title: Getting Error: "Mutations" defined in resolvers, but not in schema
body: <p>I am learning graphQL. Could somebody help me understand why Mutations are not being identified in Schema?</p>
  
  <p>Getting this error: </p>
  
  <pre><code> Error: "Mutations" defined in resolvers, but not in schema
  </code></pre>
  
  <p>Following is the code:</p>
  
  <p>Schema: </p>
  
  <pre><code>const typeDefs = gql`
   type Author {
   age: String
   name: String
   books: [String]
  }
  
  type Query {
    authors: [Author]
    author(id: String): Author
  }
  
  type Mutation {
    addAuthor(name: String, age: Int, books: [String]): Author    
  }  
  `;
  
  const schema = makeExecutableSchema({ typeDefs, resolvers });
  export default schema;
  </code></pre>
  
  <p>Resolvers:</p>
  
  <pre><code>const resolvers = {
  Query: {
      authors: () =&gt; {
          // return authors;
      },
      author: (root, { id }) =&gt; {
          // return authors.find(author =&gt; author.id === id);
      }
  },
  
  Mutations: {
      addAuthor: (root, {name, age, books}) =&gt; {
          const author = new authorModel({name, age, books});
          return author.save();
      }
    }
  }
  
  export default resolvers;
  </code></pre>