calls to angular.extend don't cause $scope.$watch to proc

Question

I'm merging a smaller object into a larger object using angular.extend.

Here's a sample of data:

$scope.bigDocument = {
    "id": 0,
    "guid": "e75ce3b3-68f6-423f-94d8-1a613cde0c59",
    "isActive": true,
    "balance": "$2,437.00",
    "picture": "http://placehold.it/32x32",
    "age": 38,
    "name": "Parks Williamson",
    "gender": "male",
    "company": "Arctiq",
    "email": "[email protected]",
    "phone": "+1 (817) 488-3119",
    "address": "252 Concord Street, Morgandale, Arizona, 1866",
    "about": "Irure nostrud nisi qui do amet nisi adipisicing dolor eiusmod do non laboris.",
    "registered": "1995-04-08T20:16:47 +05:00",
    "latitude": -6.796341,
    "longitude": 40.321499,
    "randomArrayItem": "cherry"
}

$scope.littleDocument = {
     "balance": "$3,193.00",
     "picture": "http://placehold.it/32x32",
     "age": 22, 
     "name": "Coffey Wilcox",
     "gender": "male",
     "company": "Balooba",
     "email": "[email protected]",
     "phone": "+1 (824) 518-3639",
     "address": "836 Douglass Street, Imperial, Montana, 9365",
     "about": "Ipsum dolore officia consectetur proident occaecat.",
     "registered": "1994-12-12T15:47:09 +06:00",
     "latitude": -33.474425,
     "longitude": -113.998081,
}

I'm using the following function to merge:

$scope.merge = function() {
    angular.extend($scope.bigDocument, $scope.littleDocument)

and watching for changed with the following

$scope.$watch('bigDocument', function(newVal, oldVal) {
    console.log('bigDocument Changed');
})

I get the expected output on initial hydration of $scope.bigDocument, but not after the merge function is called. The data changes correctly, but the $watch method never gets hit.

What gives?

Here's a plunker (in coffeescript) to show the full workflow: http://plnkr.co/edit/VPnxhUa6uz1zKfcufb2J

Answer 1

The $watch function accepts a third boolean parameter indicating either to compare object for equality or reference:

$scope.$watch('bigDocument', function(newVal, oldVal) {
    console.log('bigDocument Changed');
}, true);

This plunker works as expected.