One neat trick that I think is relatively lesser known is that you can select arbitrary sql expressions in ActiveRecord and those values are made available on the instances.

(Also I think the above sql needs to be tweaked since you need the votes count grouped by comment not by post)

A one to many relationship in pure SQL is an awkward fit with a Rails app as it requires serializing (at least) the many as json. Then there's this weird conceptual gotcha where one resource is an AR instance and another is a pure hash.

I'd probably make a scope and association to help out here:

    class Comment
      scope :with_vote_count, ->{ joins(:votes).select('comments.*').select('count(votes.*) as vote_count') }
    end

    class Post
      has_many :comments
      has_many :comments_with_vote_counts, ->{ with_vote_counts }, class_name: 'Comment'
    end

    # in controller
    @posts = Post.includes(:comments_with_vote_counts).limit(3).order(:created_at: :desc)

    # in view/serializer, posts and comments are both AR instances
    @posts.each do |post|
      post.comments.each do |comment|
        comment.vote_count # => Integer
      end
    end

This should give you 2 queries, one to load the posts, then one to load the comments and vote counts for the relevant posts. Controller stays nice and slim and the complexity is delegated to sql via the join scope, without any other dependencies.

* edited for HN code block syntax

Django will do something similar (possibly a little more elegantly) if one is familiar with how to use the Prefetch APIs [1]:

    Post.objects.order_by("-created_at").prefetch_related(
        Prefetch(
            "comments",
            queryset=Comment.objects.annotate(
                vote_count=Count("votes")
            ),
        )
    )[:3]

This will generate the following two queries:

    SELECT
        "post"."id",
        "post"."created_at",
        "post"."title",
        "post"."content"
    FROM "post"
    ORDER BY "post"."created_at" DESC
    LIMIT 3;

    SELECT
        "comment"."id",
        "comment"."post_id",
        "comment"."content",
        COUNT("vote"."id") AS "vote_count"
    FROM "comment"
    LEFT OUTER JOIN "vote"
        ON ("comment"."id" = "vote"."comment_id")
    WHERE "comment"."post_id" IN (3, 2, 1)
    GROUP BY
        "comment"."id",
        "comment"."post_id"

[1] https://docs.djangoproject.com/en/4.1/ref/models/querysets/#...

Personally, I find the Rails version a bit more elegant because it is declarative, reusable and composable, while Django's approach is a more utilitarian "just write the dang query when you want it." But both are a great illustration of how a well-designed ORM can give you the tools you need to get good performance.

You can do the same kind of thing in Django. The approach is a little different, but it's the same basic idea.

https://docs.djangoproject.com/en/4.1/topics/db/managers/#cu...

> To be clear, I'm not anti-ORM and use them all the time, but it really helps to understand SQL well when using them and to know when it's appropriate to switch to SQL.

When I did web development, I saw it as a "hack" and a "failure to write clean code" whenever I reached for raw SQL. This is of course not true at all, but it was a powerful psychological blocker and I'd spend too much time trying to figure how to get the ORM to do what I wanted instead of writing the SQL myself and moving on to the next problem.

> but it really helps to understand SQL well when using them and to know when it's appropriate to switch to SQL.

I agree. I often feel like I benefited by starting my web career pre-ORM and only learning to use them a few years in, so I can appreciate and use both. I sometimes wonder if it’s harder for new devs to acquire the same kind of experience.

It's pretty straightforward in Django. The key is being comfortable with writing custom Manager/QuerySet methods.

You could do something like `Post.objects.latest().annotate_comments()` which would resolve almost exactly to the query you wrote above.

Using a typical set of Post & Comment models, where Comment has a foreign key to Post, I couldn't figure out how to do this with just a single query in Django. Using prefetch_related, the 2-query version is pretty straightforward:

    from django.db.models import Count, Prefetch
    from myproject.models import Post, Comment

    # This will fetch the 3 posts first and then the comments for those posts
    query = Post.objects.prefetch_related(
        Prefetch("comments", queryset=Comment.objects.annotate(Count("votes")))
    )
    posts = query[:3]

How would you reduce this to one query using the Django ORM?

The previous query uses a JsonAgg to collect the comments