In Angular, given an array of objects and a single input, how do bind the input so that it represents a given property of all the objects?

For the sake of future reference, let me write a full answer:

A way to accomplish this in fairly modern browsers is using property getters/setters (spec). An example, proof-of-concept implementation would be:

Let's say the $scope contains the following collection:

$scope.items = [
    {id: 1, prop: "a"},
    {id: 2, prop: "a"},
    {id: 3, prop: "a"}
];

And we want to manipulate the aggregate of the item.prop property. We define another object as:

$scope.form = {
    get aggregate() {
        var res, i;
        for( i=0; i < $scope.items.length; i++ ) {
            if( typeof(res) === "undefined" ) {
                res = $scope.items[i].prop;
            }
            else if( $scope.items[i].prop !== res ) {
                return "(multiple)";
            }
        }
        return res;
    },
    set aggregate(val) {
        var i;
        for( i=0; i < $scope.items.length; i++ ) {
            $scope.items[i].prop = val;
        }
    }
};

The form.aggregate property now has a getter and setter. These function handle their values by iterating over $scope.items. The getter compares the values and returns the common one, if all are the same or "(multiple)" if at least one is different. The setter just sets the given value to all properties.

A fiddle: http://jsfiddle.net/52HE6/

And an improved (IMO) version, using a placeholder instead of the literal "(multiple)": http://jsfiddle.net/52HE6/1/

This pattern can probably be generalized/parameterized (i.e. no fixed name prop), e.g. as (WARNING: UNTESTED):

function aggregatorFactory($scope, collectionName, propName) {
    return {
        get aggregate() {
            var res, i;
            for( i=0; i < $scope[collectionName].length; i++ ) {
                if( typeof(res) === "undefined" ) {
                    res = $scope[collectionName][i][propName];
                }
                else if( $scope[collectionName][i][propName] !== res ) {
                    return "(multiple)";
                }
            }
            return res;
        },
        set aggregate(val) {
            var i;
            for( i=0; i < $scope[collectionName].length; i++ ) {
                $scope[collectionName][i][propName] = val;
            }
        }
    };
}