botkit doesn't handle the facebook checkbox plugin callback

[nishch]

Facebook has two ways to let users of your web apps, opt in to receive messages, via "checkbox plugin" and "send to messenger". Payload received for these two in the subscribed webhook is different, in case of "checkbox plugin", facebook does not send the sender id rather it sends the user_ref inside optin node.

Currently botkit handles the "send to messenger" plugin correctly but breaks for "checkbox plugin" as shown in the following stack trace:

TypeError: Cannot read property 'id' of undefined
    at Object.__dirname.Facebookbot.facebook_botkit.handleWebhookPayload (/home/nishant/work/orange/notifier/node_modules/botkit/lib/Facebook.js:320:58)
    at /home/nishant/work/orange/notifier/node_modules/botkit/lib/Facebook.js:241:29

[jonchurch]

Here are the FB docs for this: Link

The issue within botkit is that the optin event grabs the user id from the sender field, which is not present in Checkbox plugin webhook events.

ALSO! The user_id that is passed back is a "unique ref" and not the actual PSID, it can be used to message a user, but the outgoing message signature will look a lil different. Instead of sender.id it wants sender.user_ref and then that call to the send api will return the actual user PSID...