How to get Metafield for all variants of a product

[CFashion]

I have a software platform that creates products on Shopify with Rest API calls.
The variants of these products have a metafield 'supplierRef' in the 'descriptor' namespace.
When I create a product with variants with that metafield it works fine.
When I try to update an existing product variant I get an error because the metafield I try to push already exists.
When creating the product the variants in the body response do not have the metafield data attached to them.

How can I get the metafield Id of all the variants with a Rest API call?

Or how can I make the create/update call return the metafield data?

[francesco_gs]

Hi there!
in order to update a metafield on a resource you will need to pass the metafield_id along with metafield info (namespace, key, type, value).

In order to retrieve this info, you will need to ask for resource' metafields with the /metafields.json endpoint, so in your case 

GET /products/{{product.id}}/variants/{{variant.id}}/metafields.json

Unfortunately the create call cannot return the metafields data.

I strongly suggest you to switch to GraphQL for this kind of API operations, it is a definitely more efficient and powerful method.