code: function _getPK(connectionName, collectionName) {
  
          var collectionDefinition;
  
          try {
              collectionDefinition = connections[connectionName].collections[collectionName].definition;
              var pk;
  
              pk = _.find(Object.keys(collectionDefinition), function _findPK(key) {
                  var attrDef = collectionDefinition[key];
                  if (attrDef && attrDef.primaryKey)
                      return key;
                  else
                      return false;
              });
  
              if (!pk)
                  pk = 'id';
              return pk;
          }
          catch (e) {
              throw new Error('Unable to determine primary key for collection `' + collectionName + '` because ' +
                      'an error was encountered acquiring the collection definition:\n' + require('util').inspect(e, false, null));
          }
      }
docs: ********************************************************************** /* Private Methods
  ********************************************************************** 
  Lookup the primary key for the given collection
  
  @param  {String} connectionName
  @param  {String} collectionName
  @return {String}
  @api private