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