init__(*args, **kwargs)
    def get_attribute(self, obj):
        return self._constant
    def to_representation(self, value):
        return value