//the column "externalUserId" should be unique, but how to do it with doctrine?

//when calling this a new user record with externalUserId=4711 will be created
$myUser = new myUser();
$myUser->externalUserId = 4711;
$myUser->save();

//when calling this a second user record with externalUserId=4711 will be created
$myUser = new myUser();
$myUser->externalUserId = 4711;
$myUser->save();

//the second call should throw an exception "Can not create this user.
//User is already there.
//Please load the user details and change that user if you want to change the user."