class Post
  include Mongoid::Document

  embeds_many :comments
end

class Comment
  include Mongoid::Document

  field :content, type: String
  embedded_in :post
end

# query existing post
post = Post.with(collection: 'posts_2016').first
post.comments.build(content: '....')
post.with(collection: 'posts_2016').save!

# but save don't work,