The option database cannot be set on a new collection instance. The options that can be updated are [:read, :read_concern, :write, :write_concern] (Mongo::Error::UnchangeableCollectionOption)

require 'mongoid' 

Mongoid.configure do |config| 
  config.connect_to('main') 
end 

class User 
  include Mongoid::Document 
  field:name, type: String 
end 

user = User.new(name: 'John') 
user.with(database: 'alt') do |u| 
  u.save! 
end 

# 1st try: reusing the same user object 
user.with(database: 'alt') do |u|   
  u.update(name:'John Doe 2') # raises Mongo::Error::UnchangeableCollectionOption 
end 

# 2nd try: using a new user object 
User.with(database: 'alt') do |clazz| 
  u=clazz.find_by(name:'John') 
  u.update(name:'John Doe 1') # raises Mongo::Error::UnchangeableCollectionOption 
end