title: FeathersJs: Check if user is item creator or creator of associated item
body: <p>I have two services: <code>posts</code> and <code>comments</code>. Every post can have multiple comments. User 1 creates a post, user 2 creates a comment inside this post. Both users should be able to update, patch or delete the created comment. How do I correctly qualify both users to modify the comment created by user 2?</p>
  
  <p><strong>Posts model</strong></p>
  
  <pre class="lang-js prettyprint-override"><code>{
    userId: { type: DataTypes.INTEGER, allowNull: false },
    text: { type: DataTypes.TEXT, allowNull: false },
  };
  
  posts.associate = function (models) {
    posts.hasMany(models.comments, { onDelete: 'CASCADE' });
  }
  </code></pre>
  
  <p><strong>Comments model</strong></p>
  
  <pre class="lang-js prettyprint-override"><code>{
    userId: { type: DataTypes.INTEGER, allowNull: false },
    postId: { type: DataTypes.INTEGER, allowNull: false },
    text: { type: DataTypes.TEXT, allowNull: false },
  };
  
  comments.associate = function (models) {
    comments.belongsTo(models.posts);
  }
  </code></pre>
  
  <p>Posts can only be deleted by the post-creator, so <code>setField</code> from the <code>feathers-authentication-hooks</code> does a good job in this case. But how do I qualify the post-creator to manage comments inside his post?</p>