require "logger"
require "mongoid"
Mongoid.configure do |config|
  config.clients.default = {
    hosts: ['localhost:27017'],
    database: 'my_db',
    options: {
      connect: :direct  
    }
  }
  config.log_level = :warn
end
class Parent
  include Mongoid::Document
  has_and_belongs_to_many :children, class_name: "Child", inverse_of: :parents
end
class Child
  include Mongoid::Document
  has_and_belongs_to_many :parents, class_name: "Parent", inverse_of: :children
end
p Mongoid::VERSION
parent = Parent.new()
parent.save!
parent2 = Parent.new()
parent2.save!
child_one = Child.new()
child_one.parents = [parent]
child_one.save!
child_two = Child.new()
child_two.parents = [parent2]
parent_loaded = Parent.find(parent.id)
p "BEFORE"
p parent_loaded.children.size
p parent_loaded.children.length
p parent_loaded.children.count
parent_loaded.children = [child_one, child_two]
p "AFTER"
p parent_loaded.children.size
p parent_loaded.children.length
p parent_loaded.children.count

"8.0.9"
"BEFORE"
1
1
1
"AFTER"
1
1
2 

source 'https://rubygems.org'
ruby '3.2.2'

gem 'mongoid', '8.0.9'
gem 'rails', '~> 6.1'