batch.rb

require "set"

class Batch
  def initialize(results = {}, &blk)
    @ids = ::Set.new
    @blk = blk

    @promises = {}
    @results = results
  end

  def load(id)
    @ids << id

    @promises[id] ||= -> {
      sync unless @results[id]
      @results[id]
    }
  end

  private

  def sync
    loader = ->(id, result) {
      @results[id] = result
    }
    @blk.call(@ids, loader)
    @ids.clear
  end
end

# main
my_loader = Batch.new { |ids, loader|
  puts "Loading for #{ids.inspect}"
  ids.map { |id| loader.call(id, "cool #{id}") }
}

a = my_loader.load(1)
b = my_loader.load(2)
c = my_loader.load(2)

puts "a: #{a.call.inspect}"
puts "b: #{b.call.inspect}"
puts "c: #{c.call.inspect}"

d = my_loader.load(4)
puts "d: #{d.call.inspect}"

# Output
# Loading for #<Set: {1, 2}>
# a: "cool 1"
# b: "cool 2"
# c: "cool 2"
# Loading for #<Set: {4}>
# d: "cool 4"