Automatically increment order numbers

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12

This is what I’ve done:

Every collection needing an “autoincrement” gets an extra document added: {_id: "autoincrement", value: 0}. I usually put the insert(s) in the server-side Meteor.startup():

Meteor.startup(function() {
  try {
    MyCollection.insert({_id: 'autoincrement', value: 0});
  } catch(err) {
    // Will always get here once the doc's in place, so just ignore
  }
  try {
    AnotherCollection.insert({_id: 'autoincrement', value: 0});
  } catch(err) {
    // Will always get here once the doc's in place, so just ignore
  }
});

Define a helper function to do the autoincrement on a collection (note the node syntax):

const doAutoincrement = function(collection, callback) {
  collection.rawCollection().findAndModify({
    _id: 'autoincrement'
  }, [], {
    $inc: {
      value: 1
    }
  }, {
    'new': true
  }, callback);
}

and define a (synchronous) function to get the next autoincrement value for a collection:

const nextAutoincrement = function(collection) {
  return Meteor.wrapAsync(doAutoincrement)(collection).value;
}

Then, when I want the next autoincrement value:

let nextValue = nextAutoincrement(MyCollection); // 1
nextValue = nextAutoincrement(MyCollection); // 2
nextValue = nextAutoincrement(MyCollection); // 3
let anotherNextValue = nextAutoincrement(AnotherCollection); // 1

Using findAndModify is atomic and is safe even when multiple server instances are being used.