Backbone.sync.parsers({
  MyDocumentsModel: {
    url: "/documents",
    parse: function(response){
      return response.documents;
    }
  },
  ...
});

// actually, this would probably be a better idea so we don't get such a high level of cohesion between names and strings...

Backbone.sync.parsers[MyDocumentsModel] = {
  url: "/documents",
  parse: function(response){ return response.documents; }
}

Backbone.sync.parsers[MyTags] = {
  url: "/documents",
  parse: function(response){ return _.map(response.documents, function(d){ return d.tags }); }
}