Google Calendar API - Fetching Group calendars

I’m currently working on writing a sync application to migrate Group calendars from Google G Suite to MS O365, and after reading up on a lot of APIs etc, I thought I’d post a short summary of my findings below.

Unfortunately, Google’s Calendar API isn’t as flexible as I’d like it to be, so this is a much more complex process than it arguably should be.

The following are some caveats/“gotchas” that I’ve come across so far.

API caveats

• Google’s API doesn’t allow you to impersonate a Group, even with domain-wide delegation.
  • As a result, you can’t simply get a CalendarList for a Group email, as you would for a user.
• Google’s API doesn’t allow you to fetch calendar lists on an organisational-level.
  • So if you want to get all calendars in an organisation, you’ll have to iterate through each user and build up an array of unique calendars manually.

Group caveats

• Groups don’t have calendars - Users do, and they’re shared with a group.
  • Each event has an Organizer attribute with the calendar’s unique ID as the id, and the calendar’s unique email address (i.e. [email protected]) as the email. There’s no reference to the group’s normal email (i.e. [email protected]). As expected, the creator attribute reflects the user who created the event.
• Each user who has a Group’s calendar in their Calendar list retrieves the same Calendar ID. This can be used to cross-reference calendars between users.
  • e.g. with the Ruby client API, the Google::Apis::CalendarV3::CalendarListEntry object will have the same etag and id for different users.
• There does not appear to be a way to programmatically determine the associated group email address, from a CalendarList.

Based on this, to be able to get a list of unique shared calendars within an organisation, you’d need to:

1. Create an array or Hash map to be used to track the shared calendars within an organisation.
2. Get a list of users.
3. For each of the users
   • Get all calendar lists for this user,
   • Filter the calendar lists by those with ids ending with @group.calendar.google.com,
   • Add these into the aforementioned array or Hash map.