I am using Mongodb 6 with Mongoid (Ruby ORM). I have tested this with Mongoid 7.5/8.1.5. Other dependencies:

• Mongo: 2.19.3
• Rails: 7.0.8.1
• Ruby: 3.2.3

Reproduce:

1. Set up a new model:

2. class Company
     # include ...
   
     has_many :connections, validate: false
   
     def default_connections
       connections.find { |c| c.default? }
     end
   end
   
   class Connection
     # include ...
   
     belongs_to :company
   
     field :default, type: Mongoid::Boolean, default: false
   end

1. Run a test rake task

2. namespace :issue do
     desc 'Reproduce association issue'
     task({ :association => :environment }) do |task, args|
       company = Company.new
   
       connection = company.connections.build(default: true)
   
       if company.default_connections == nil
         puts "company.default_connection is nil!!"
       end
   
       puts ''
       puts "But connection is here."
       p company.connections
   
       puts ''
       puts "#detect works fine"
       p company.connections.detect { |c| c.default? }
   
       puts ''
       puts "#method(:find).call works fine"
       p company.connections.method(:find).call { |c| c.default? }
     end
   end