if (!_.isBoolean(pkAttrDef.required)) {
    throw new Error('Consistency violation: The `primaryKey` (`'+WLModel.primaryKey+'`) in the referenced Waterline model (`'+modelIdentity+'`) corresponds with a CORRUPTED attribute definition '+util.inspect(pkAttrDef, {depth:5})+'\n(^^this should have been caught already!  `required` must be either true or false!)');
  }