Checkbox is not working properly with ng-repeat, AngularJs

I want to make multiselect checkbox using ng-repeat. It is working fine if I don't have pre-selected checkbox. But when I have preselected checkbox then its behaviour is totally unusual.

<div ng-repeat="account in accounts">
    <input ng-model="selectedAccounts[account.id]"              
    ng-checked="" type="checkbox">       
</div>

In Controller I got selected id as:

$scope.selectedAccounts = [];
angular.forEach($scope.selectedAccounts, function(value, key) {
    if(value == true) {
        selectedIds.push(key);
    }
}); 

The problem here is that I have to initialise selectedAccounts with initial array. If I don't do this then it gives me undefined. When I have 'lastchecked' true for some accounts then it shows pre-checked values according to lastchecked but when I try to retreive $scope.selectedAccounts it give me empty array. But when I manually check/uncheck each option then $scope.selectedAccounts give me correct result.