// This is how you could use Keystone's features to simplify
// managing the relationship. Posts have gained authors.

var Post = new keystone.List({
  autokey: { path: 'slug', from: 'title', unique: 'true' }
}).add({
  title: String,
  contents: keystone.Types.Markdown,
  author: { type: keystone.Types.Relationship, ref: 'User' },
  categories: { type: keystone.Types.Relationship, ref: 'Category', many: true }
});
Post.register();

var Category = new keystone.List().add({
  name: String
});
Category.relationship({ path: 'posts', ref: 'Post', refPath: 'categories' });
Category.register();

// we can populate categories on posts using mongoose's populate
Post.model.find().populate('categories').exec(function(err, posts) { /* ... */ });

// there's one more step, but we can use keystone's populateRelated to achieve a similar effect for posts in categories
Category.model.find().exec(function(err, categories) {
  keystone.populateRelated(categories, 'posts', function(err) {
    // ... you have categories with posts
  });
});

// if you've got a single document you want to populate a relationship on, it's neater
Category.model.findOne().exec(function(err, category) {
  category.populateRelated('posts', function(err) {
    // posts is populated
  });
});

// if you also wanted to populate the author on the posts loaded for each category,
// you can do that too - it uses mongoose's populate method because the relationship
// is stored in a path on the Post
Category.model.findOne().exec(function(err, category) {
  category.populateRelated('posts[author]', function(err) {
    // posts is populated, and each author on each post is populated
  });
});