Barrels module
@param {string} sourceFolder defaults to <project root>/test/fixtures
function Barrels(sourceFolder) {
  if (!(this instanceof Barrels))
    return new Barrels(sourceFolder);

  // Fixture objects loaded from the JSON files
  this.data = {};

  // Map fixture positions in JSON files to the real DB IDs
  this.idMap = {};

  // The list of associations by model
  this.associations = {};

  // Load the fixtures
  sourceFolder = sourceFolder || process.cwd() + '/test/fixtures';
  var files = fs.readdirSync(sourceFolder);

  for (var i = 0; i < files.length; i++) {
    if (['.json', '.js'].indexOf(path.extname(files[i]).toLowerCase()) !== -1) {
      var modelName = path.basename(files[i]).split('.')[0].toLowerCase();
      this.data[modelName] = require(path.join(sourceFolder, files[i]));
    }
  }

  // The list of the fixtures model names
  this.modelNames = Object.keys(this.data);
}