Cart Transform: Update Lines Operation

Modify the price, title, and image of cart line items

Overview

The Update Lines block modifies the properties of cart line items — including their price, title, and image. Changes appear immediately in the customer’s cart.

• Dynamic pricing — apply custom prices based on metafields, attributes, or mapping rules
• Product customization — change titles and images based on customer selections
• Personalization — display customized product information in the cart
• Conditional pricing — apply different prices based on product attributes

Two Methods

The Update Lines block supports two configuration methods:

• Manual configuration — set price, title, and image individually using static values or linked fields
• Attribute-based configuration — load all values dynamically from a single JSON cart line attribute

Setup

Settings

Cart Line Groups

Required

Select which groups of cart lines should be updated. Groups must be defined earlier in your function using a Define Cart Line Groups block.

Price Mapping

Optional

Select a mapping variable to apply dynamic prices based on mapping rules. When selected, this overrides any static or field-based price settings.

This dropdown shows output variables created by Variant Price Mapping blocks placed earlier in your function. When a mapping is selected, per-unit prices are looked up from a CSV-format metafield at runtime.

If no Variant Price Mapping blocks exist in your function, the UI shows “No mapping variables available.”

Load All Fields from Attribute

When enabled, price, title, and image are all read from a JSON attribute on each cart line instead of being set individually. The individual field settings below are hidden.

See Attribute-Based Configuration for the JSON format.

Attribute Key

Shown when Load All Fields from Attribute is enabled

The cart line attribute key to read data from. Defaults to _update.

Load Price from Field

Shown when not loading from attribute

When enabled, the price value is read from a metafield, cart attribute, or other field instead of using a fixed value. The fixed price input is replaced with a field selector.

Supported field sources:

• Product Metafields — read price from a product-level metafield
• Variant Metafields — read price from a variant-level metafield
• Cart Line Attributes — read price from a line item property
• Standard Fields — use a built-in numeric field (e.g., quantity, unit price)

Price

Shown when not loading from attribute or field

Set a fixed price per unit for all items in the selected groups. Leave empty to keep the original price.

Title

Optional · Shown when not loading from attribute

Override the product title shown to customers. Leave empty to keep the original title.

Image

Optional · Shown when not loading from attribute

Select a custom product image. Leave empty to keep the original image.

Attribute-Based Configuration

For maximum flexibility, load all fields from a single cart line attribute containing a JSON object.

JSON Structure

{
  "price": "34.56",
  "title": "Custom Product Title",
  "image": "https://cdn.shopify.com/path/to/image.png"
}

All fields are optional — omit any field to keep its original value.

price

Optional

New price per unit. Must be a string representation of a decimal number (e.g., "29.99").

title

Optional

New display title for the cart line.

image

Optional

New image URL for the cart line.

Setting the Attribute

In Product Forms (Liquid Templates)

<input type="hidden" name="properties[_update]" value='{"price":"34.56","title":"Custom Product"}'>

Via Ajax Cart API

{
  "id": "12345",
  "qty": 1,
  "properties": {
    "_update": '{"price":"34.56","title":"Custom Product","image":"https://example.com/image.png"}'
  }
}

Through apps or scripts — set the property when adding items to cart programmatically. Useful for product configurators and customization tools.

Examples

Example 1: Fixed Price Override

Apply a promotional price to all items in a specific collection.

Example 2: Price from Variant Metafield

Use a custom price stored in a variant metafield.

Example 3: Dynamic Customization with Attributes

A customer personalizes a product with custom text and image.

Example 4: Price Mapping from Variant Metafield

Use a Variant Price Mapping block to apply dynamic pricing from a CSV metafield.

Best Practices

Pricing Strategy

• Use metafield-based pricing when different products need different prices without creating separate functions
• Use Price Mapping when you need quantity-based or market-based pricing tiers from a CSV metafield
• Set prices as string values in attribute JSON (e.g., "29.99" not 29.99)

Attribute-Based Loading

• Use a consistent attribute key across your store (the default _update works well)
• Validate JSON format before setting the attribute — invalid JSON is silently ignored
• Only include fields you want to change — omit fields to keep original values

Customer Experience

• Use meaningful titles so customers understand any price changes
• Provide updated images when the visual appearance of the product changes
• Test the checkout experience with updated lines

Important Considerations

Plan Requirements

• Update operations require Shopify Plus
• The block shows a warning banner if used on a non-Plus store

Cart Behavior

• Updated values appear immediately in the customer’s cart
• Original product data is preserved in Shopify — only the cart presentation changes
• Cart totals reflect the updated pricing
• Leave fields empty to keep original values

Price Mapping Precedence

• When a Price Mapping variable is selected, it overrides both static prices and field-based prices
• If the mapping doesn’t find a match for a given line, the line’s price is not changed

Function Flow Requirements

• Cart line groups must be defined before the Update Lines block
• Variant Price Mapping blocks must be placed before the Update Lines block if using price mapping
• Updates apply to the current cart session

Troubleshooting

Updates Not Applying

• Check your groups — make sure cart line groups are defined before the Update Lines block
• Verify plan — Update operations require Shopify Plus
• Check for empty values — empty fields are intentionally ignored (original value kept)

Price Not Changing

• Check Price Mapping — if a mapping variable is selected, it overrides static and field-based prices. Try setting the mapping dropdown to “No mapping” to test
• Verify field source — when using “Load price from field,” ensure the metafield exists and has a numeric value
• Check attribute format — when using attribute-based loading, price must be a string (e.g., "29.99")

Attribute-Based Loading Not Working

• Check JSON format — ensure the attribute value is valid JSON
• Verify attribute key — confirm the key matches what’s configured (default: _update)
• Check attribute is set — verify the property is actually present on the cart line
• Test individual fields — try setting only price first to isolate issues