select2_extended_ajax_adapter.js

$.fn.select2.amd.define('select2/data/extended-ajax',['./ajax', './tags', '../utils', 'module', 'jquery'], function(AjaxAdapter, Tags, Utils, module, $) {
  function ExtendedAjaxAdapter ($element,options) {
    //we need explicitly process minimumInputLength value 
    //to decide should we use AjaxAdapter or return defaultResults,
    //so it is impossible to use MinimumLength decorator here
    this.minimumInputLength = options.get('minimumInputLength');
    this.defaultResults     = options.get('defaultResults');

    ExtendedAjaxAdapter.__super__.constructor.call(this,$element,options);
  }

  Utils.Extend(ExtendedAjaxAdapter,AjaxAdapter);
  
  //override original query function to support default results
  var originQuery = AjaxAdapter.prototype.query;

  ExtendedAjaxAdapter.prototype.query = function (params, callback) {
    var defaultResults = (typeof this.defaultResults == 'function') ? this.defaultResults.call(this) : this.defaultResults;
    if (defaultResults && defaultResults.length && (!params.term || params.term.length < this.minimumInputLength)) {
      var processedResults = this.processResults(defaultResults,params.term);
      callback(processedResults);
    } else {
      originQuery.call(this, params, callback);
    }
  };

  if (module.config().tags) {
    return Utils.Decorate(ExtendedAjaxAdapter, Tags);
  } else {
    return ExtendedAjaxAdapter;
  }
});

example

<script src="/js/select2.full.js"></script>
<script src="/js/select2_extended_ajax_adapter.js"></script>
<select id="myselect"></select>
<script>
(function() {
  $.fn.select2.amd.require.config({
    config: {
      'select2/data/extended-ajax': {
        tags: true
      }
    }
  });

  $('#myselect').select2({
    dataAdapter: $.fn.select2.amd.require('select2/data/extended-ajax'),
    defaultResults: [
      {id: 1, text: 'default_options_1'},
      {id: 2, text: 'default_options_2'},
      {id: 3, text: 'default_options_3'}
    ],
    tags: true,
    ajax: {
      // your ajax options
    }
  });
})();
</script>