class ModelMetaclass (type):
    """
    ModelMetaclass makes changes to Model sub-classes to make them more user friendly.
    """

    def __init__ (cls, name, bases, members):
        """
        Create a new ModelMetaclass instance.

        @param cls:     The metaclass instance.
        @param name:    The class name.
        @param bases:   The class base class and interfaces.
        @param members: The class members.

        @raise ModelException: If the class cannot be built for any reason.
        """

        if name == "Model" and ModelMetaclass.__module__ == cls.__module__:
            return type.__init__(cls, name, bases, members)

        if Model not in bases:
            raise ModelException("%s must sub-class Model in order to use Metaclass" % name)

        # meta details
        if not hasattr(cls, "Meta"):
            cls.Meta = new.classobj("Meta", (object,), {})

        if not hasattr(cls.Meta, "required") or type(cls.Meta.required) != bool:
            cls.Meta.required = True

        cls.Meta.default   = {}
        cls.Meta.fields    = {}
        cls.Meta.groups    = {}
        cls.Meta.read_only = []

        # iterate class instances
        for field_name in dir(cls):
            field_inst = getattr(cls, field_name)

            if not isinstance(field_inst, Field):
                continue

            field_inst.field = field_name
            field_inst.model = cls

            if field_inst.default is None:
                cls.Meta.default[field_name] = field_inst.default

            if field_inst.read_only:
                field_inst.required = False

                cls.Meta.read_only.append(field_name)

            if field_inst.required is None:
                field_inst.required = cls.Meta.required

            if field_inst.group and field_inst.group not in cls.Meta.groups:
                if not hasattr(cls, "group_" + field_inst.group):
                    raise ModelException("%s is missing group validator group_%s()" % (name, field_inst.group))

                cls.Meta.groups[field_inst.group] = None

            cls.Meta.fields[field_name] = field_inst

            delattr(cls, field_name)

        return type.__init__(cls, name, bases, members)