update_variant_inventory.rb

# Takes a product id, and a map of variant ids and quantities
def update_inventory(id, vid_quantities)
  variants = vid_quantities.map do |vid, qty|
    { "id" => vid, "inventory_quantity" => qty }
  end
  data = {
    "product" => {
      "variants" => variants
    }
  }
  request(:put, "products/#{id}.json", body: data.to_json)
end

update_inventory(1234567, {2222222 => 100, 3333333 => 200})