gistfile1.js

var AlbumSchema = new Schema({
  parent: {
    type: Schema.Types.ObjectId,
    ref: 'Album'
  },
  children: [{
    type: Schema.Types.ObjectId,
    ref: 'Album'
  }],
  createdAt: {
    type: Date,
  },
  updatedAt: {
    type: Date,
  }
});

AlbumSchema.pre('save', function (next) {
  var now = Date.now()
    , album = this
    , Album = mongoose.model('Album');

  // only set createdAt if it's a new object
  if (album.isNew) album.createdAt = now;
  // always set updatedAt each time we touch it
  album.updatedAt = now;

  // here comes the tricky bit
  async.series([
    function (callback) {
      // just grab every album 
      // and remove this one from their 'children' array
      Album.update({}, {
        $pull: {children: album}
      }, {multi: true}, callback);
    },
    function (callback) {
      // grab the album that we have set as this ones parent
      // and stick this album into *that* albums children
      Album.update({_id: album.parent}, {
        $push: {children: album}
      }, callback);
    }
  ], next);
});


AlbumSchema.pre('remove', function (next) {
  var album = this
    , Album = mongoose.model('Album');

  async.parallel([
    function (callback) {
      // grab the album that has this one as a child
      // and remove this album from its children
      Album.update({children: album}, {
        $pull: {children: album}
      }, callback);
    },
    function (callback) {
      // grab the album(s) that have this one as its parent
      // and remove it as parent
      Album.update({parent: album}, {
        parent: null
      }, {multi: true}, callback);
    }
  ], next);
});