habitica/migrations/20131105_remove_history_ids.js

function deleteId(h){
  delete h._id;
}

db.users.find({},{habits:1,dailys:1,history:1}).forEach(function(user){
  if (user.history) {
    _.each(['todos','exp'], function(type){
      if (user.history[type]) {
        _.each(user.history.exp, deleteId);
      }
    })
  } else {
    user.history = {exp:[],todos:[]};
  }

  _.each(['habits', 'dailys'], function(type){
    _.each(user[type].history, deleteId);
  });

  db.users.update({_id:user._id}, {$set:{history: user.history, habits: user.habits, dailys: user.dailys}});
});