Active Record includes with limit?

[bkoshy (Ben Koshy)]

hi all

Was wondering how you would handle this situation:

# psuedo code - to illustrate the concept
# not technically correct

* parent has_many: children

# # parents_controller.rb
def index
   @parents = Parent.all.includes(:children) 
end

<-- parents/index.html.erb -->
<% @parents.each do |parent| %>
   <%= parent.name   %>
   <%= parent.child.name   %> <-- How can I get the latest child only, given the above query?  -->
   <%#= parent.children.last.name   %> <-- I could drop the includes statement in the query and fire off an additional query like this in the view, but would prefer not to -->
<% end %>

• As I understand it, if a parent has 500 children, all 500 will be loaded in memory? Many parents may have many children. In this situation though, I would want only the latest child – or perhaps the latest 2 children for each parent – is there any way I can load only a few children in the one query and present them

Am wondering how you folks would handle this type of situation?

Ben

Appreciate your pointers.

[bhserna (Benito Serna)]

Hi Ben!

I have had a similar problem…

Maybe this two post can help you…

• 5 ways to fix the latest-comment n+1 problem
• 5 ways to fetch the latest-N-of-each record on Rails

There are different ways of doing it, and you have different options id you need just one vs more than one record.

I hope this can help you

[bkoshy (Ben Koshy)]

@bhserna Thank you! I will study your links

[akaspick]

has_many :children 
has_many :limited_children, -> { limit(3) }, class_name: Child

Parent.includes(:limited_children).each do |parent|
  # only 3 children (max) will be in the limited_children collection
  parent.limited_children.each do |child|
    # use child
  end
end

[bhserna (Benito Serna)]

As far as I understand, this will not work because it will fetch only 3 children in total, instead of 3 children per parent.

I had one example at hand and tried this…

class Post < ActiveRecord::Base
  has_many :comments
  has_many :latest_comments, -> { order(id: :desc).limit(3) }, class_name: "Comment"
end

class Comment < ActiveRecord::Base
  belongs_to :post
end

Post.includes(:latest_comments).limit(10).map do |post|
  post.latest_comments.each do |comment|
    puts comment.body
  end
end

And it actually printed just 3 comments… and execute this sql

Post Load (3.7ms)  SELECT "posts".* FROM "posts" LIMIT 10
Comment Load (35.8ms)  SELECT "comments".* FROM "comments" WHERE "comments"."post_id" IN (...) ORDER BY "comments"."id" DESC LIMIT 3

[akaspick]

Yep, you’re right. The limit appears to limit the total children overall, not per parent. I double checked here myself on my own example and it’s the same as you’re showing.

Now that I think about it, it makes sense it does it that way when eager loading.