Cart Transform: Supply a different discount to each line item

[Jay-Plumb]

The bundle Shopify function accepts a discount per bundle which does not solve a client goal of ours which currently uses shopify scripts to first:

1. Apply the correct discount for each line item
2. Bundle the line items into a single entity

The discount attribute on a shopify bundle just discounts the subtotal combined and so the discount would not match with the discounts they have with their wholesale partners.

The workaround that I thought could work would be to:

1. Use a discount function to apply the correct discount per line item (pending a solution for: Impossible to return multiple discounts from the same function #194)
2. Bundle the items together now that the correct discounts have been applied

Actual behavior

For testing purposes, I applied one discount to one line item and then added a non discounted item to the cart. Two issues observed:

1. The bundling does not always contain the correctly added quantities for each line item bundled together.
2. The discount is not applied

While I have opened the ticket as a bug, I am okay if Shopify want to track the story as an enhancement should they wish to allow the ability to apply a discount for each line item rather than then existing functionality per bundle: https://shopify.dev/docs/api/functions/reference/cart-transform/graphql/common-objects/priceadjustment

Reproduction steps

1. index.js

// @ts-check

/**
 * @typedef {import("../generated/api").InputQuery} InputQuery
 * @typedef {import("../generated/api").FunctionResult} FunctionResult
 */

/**
 * @type {FunctionResult}
 */
let output = {
  operations: [],
};

const fetchUniqueAttributes = (lines) => {
  const attributes = [];

  lines.forEach((line) => {
    const attribute = line?.attribute?.value;
    if (attribute) {
      attributes.push(attribute);
    }
  });

  return [...new Set(attributes)];
}

export default /**
 * @param {InputQuery} input
 * @returns {FunctionResult}
 */
  (input) => {
    const lines = input.cart.lines;

    // For line attributes with no bundle attribute, just return the line item
  
  
    if (lines?.length >= 2) {
    // Front end will have an attribute/property to indicate which line item the bundle belongs to and the value will indicate which bundle ite belongs to
    // e.g. bundle-group: bundle-1, bundle-group: bundle-2 etc.

    // Create a bundle based on which property we have on the line item
    
    // Fetch all unique attributes
    const bundleAttributes = fetchUniqueAttributes(lines); // [bundle1, bundle2]

    // loop through each uniqueBundles
    bundleAttributes.forEach((bundleAttribute, index) => {
      // filter lines based on the bundleAttribute
  
      const cartLinesPerBundle = lines.filter((line) => {
        const attribute = line?.attribute?.value;
        return attribute === bundleAttribute;
      });


      if (cartLinesPerBundle?.length > 1) {
  
        let lineItemBundles = [];
        // Loop through each cartLinesPerBundle
        cartLinesPerBundle.forEach((linePerBundle) => {
          lineItemBundles.push({
            "cartLineId": linePerBundle.id,
            "quantity": linePerBundle.quantity
          }) 
        });

        const operation = {
          "merge": {
            "cartLines": lineItemBundles,
            "title": `Bundle ${index + 1}`,
            "parentVariantId": "gid://shopify/ProductVariant/46450132943136", // TODO: 
          }
        }
      
        output.operations.push(operation);
      }
    });
    
    return output;
  }
}

2. input.graphql

query Input {
  cart {
    lines {
      id
      quantity
      attribute(key: "bundle-group") {
        value
      }
    }
  }
}

3. Run mutation query on the store to enable bundling

4. Supply basic discount shopify function from the tutorials

5. Confirm that the discount is not applied when bundling with incorrect quantities added to the bundle