I was still unable to create rules for my existing Atlas collections after unlinking and relinking. I tried two methods of linking the data source:
1) Automatically by creating a new Realm app
When creating a new Realm app, I am prompted to select the existing cluster as the data source, and a link with the service name
mongodb-atlas is automatically configured for me. From the Realm UI, this data source shows my existing collections from Atlas. However, when I attempt to create a rule for any of these collections, I get the same error:
Failed to add rule: error processing request
I also get this error when I attempt to create rules for a new (non-existent) collection on this data source, as @Jay appears to have been doing in the original post above.
2) Manually using the “Link a Data Sources” tool under the “Manage” section of the side bar
Although I can select my Atlas cluster from this tool, once it is linked it does not find any of my existing collections. On this service, however, I can create a new rule on a “new” database and collection - i.e. I can successfully make a rule for a collection that doesn’t exist on a database that doesn’t exist on the Atlas cluster. Not really sure what to make of this.