Commanded multi-entity aggregates

article.ex

defmodule Article do
  defstruct [:content, comments: []]
  
  # public commands
  
  def execute(%Article{}, %PublishArticle{content: content}) do
    %ArticlePublished{content: content}
  end
  
  def execute(%Article{}, %CommentOnArticle{} = comment) do
    Comment.new() |> Comment.execute(comment)
  end
  
  # state mutators
  
  def apply(%Article{}, %ArticlePublished{content: content}) do
    %Article{content: content}
  end
  
  def apply(%Article{comments: comments} = article %ArticleCommented{} = commented) do
    comment = Comment.new() |> Comment.apply(commented)
    
    %Article{article |
      comments: [comment | comments]
    }
  end
 end

comment.ex

defmodule Comment do
  defstruct [:comment]
  
  def new, do: %Comment{}

  def execute(%Comment{}, %CommentOnArticle{comment: comment}) do
    %ArticleCommented{comment: comment}
  end

  def apply(%Comment{}, %ArticleCommented{comment: comment}) do
    %Comment{comment: comment}
  end
end