Stale attribute overwrite during concurrent Backbone.Model.save calls

Answer

Backbone.Model does not provide a built‑in way to discard server responses that belong to outdated save requests. You must add your own request‑sequencing logic (e.g., a monotonic ID or timestamp) and ignore any response whose identifier is not the most recent.

Confirmed facts

• By default Backbone.Model.save updates the model’s attributes immediately with the data sent to the server ({wait:false}).
• The success callback of the underlying AJAX request then replaces those attributes with the server’s response.
• If two save calls overlap, the slower response can arrive after a faster one and overwrite the newer data, causing a stale overwrite.
• Setting {wait:true} defers the local attribute update until the server responds, but it does not prevent an older response from winning the race if it arrives later.

Likely explanation & mitigation steps

1. Add a request counter (or timestamp) to the model, e.g., this._saveSeq = (this._saveSeq || 0) + 1 each time save is called.
2. Pass the current sequence number to the save call via the options object, e.g., model.save(attrs, {seq: this._saveSeq}).
3. Override Backbone.sync (or the model’s save method) to forward the sequence number to the server, or store it locally and compare it in the success handler.
4. In the success handler, ignore the response if its sequence number is less than or equal to the model’s last processed sequence: if (resp.seq <= this._lastSeq) return; this._lastSeq = resp.seq; then apply the attributes.
5. Optionally combine this with {wait:true} so that the model is not updated with stale data before the check.

Diagnostic question

Are you currently using {wait:true} or a custom Backbone.sync implementation? Knowing this determines whether you need only the request‑ID check or also need to adjust how the model updates locally.