A data structure to put stuff in and take it out again. NIH syndrome?

bag.rb

class Bag
  def initialize
    @contents = Hash.new(0)
  end

  def put_in(names, amount)
    @contents[[names].flatten] += amount
  end

  def take_out(name, amount)
    bag_key = @contents.keys.find{|k| k.include?(name)}
    available = [amount, @contents[bag_key]].min
    @contents[bag_key] -= available
    available
  end
end

bag_spec.rb

describe Bag do
  let(:bag) { Bag.new }

  it 'only lets me take out what i put in' do
    bag.put_in :tshirt, 2

    expect(bag.take_out(:tshirt, 3)).to eql(2)
  end

  it 'returns 0 if there is nothing left' do
    bag.put_in :tshirt, 2

    bag.take_out(:tshirt, 2)

    expect(bag.take_out(:tshirt, 1)).to eql(0)
  end

  it 'lets me put in items under multiple names' do
    bag.put_in [:tshirt, :polo], 2

    expect(bag.take_out(:tshirt, 1)).to eql(1)
    expect(bag.take_out(:polo, 1)).to eql(1)

    expect(bag.take_out(:tshirt, 1)).to eql(0)
    expect(bag.take_out(:polo, 1)).to eql(0)
  end
end