Add non-model fields back to validated_data by overwriting the to_internal_value function. Then it will be available in update/create methods

serializers.py

def to_internal_value(self, data):
    internal_value = super(SearchPresetSerializer, self).to_internal_value(data)
    user = data.get("user")
    internal_value.update({
        "user": user
    })
    return internal_value