HOWTO: Do you know that you can easily create dataset index fields for nested datasets for Entity?

knowhow_indexFieldOnEntityNestedDatasets.js

basis.require('basis.entity');
basis.require('basis.data.index');

// create type with single field, that used for EntitySet
var Item = basis.entity.createType('SomeType', {
  value: Number
});

// main Entity type
var DemoType = basis.entity.createType('DemoType', {
  items: basis.entity.createSetType(Item),          // nestes dataset
  sum: basis.entity.calc('items', function(items){  // key moment here, we create calc field that
    if (items)                                      // create index on items property
      return basis.data.index.sum(items, 'data.value');
  })
});

// create instance of DemoType
var dataObject = DemoType({
  items: [
    { value: 1 },
    { value: 2 },
    { value: 3 }
  ]
});

// dataObject.data.sum -> instance of basis.data.index.Sum
// dataObject.data.sum.value === 6

// demo view to show how bind index value to template
// as you can see no difference for simple cases because templates 
var view = new basis.ui.Node({
  container: document.body,
  delegate: dataObject,

  template: '<span>Items sum is {sum}</span>',
  binding: {
    sum: 'data:'
  }
});