Guarantee short yet unique slugs for any model

short_slugs_for_mongoose.js

// Extend any Mongoose model that has a unique index on a 'slug' field so that
// if a unique index error occurs on 'slug', a random digit is added to the slug
// and the save operation is retried until it works. This is concurrency-safe even 
// if you have lots of inserts going on from multiple machines etc.
//
// By Tom Boutell, tom@punkave.com
//
// NOTE: YOU MUST HAVE THIS PULL REQUEST... https://github.com/LearnBoost/mongoose/pull/837
//
// (As of this writing that commit is not in LearnBoost/mongoose yet.)
//
// Here's how to use it:

// Make your model the usual way
// var MediaItem = mongoose.model('MediaItem', mediaItemSchema);
// Call my function to install the extensible slug behavior
// modelExtendSlugOnUniqueIndexError(MediaItem);

// ALSO: note that it is also very important to make sure your indexes have really 
// been applied before you try to do things like inserts! Otherwise you won't get 
// your unique index error until it is too late.
 
// MediaItem.on('index', function()
// {
//   nowICanSafelyInsertThings();
// });

// Call this function with a model object to install the automatic slug extension behavior

function modelExtendSlugOnUniqueIndexError(model)
{
  // Stash the original 'save' method so we can call it
  model.prototype.saveAfterExtendSlugOnUniqueIndexError = model.prototype.save;
  // Replace 'save' with a wrapper
  model.prototype.save = function(f)
  {
    var self = this;
    // Our replacement callback
    var extendSlugOnUniqueIndexError = function(err, d)
    {
      if (err) 
      { 
        // Spots unique index errors relating to the slug field
        if ((err.code === 11000) && (err.err.indexOf('slug') !== -1))
        {
          self.slug += (Math.floor(Math.random() * 10)).toString();
          self.save(extendSlugOnUniqueIndexError);
          return;
        }
      }
      // Not our special case so call the original callback
      f(err, d);
    };
    // Call the original save method, with our wrapper callback
    self.saveAfterExtendSlugOnUniqueIndexError(extendSlugOnUniqueIndexError);
  }
}