Shopify API update variant inventory - https://www.shopify.com/partners/blog/84051654-7-tips-to-maximize-your-use-of-the-shopify-api

shopify-api-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})