docs: Clarify potential gotcha when overriding serialize in rtdb

[julienr]

TLDR: The rtdb childRemoved handler uses indexForKey to find the element to delete. This function assumes that a .key property is defined on objects, which won't be the case if you override the serialize option. This can lead to bugs when removing from array.

For example, I had the following, which doesn't define .key:

      await bindFirebaseRef('events', db.ref(FB_EVENT_PREFIX + aoiName + '/'), {
        serialize: (snapshot: firebase.database.DataSnapshot) => {
          const val = snapshot.val()
          val.date = new Date(val.date)
          return val
        }
      })

With this code, whenever I deleted an element from my array, it was always the last one that got removed. Took me a bit of time to figure out why deletion wasn't working, hence this documentation improvement.

The reason is that if .key is not defined, indexForKey will return -1 which ultimately will gets passed to Array.slice which ends up removing the last item.

A better fix might be to throw an error instead of returning -1 in indexForKey. Happy to work on this if that's deemed better.

[posva]

Thanks for this, I added a similar comment to the new docs