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It looks like you found the solution to your problem using the Shopify GraphQL API. To get the default variant ID of a product, you can use the productVariants query with a filter on the product ID. Here’s the corrected version of your query that you found:
This query will return the first 10 variants of the product with the specified product ID (123456789) and the response will contain the IDs of these variants, from which you can extract the default variant ID. Here how you can generally structure your query to get all variants of a product using its ID:
This will return the list of variants for the product, including the default variant. You can then use the ID of the default variant to set the price or perform other operations.
Hope that helps anybody else trying to solve the same issue!