mongo/jstests/core/ref2.js

t = db.ref2;
t.drop();

a = {
    $ref: "foo",
    $id: 1
};
b = {
    $ref: "foo",
    $id: 2
};

t.save({name: "a", r: a});
t.save({name: "b", r: b});

assert.eq(2, t.find().count(), "A");
assert.eq(1, t.find({r: a}).count(), "B");
assert.eq(1, t.find({r: b}).count(), "C");