mongodb/jstests/autoid.js

f = db.jstests_autoid;
f.drop();

f.save( {z:1} );
a = f.findOne( {z:1} );
f.update( {z:1}, {z:2} );
b = f.findOne( {z:2} );
assert.eq( a._id.str, b._id.str );
c = f.update( {z:2}, {z:"abcdefgabcdefgabcdefg"} );
c = f.findOne( {} );
assert.eq( a._id.str, c._id.str );