********************************************************************** /* Private Methods
********************************************************************** 
Lookup the primary key for the given collection

@param  {String} connectionName
@param  {String} collectionName
@return {String}
@api private
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));
        }
    }