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