How do I save the creation of a new invoice (with line items) as a single transaction? As far as I can tell, you can't without breaking from Ember Data's very strict 'this is the way things should be done' model.

You have to create a new Invoice, then create the InvoiceLineItems doing something like this:

  var self = this;
  self.get('store').createRecord('invoice', {
    .. attributes ..
  }).save().then(function (invoice) {
    return Ember.RSVP.all(self.get('lineItems').map(function (item) {
       return self.get('store').createRecord('invoiceLineItem', {
           invoice: invoice,
           .. attributes ..
       }).save();
    });
  }).then(function (invoiceLineItems) {
    // do something
  }).then(null, function (error) {
    console.error(error);
  });

The 'other' way to do it is to create just an Invoice model, and have a lineItems attribute of type 'raw' and just create a RawTransform that's just as pass-through of whatever the JSON has for that attribute. It works, but at the same time, feels like it's going against the Ember Data Way, especially if you have a need for an InvoiceLineItem to be its own model (i.e. to be able to maintain relationships to other things and look one up by ID via the API).

Then you might have something like:

- Invoice and InvoiceLineItem models

- A RawTransform:

  App.RawTransform = DS.Transform.extend({
      deserialize: function (data) { return data; },
      serialize:   function (data) { return data; },
  });

  lineItems: DS.attr('raw'),

  var self = this;
  self.get('store').find('invoice', invoiceId).then(function (invoice) {
    self.get('store').pushPayload('invoiceLineItem', { invoiceLineItems: invoice.get('lineItems') });
  });