if(!_.isString(attrDef.type) || attrDef.type === '') {
      throw new Error('Consistency violation: The referenced attribute (`'+attrName+'`, from model `'+modelIdentity+'`) has an invalid `type` property.  If specified, `type` should be a non-empty string.  But instead, got: '+util.inspect(attrDef.type, {depth:5})+'');
    }