ruby 37 lines · 7 steps

Merging a guest cart into a user cart in Rails

A service object that folds an anonymous cart's items into a logged-in user's cart atomically, respecting stock limits.

Explained by highlit

Rails Intermediate service object transactions idempotency data merging stock limits

1class CartMergeService
def initialize(guest_cart:, user_cart:)
  @guest_cart = guest_cart
  @user_cart = user_cart
end
6 
def call
  return @user_cart if @guest_cart.nil? || @guest_cart == @user_cart
9 
  ActiveRecord::Base.transaction do
    @guest_cart.line_items.includes(:variant).find_each do |guest_item|
      merge_line_item(guest_item)
    end
14 
    @guest_cart.destroy!
  end
17 
  @user_cart.reload
end
20 
private
22 
def merge_line_item(guest_item)
  existing = @user_cart.line_items.find_by(variant_id: guest_item.variant_id)
25 
  if existing
    new_quantity = [existing.quantity + guest_item.quantity, guest_item.variant.stock].min
    existing.update!(quantity: new_quantity)
  else
    @user_cart.line_items.create!(
      variant: guest_item.variant,
      quantity: [guest_item.quantity, guest_item.variant.stock].min,
      price: guest_item.variant.current_price
    )
  end
end
37end

01 / 01

STEP 01

Walkthrough

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Three takeaways

1Wrapping multi-step writes in a transaction keeps the merge all-or-nothing, so a failure never leaves both carts half-updated.
2Guarding against nil and identical inputs up front makes the operation safe to call repeatedly without side effects.
3Clamping quantities to available stock at merge time prevents the combined cart from exceeding what can actually ship.

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