If your theme has built-in support for displaying ratings in product grid, you can use OnVoard to sync rating and rating count metadata. 

 

Auto sync product metafields

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Go to reviews config page and turn on Sync Metafields. If this is turned on, we will update product metafields whenever a linked a review is created/updated. 

 

Sync metafields for all products

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To sync metafields for all products, go to Shopify actions page and click Sync Metafields.