Calling new on an AR class, from within the context of an ActiveRelation on a subclass of said class, assumes the "id" attribute defined in the relation's where clause

ar_weirdness.rb

#!/usr/bin/env ruby

class ArWeirdnessUsage
  def self.run
    message = <<-EOF
    Usage:
     0) make this file executable and run it in order to see these instructions
     1) Create a Rails Project (rails new foo) and add this file: cp %s ./foo/lib/
     2) rails g scaffold TestClass name && RAILS_ENV=test rake db:create db:migrate
     3) echo 'TestClass; ArWeirdness.test_suite' \\
        | rails c --environment=test

     4) Cleanup:
        echo 'ActiveRecord::Base.connection.execute("drop table test_classes")' \\
        | rails c --environment=test && rails destroy scaffold TestClass

     You may also want to remove stray files like:
         db/schema.rb
         app/assets/stylesheets/scaffolds.scss
     with either 'git checkout ...' or 'rm ...'
    EOF
    return(message % $PROGRAM_NAME)
  end
end

if __FILE__ == $PROGRAM_NAME
  puts ArWeirdnessUsage.run
else
  class ArWeirdness
    COMMON_ATTRS = { } # <-- put any required attributes here; sorry, can't handle attributes that must be unique.

    def self.test_setup
      ::TestClass.create!(COMMON_ATTRS.merge(name: 'foo'))
      ::TestClass.create!(COMMON_ATTRS.merge(name: 'bar'))
    end

    def self.test_cleanup
      ::TestClass.delete_all
    end
    module ExtraFunctionality
      def create_instance_of_test_class(attrs={})
        ret_val = BROKEN_TEST
        new_record = ::TestClass.new(attrs) # this new record shouldn't have an ID!
        if new_record.id.present?
          ret_val = BUG
          puts " ------------> huh: generated a new instance with an ID assigned?!" # the id of the activerecord relation got in our way!
        else
          ret_val = BUG_FREE
          puts " ------------> ok"
        end
        puts "\tWe have #{new_record.inspect}"
        puts "\tfrom: #{attrs.inspect}"
        return ret_val
      end
    end
    class SubsetOfTestClass < ::TestClass
      extend ExtraFunctionality
    end

    def self.test_suite
     test_ok
     test_huh
    end

    BUG = -1; BROKEN_TEST = 0; BUG_FREE = 1
    def self.test_ok # this WORKS ...it creates a new object without picking-up the "id" of the collection
      test_setup
      ids = ::TestClass.pluck(:id)
      @id = ids.first

      @filtered_collection = ::TestClass.where(id: [@id]) #<-- the use-case: to present a filtered_version of TestClass with only the current user's entries
      @filtered_collection.send(:extend, ExtraFunctionality)

      result = @filtered_collection.create_instance_of_test_class(COMMON_ATTRS.merge(name: 'foo-bar-baz-OK'))
      test_cleanup
      return result
    end

    def self.test_huh # this FAILS ...it creates a new object, that ?magically? picks-up the id of the ARRelation within which it's defined
      test_setup
      ids = ::TestClass.pluck(:id)
      @id = ids.first

      @filtered_collection = SubsetOfTestClass.where(id: [@id]) #<-- the use-case: to present a filtered_version of TestClass with only the current user's entries

      result = @filtered_collection.create_instance_of_test_class(COMMON_ATTRS.merge(name: 'foo-bar-baz-HUH'))
      test_cleanup
      return result
    end
  end
end