class ActiveRecord::Associations::Preloader::Batch

def call
  branches = @preloaders.flat_map(&:branches)
  until branches.empty?
    loaders = branches.flat_map(&:runnable_loaders)
    loaders.each { |loader| loader.associate_records_from_unscoped(@available_records[loader.klass.base_class]) }
    if loaders.any?
      future_tables = branches.flat_map do |branch|
        branch.future_classes - branch.runnable_loaders.map(&:klass)
      end.map(&:table_name).uniq
      target_loaders = loaders.reject { |l| future_tables.include?(l.table_name)  }
      target_loaders = loaders if target_loaders.empty?
      group_and_load_similar(target_loaders)
      target_loaders.each(&:run)
    end
    finished, in_progress = branches.partition(&:done?)
    branches = in_progress + finished.flat_map(&:children)
  end
end

def group_and_load_similar(loaders)
  loaders.grep_v(ThroughAssociation).group_by(&:loader_query).each_pair do |query, similar_loaders|
    query.load_records_in_batch(similar_loaders)
  end
end

def initialize(preloaders, available_records:)
  @preloaders = preloaders.reject(&:empty?)
  @available_records = available_records.flatten.group_by { |r| r.class.base_class }
end