Apollo Query Reducer

reducer.js

const CommentsPageWithData = graphql(CommentsPageQuery, {
  props({ data }) {
    // ...
  },
  options({ params }) {
    return {
      reducer: (previousResults, action, variables) => {

        let newResults = previousResults;

        // check if a document belongs to the current query
        const belongsToQuery = (documents) => { /* ... */ }

        // check if a document already exists in a results object
        const existsInResults = (results, document) => { /* ... */ }

        // remove a document from a results object, used by edit and remove cases below
        const removeFromResults = (results, document) => { /* ... */ }

        // add document to a results object
        const addToResults = (results, document) => { /* ... */ }

        // reorder results
        const reorderResults = (results) => => { /* ... */ }

        switch (action.operationName) {

          case "new":

            // get new document
            const newDocument = action.result.data["new"];

            // if new document belongs to current results, add it
            if (belongsToQuery(newDocument)) {
              newResults = addToResults(previousResults, newDocument);
              newResults = reorderResults(newResults);
            }

            break;

          case "edit":

            // get edited document
            const editedDocument = action.result.data[editMutationName];

            // see if it still belongs to the current results
            if (belongsToQuery(previousResults, editedDocument)) {

              // if document wasn't already in results, add it
              if (!existsInResults(previousResults, editedDocument)) {
                newResults = addToResults(previousResults, editedDocument);
              }

              // reorder results
              newResults = reorderResults(newResults, options.sort);

            } else {
              
              // if edited doesn't belong to current list anymore, remove it
              newResults = removeFromResults(previousResults, editedDocument);
            
            }

            break;

          case "remove":

            // get removed document
            const removedDocument = action.result.data[removeMutationName];
            
            // remove it from results object
            newResults = removeFromResults(previousResults, removedDocument);
          
            break;

          default: 

            // default case: just return previous results unchanged
            return previousResults;
        
        }

        // copy over arrays explicitely to ensure new sort is taken into account
        return {
          [listResolverName]: [...newResults[listResolverName]],
          [totalResolverName]: newResults[totalResolverName],
        }

      }
    };
  },
})(CommentsPage);