meteor/packages/minimongo/wrap_transform.js

// Wrap a transform function to return objects that have the _id field
// of the untransformed document. This ensures that subsystems such as
// the observe-sequence package that call `observe` can keep track of
// the documents identities.
//
// - Require that it returns objects
// - If the return value has an _id field, verify that it matches the
//   original _id field
// - If the return value doesn't have an _id field, add it back.
LocalCollection.wrapTransform = function (transform) {
  if (!transform)
    return null;

  return function (doc) {
    if (!_.has(doc, '_id')) {
      // XXX do we ever have a transform on the oplog's collection? because that
      // collection has no _id.
      throw new Error("can only transform documents with _id");
    }

    var id = doc._id;
    // XXX consider making tracker a weak dependency and checking Package.tracker here
    var transformed = Tracker.nonreactive(function () {
      return transform(doc);
    });

    if (!isPlainObject(transformed)) {
      throw new Error("transform must return object");
    }

    if (_.has(transformed, '_id')) {
      if (!EJSON.equals(transformed._id, id)) {
        throw new Error("transformed document can't have different _id");
      }
    } else {
      transformed._id = id;
    }
    return transformed;
  };
};