code: function timestampsPlugin(schema, options) {
  
    // Add the fields to the schema
    schema.add({
      createdAt: {
        type: Date,
        'default': Date.now
      },
      updatedAt: {
        type: Date,
        'default': Date.now
      },
      deletedAt: {
        type: Date,
        sparse: true
      }
    });
  
    // Define the pre save hook
    schema.pre('save', function (next) {
      this.updatedAt = new Date();
      next();
    });
  
    // Create an index on all the paths
    schema.path('createdAt').index(true);
    schema.path('updatedAt').index(true);
    schema.path('deletedAt').index(true);
  }
docs: `Timestamps` Plugin for Mongoose.
  
  @param {mongoose.Schema} schema -
  @param {Object} options - some options
  @api public