lazy_ids.rb

# This is a workaround for the rails issue described in
# https://github.com/rails/rails/issues/17368. If you use this,
# make sure you understand its code and what it does and what it
# doesn't.
#
# See below for an example.

# lib/lazy_ids.rb (i.e.)
module LazyIds
  extend ActiveSupport::Concern

  included do
    after_save :persist_lazy_ids
  end

  def persist_lazy_ids
    return unless @_lazy_ids
    @_lazy_ids.each do |association, ids|
      send(:"eager_#{association.to_s.singularize}_ids=", ids)
    end
    @_lazy_ids = {}
  end

  module ClassMethods
    def lazy_ids(association)
      alias_method :"eager_#{association.to_s.singularize}_ids=", :"#{association.to_s.singularize}_ids="
      
      define_method "#{association.to_s.singularize}_ids=" do |ids|
        @_lazy_ids ||= {}
        @_lazy_ids[association] = ids
      end

      define_method association.to_s do
        if @_lazy_ids && @_lazy_ids[association]
          return self.class.reflect_on_association(association).klass.find(@_lazy_ids[association])
        else
          return super()
        end
      end
    end
  end
end

# Example:

class User
  include LazyIds
  
  has_many :groups_users
  has_many :groups, through: :groups_users
  
  lazy_ids :groups
end

u = User.first
u.group_ids = [1, 2, 3] # Aah, it does not save :)
u.save! # Now it does!

Thanks! I needed to change line 37 from return super to return super() due to error "implicit argument passing of super from method defined by define_method() is not supported. Specify all arguments explicitly.". (Ruby 2.5.1, Rails 5.2.4.1).

Thank you, the gist is updated!