Mutation and update cache

Open components/Todo/TodoPrivateList.js and import useMutation React hook and add the bulk delete mutation.

components/Todo/TodoPrivateList.js

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  import React, { Fragment, useState } from "react";
+ import { useQuery, useMutation, gql } from "@apollo/client";

  import TodoItem from "./TodoItem";
  import TodoFilters from "./TodoFilters";

const GET_MY_TODOS = gql`
  query getMyTodos {
    todos(
      where: { is_public: { _eq: false } }
      order_by: { created_at: desc }
    ) {
      id
      title
      created_at
      is_completed
    }
  }
`;

+  // Remove all the todos that are completed
+  const CLEAR_COMPLETED = gql`
+    mutation clearCompleted {
+      delete_todos(where: {is_completed: {_eq: true}, is_public: {_eq: false}}) {
+        affected_rows
+      }
+    }
+  `;

Use the useMutation React hook and update the clearCompleted function as below

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const filterResults = filter => {
    setState({
      ...state,
      filter: filter
    });
  };

+ const [clearCompletedTodos] = useMutation(CLEAR_COMPLETED);

clearCompleted() {
+  clearCompletedTodos({
+    optimisticResponse: true,
+    update: (cache, {data}) => {
+      const existingTodos = cache.readQuery({ query: GET_MY_TODOS });
+      const newTodos = existingTodos.todos.filter(t => (!t.is_completed));
+      cache.writeQuery({query:GET_MY_TODOS, data: {todos: newTodos}});
+    }
+  });
}

That's a wrap of the todo app.