Snippet from the Advanced GraphQL workshop I run at OK GROW! - https://www.okgrow.com/graphql

subscriptions.js

this.props.subscribeToMore({
  // a graphql document (`subscriptions 
  document: LIST_SUBSCRIPTION_UPDATE,
  // how to update the cache when new data comes from the subscriptions
  updateQuery: (prev, { subscriptionData }) => {
    const { placeUpdated } = subscriptionData;
    
    if (!placeUpdated) {
      return prev;
    }

    const index = prev.places.findIndex(
      place => place.id === placeUpdated.id
    );
    
    // change the "updated place" in the list of places present in the cache
    return {
      ...prev,
      places: [
        ...prev.places.slice(0, index),
        placeUpdated,
        ...prev.places.slice(index + 1),
      ],
    };
  },
});