Sync Gateway. Add the channel to the document via Admin REST API

“You can’t; documents can only be added to channels when they are updated since the channel assignment is part of the revision. The sync function runs while the revision is being added to the database, so it can assign channels.”
(I do not understand how can a quotation mark)

Ok, but can you assign channels to update the document - update creates a new revision, when creating a revision updated channels.
Update the document possible through the Rest API. Or am I wrong?
This, of course, too workaround, but as far as I understood, the design mistakes were made, which are not easy to get around by …

“You’ll need to find another way to do it, then. You may be thinking about the problem in ways that make sense for a relational database but don’t make sense for Couchbase Mobile.”

Relationships can be implemented and using Couchbase. Look at the documentation, there are described scenario implementation.