onSave(): void {
   let variables = {};
   let mutation = {};
   let msg = "";

   if (this.model.id) {
      // update
      variables = {
         id: this.model.id,
         name: this.model.name,
         genre: this.model.genre,
         authorId: this.model.authorId
      };
      mutation = updateBookMutation;
      msg = "Book updated";
    } else {
      // create
      variables = {
         name: this.model.name,
         genre: this.model.genre,
         authorId: this.model.authorId
      };
      mutation = addBookMutation;
         msg = "Book Added";
   }

   this.apollo.mutate({
      mutation: mutation,
      variables:variables,
      refetchQueries: [{
         query: getBooksQuery
      }]
    }).pipe(
      map ( results => mutation === updateBookMutation ? results.data['updateBook'] : results.data['addBook'] )
    ).subscribe( ({ id, name }) => {
        console.log(`${msg}:\n -Id (${id}) \n -Name (${name})`);
      });
   this.reset();
 }