Why am I receiving an upgrade compatibility check error?

In my main actor file

stable let customerConfigurationStore = CustomerConfiguration.init();
// In CustomerConfiguration module...
// public func init() : Types.CustomerConfigurationStore
  private type CustomerId = Principal;
  // Map is from https://github.com/ZhenyaUsenko/motoko-hash-map
  public type Map<K, V> = (
    root: Entry<K, V>,
    size: [var Nat32],
  public type CustomerConfigurationStore = Map.Map<CustomerId, CustomerRecord>;
  public type CustomerRecord = {
    var email : ?CustomerEmail;
    canisters : Map.Map<CanisterId, CanisterConfig>;
    var customerNotifications : CustomerNotificationSettings;
    createdTimestamp : ?Nat;

    // <- I just added this, then attempted to re-deploy
    var onboardingCompletedTimestamp : ?Nat;

I received the following warning on dfx deploy

Stable interface compatibility check failed for canister 'xxxx'.
Upgrade will either FAIL or LOSE some stable variable data.

(unknown location): Compatibility error [M0170], stable variable customerConfigurationStore of previous type
  (Entry<CustomerId__2, CustomerRecord>, [var Nat32])
cannot be consumed at new type
  (Entry__1<CustomerId__2__1, CustomerRecord__1>, [var Nat32])

Do you want to proceed? yes/No

Isn’t adding this property a subtype here?

var onboardingCompletedTimestamp : ?Nat;

The confusing part is that I could have sworn we’ve added additional fields to this type previously…for example, we added createdTimestamp : ?Nat; last month with no issues (I just checked git blame)

1 Like

That error is correct. You cannot add a field, even optional, to a stable record type that needs to be consumed during upgrade.

Maybe the last time you did this the addition was to a record type in the candid interface, where this sort of thing is allowed.