Firestore -- Cart Margin Value (sGTM Variable)

Overview

This Server-Side GTM variable retrieves product margin data from Google Firestore, calculates the total cart margin from event items, and returns the result as a string.

Supports:

• Custom (non-Shopify) setups
• Shopify integrations
• Multiple calculation methods
• Automatic fallback when data is missing

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How It Works

1. Reads items from event data
2. Fetches product data from Firestore
3. Calculates margin per item
4. Applies fallback if needed
5. Returns total cart margin (rounded to 2 decimals)

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Firestore Structure

Non-Shopify

Path

<collectionId>/<productId>

Example document

{
  "margin": 12.5,
  "return_rate": 0.1
}

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Shopify

Path

shopify/shops/shop_<SHOPIFY_ID>/collections/products/<item_id>-<variant_id>

Margin logic

price - unitCost

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Configuration

Required

• Data Source: Other or Shopify
• Shopify ID (if Shopify selected)
• Collection ID (if not Shopify)

Optional

• GCP Project ID (otherwise uses GOOGLE_CLOUD_PROJECT)
• Custom item field names
• Fallback percentage

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Calculation Modes

Value

margin × quantity

Return Rate

(1 - return_rate) × margin × quantity

Value with Discount

(margin - discount) × quantity

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Fallback Behavior

If a product is not found in Firestore:

fallbackPercent × price × quantity

Default value

0.5 (50%)

The value must be between 0 and 1.

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Event Requirements

Example event structure

{
  "items": [
    {
      "item_id": "123",
      "price": 100,
      "quantity": 2,
      "discount": 0
    }
  ]
}

Field names are configurable in the template.

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Return Value

• Returned as a string
• Rounded to 2 decimals
• Represents the total cart margin

Example

"42.37"

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Use Cases

• Profit-based bidding
• Server-side ecommerce tracking
• Margin reporting
• Advanced attribution models