Add RecordArray#filterBy which contains a live, filtered subset

This method takes the parameters `key` and an optional `value` and
returns an updating list of records of the `RecordArray`, which'
property value of `key` matches `value`.

packages/ember-data/lib/system/record-arrays/record-array.js

@@ -8,6 +8,21 @@ import SnapshotRecordArray from "ember-data/system/snapshot-record-array";
 var get = Ember.get;
 var set = Ember.set;
 
+var FilteredSubset = Ember.ArrayProxy.extend({
+  init: function() {
+    this._super(...arguments);
+
+    var { filterByArgs, recordArray } = this.getProperties('filterByArgs', 'recordArray');
+    var [key] = filterByArgs;
+
+    var path = `recordArray.@each.${key}`;
+    Ember.defineProperty(this, 'content', Ember.computed(path, function() {
+      return this.filterBy.apply(recordArray, filterByArgs);
+    }));
+  }
+});
+
+
 /**
   A record array is an array that contains records of a certain type. The record
   array materializes records as needed when they are retrieved for the first
@@ -96,6 +111,44 @@ export default Ember.ArrayProxy.extend(Ember.Evented, {
     return internalModel && internalModel.getRecord();
   },
 
+  /**
+    Get a filtered subset of the underlying `RecordArray`.
+    The subset updates when a record would match or mismatch the
+    specified filter parameters.
+
+    Example
+
+    ```javascript
+    var allToms = store.all('person').filterBy('name', 'Tom');
+
+    allToms.get('length'); // 0, since no toms yet in store
+
+    var tom = store.push('person', { id: 1, name: 'Tom' });
+    allToms.get('length'); // Tom is added
+
+    tom.set('name', 'Thomas');
+    allToms.get('length'); // 0, since no more records with name === 'Tom'
+    ```
+
+    @method filterBy
+    @param {String} key property path
+    @param {*} value optional
+
+  */
+  filterBy: function(key, value) {
+    // only pass value to the arguments if it is present; this mimics the same
+    // behavior for `filterBy`: http://git.io/vIurH
+    var filterByArgs = [key];
+    if (arguments.length === 2) {
+      filterByArgs.push(value);
+    }
+
+    return FilteredSubset.create({
+      filterByArgs,
+      recordArray: this
+    });
+  },
+
   /**
     Used to get the latest version of all of the records in this array
     from the adapter.

packages/ember-data/tests/unit/record-array-test.js

@@ -284,3 +284,91 @@ test("a record array should return a promise when updating", function() {
   });
   ok(promise.then && typeof promise.then === "function", "#update returns a promise");
 });
+
+test('filterBy - returns a filtered subset', function() {
+  var store = createStore({
+    person: Person
+  });
+
+  run(function() {
+    store.pushMany('person', [{ id: '1', name: "Tom" }, { id: '2', name: "Yehuda" }, { id: '3', name: "Alex" }]);
+  });
+
+  var all = store.all('person');
+  var toms = all.filterBy('name', 'Tom');
+  equal(toms.get('length'), 1);
+  deepEqual(toms.getEach('id'), ['1']);
+
+  // a new record is added if filter matches
+  run(function() {
+    store.push('person', { id: '4', name: "Tom" });
+  });
+  equal(toms.get('length'), 2);
+  deepEqual(toms.getEach('id'), ['1', '4']);
+
+  // a new record is not added if filter doesn't match
+  run(function() {
+    store.push('person', { id: '5', name: "Igor" });
+  });
+  equal(toms.get('length'), 2);
+  deepEqual(toms.getEach('id'), ['1', '4']);
+
+  // changing the filtered value remvoves the record from the list
+  run(function() {
+    // we are using a private method here to get the record immediatly
+    store.recordForId('person', '1').set('name', "Thomas");
+  });
+  equal(toms.get('length'), 1);
+  deepEqual(toms.getEach('id'), ['4']);
+
+  // change value back to original
+  run(function() {
+    store.recordForId('person', '1').set('name', "Tom");
+  });
+  equal(toms.get('length'), 2);
+  deepEqual(toms.getEach('id'), ['1', '4']);
+});
+
+test('filterBy - value is optional', function() {
+  var store = createStore({
+    person: Person
+  });
+
+  run(function() {
+    store.pushMany('person', [{ id: '1', name: "Tom" }, { id: '2' }]);
+  });
+
+  var all = store.all('person');
+  var allWithNames = all.filterBy('name');
+  equal(allWithNames.get('length'), 1);
+  deepEqual(allWithNames.getEach('id'), ['1']);
+
+  // a new record is added if filter matches
+  run(function() {
+    store.push('person', { id: '3', name: "Igor" });
+  });
+  equal(allWithNames.get('length'), 2);
+  deepEqual(allWithNames.getEach('id'), ['1', '3']);
+
+  // a new record is not added if filter doesn't match
+  run(function() {
+    store.push('person', { id: '4' });
+  });
+  equal(allWithNames.get('length'), 2);
+  deepEqual(allWithNames.getEach('id'), ['1', '3']);
+
+  // changing the filtered value remvoves the record from the list
+  run(function() {
+    // we are using a private method here to get the record immediatly
+    store.recordForId('person', '1').set('name', null);
+  });
+  equal(allWithNames.get('length'), 1);
+  deepEqual(allWithNames.getEach('id'), ['3']);
+
+  // change value back to original
+  run(function() {
+    store.recordForId('person', '1').set('name', "Tom");
+  });
+  equal(allWithNames.get('length'), 2);
+  deepEqual(allWithNames.getEach('id'), ['1', '3']);
+});