var res;
t = db.jstests_update_arraymatch6;
t.drop();

function doTest() {
    t.save( {a: [{id: 1, x: [5,6,7]}, {id: 2, x: [8,9,10]}]} );
    res = t.update({'a.id': 1}, {$set: {'a.$.x': [1,1,1]}});
    assert.writeOK( res )
    assert.eq.automsg( "1", "t.findOne().a[ 0 ].x[ 0 ]" );
}

doTest();
t.drop();
t.ensureIndex( { 'a.id':1 } );
doTest();