Mongoid3 Cleaner can not remove a collection that contains the "system" in name.
collection name e.g. user_systems, system_settings
Cause
Moped::Truncation.collections
def collections
session['system.namespaces'].find(:name => { '$not' => /system|\$/ }).to_a.map do |collection|
_, name = collection['name'].split('.', 2)
name
end
end
I think. To fix this, change the RegExp.
e.g.
/^system\./