Question

Simple task: given that an article has many comments, be able to display in a long list of articles how many comments each article has. I m trying to work out how to preload this data with Arel.

The "Complex Aggregations" section of the README file seems to discuss that type of situation, but it doesn t exactly offer sample code, nor does it offer a way to do it in two queries instead of one joined query, which is worse for performance.

Given the following:

class Article
  has_many :comments
end

class Comment
  belongs_to :article
end

How can I preload for an article set how many comments each has?

Answer 1

You can do something nasty using SQL like:

default_scope :select =>  articles.*, (select count(comments.id) from comments where comments.article_id = articles.id) as count_comments

and then you would have access to Article.first.count_comments.

Another (nicer) method to do it is to use the counter_cache feature/option from belongs_to association.

Answer 2

Can t you use counter cache for this?

belongs_to :article, :counter_cache => true

You also need to have a migration that adds the column comments_count

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