Option to deconstruct session to select which id to use for externalId (better-auth)

[MatthewDavidTravers]

Currently all interactions when using the better-auth plugin are done via the user.id which references the customer.externalId in Polar. Better-auth allows the concept of organisations through the organisation plugin. It would be good to allow the polar plugin to decide what will be used as the referencing id.

A possible solution could be to give a callback function in the configuration of the plugin that allows the user to decide how we deconstruct the session to get the ID.

something like:

export const auth = betterAuth({
  plugins: [
    polar({
      getExternalId(session) {
        return session.session.activeOrganizationId;
      }
    })
  ]
});

if this is not possible are there other ways this could be achieved currently?

[pieterbeulque]

Great issue, and we'll definitely need this once we roll out seat-based billing (cc @psincraian).

I've opened a PR in #372 that should cover this, but I want to run some extra tests before I roll this out.

[pieterbeulque]

#372 turns out to be a dead end. We'll have to roll out a new major version that supports the organizations approach properly to work with seat-based billing, and it'll introduce some breaking changes compared to the current setup.

This will likely ship later this month, but waiting for some things with seat-based billing to finalize. I'll keep you posted!