gistfile1.rb

BlogsDatasource.new(Blog.all)
.select(:id, posts: { scope: Post.all, select: [:id, :author_name] }).results
# (0.1ms)  SELECT blogs.id FROM "blogs"
# (0.2ms)  SELECT posts.id, posts.blog_id, (posts.author_first_name || ' ' || posts.author_last_name) as author_name FROM "posts"  WHERE (posts.blog_id IN (1))

PostsDatasource.new(Post.all)
.select(:id, :title, :author, :author_name).results
# (0.1ms)  SELECT posts.id, posts.title, posts.author_first_name, posts.author_last_name, (posts.author_first_name || ' ' || posts.author_last_name) as author_name FROM "posts"