binding.js

// in apps/your_app/core.js, add: require('ext/binding');
// place this in apps/your_app/ext/binding.js:

SC.mixin(SC.Binding, {
  firstObject: function() {
    return this.transform(function(value, isForward) {
      if (value && value.isEnumerable) {
        value = value.firstObject();
      }
      return value;
    });
  }
});

// Usage:

MyApp.objController = SC.ObjectController.create(
/** @scope MyApp.objController.prototype */ {

  contentBinding: 'Admin.arrController.selection',
  contentBindingDefault: SC.Binding.firstObject()

}) ;