FeathersJs: Check if user is item creator or creator of associated item

Question

I have two services: posts and comments. 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?

Posts model

{
  userId: { type: DataTypes.INTEGER, allowNull: false },
  text: { type: DataTypes.TEXT, allowNull: false },
};

posts.associate = function (models) {
  posts.hasMany(models.comments, { onDelete: 'CASCADE' });
}

Comments model

{
  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);
}

Posts can only be deleted by the post-creator, so setField from the feathers-authentication-hooks does a good job in this case. But how do I qualify the post-creator to manage comments inside his post?