---
title: Nested Comments with Ruby on Rails
slug: nested-comments-with-ruby-on-rails
published_at: 2021-08-27 14:00:18 +0000
updated_at: 2026-03-04 20:13:58 +0000
summary: 
description: In this episode, we&#39;ll improve our comment system to allow users to reply to existing comments in a thread. #rails #rubyonrails
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, nested comments, comments, ruby on rails, ruby on rails tutorial, comments tutorial, threaded comments]
views: 3807
author: CJ Avilla
url: https://www.cjav.dev/videos/nested-comments-with-ruby-on-rails
youtube_url: https://www.youtube.com/watch?v=mtIBFvWZuwI
youtube_id: mtIBFvWZuwI
embed_url: https://www.youtube.com/embed/mtIBFvWZuwI
thumbnail_url: https://i.ytimg.com/vi/mtIBFvWZuwI/hqdefault.jpg
type: video
---

# Nested Comments with Ruby on Rails

*Published: August 27, 2021*
*Views: 3807*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=mtIBFvWZuwI)

[![Nested Comments with Ruby on Rails](https://i.ytimg.com/vi/mtIBFvWZuwI/hqdefault.jpg)](https://www.youtube.com/watch?v=mtIBFvWZuwI)

## Description

In this episode, we&#39;ll improve our comment system to allow users to reply to existing comments in a thread.
#rails #rubyonrails

## Transcript

what&#39;s up welcome back let&#39;s keep jamming we&#39;re working through building a blog system with comments in the last couple episodes we went through building out a flat set of comments here and going through some performance improvements addressing the n plus one problem so if you haven&#39;t seen those videos head over and check them out in this episode you&#39;ll learn all about how to make nested comments so just like we have here on um on hacker news we&#39;ve got uh the ability to click reply and then you can sort of see an input box and reply to a specific comment and they could be net like deeply nested like this where we have several comments replying to another comment um so we&#39;ll look at how we can do that so let&#39;s get started so if we think about how we might make this reply work it sounds like or it seems like we might want to link here and when you click on the link it will display sort of the same form or input that we had below for adding a comment at the top level so we&#39;ll call this sort of the top level meaning like the left most oriented comments the left most aligned comments will be about the post itself those will be sort of like original comments or yeah top level comments and then any comment that&#39;s nested under it will need to actually keep track of which comment that comment is in reply to so in that case we kind of have like a parent-child relationship between some sub-comment and its parent and so for instance like this comment here is a the child of this comment and this comment is the parent of this comment and so when we&#39;re building out these associations if you look at this closely this this comment here can have many sub comments and so when we&#39;re thinking about how the data might be modeled i think what we want to do is store the id of the parent comment in the child comment that way we can have a child comment belongs to its parent and a parent comment can have many children comments so let&#39;s make that adjustment to our database first so that we can support nested comments so we&#39;ll come over here rails g migration add parent to comments so we&#39;re going to store it we&#39;ll store the id of the parent comment in a column called parent id so let&#39;s see what that looks like so we&#39;re going to jump in here we&#39;re going to say add column to comments we&#39;re going to call it parent id and if we think about whether or not this can be null it must be nullable because when you have a comment at the very top level it won&#39;t have a parent id and i guess that might be all we need technically we could enforce referential integrity and some other stuff and add foreign keys but we&#39;ll keep it simple and we&#39;ll just say we&#39;re adding a column called parent id it is an integer and it&#39;s nullable and yeah i think that&#39;s all we need rails db migrate now typically or throughout all of this entire application we have been following the rails convention where our association columns are named the correct way so that we can get away with using all of the rails conventions now in this case we need to have our comment belong to its parent now in order to belong to the parent we need to specify the type of the parent because we don&#39;t have a parent class right so when we say belongs to a user rails is assuming there&#39;s a class called user we say belongs to posts rails is assuming there&#39;s a class called post there is no class called parent so when we&#39;re saying that this comment belongs to its parent we need to say that the class name is parent or is comment so we need to specify the class name the other thing that we need to do is we we have to say if a comment has many comments or has many sub comments or something right so has many comments so this will be like all of the children comments that a specific comment has then again it won&#39;t know which column to come back to this comment class for so we need to specify the foreign key in order to derive the list of comments for the comment so here we can say foreign foreign key is parent id i think this might work i think this might work so let&#39;s go over to our rails console we&#39;re going to say reload and i think we ran our migration so we should be able to say like comment dot last and then say c dot comments so there&#39;s no comments that&#39;s great c dot parent that should be nil great now let&#39;s create a child comment so c dot comments dot create we&#39;re going to pass in post id i think we still want to have the post id even if it&#39;s a child comment um yeah i don&#39;t know that seems seems like a good practice and then we will also pass in the user id is for and the content is i&#39;m a child and we&#39;ll say create so we created a comment it has a parent so c c is i sees id is eight now this new comment is id nine but its parent is eight so this comment here i am a child has a parent comment eight so if we go back to our um to our view here and refresh you&#39;ll see that this comment is showing up at the top level it&#39;s not showing up nested under its parent so let&#39;s actually say c dot content is equal to like i&#39;m the parent comment c dot save yup c dot save oh okay so the parent it says the parent must exist okay so this is something that we have to do so in addition to saying it belongs to the parent we also need to say that um it&#39;s nullable um allow nil true i think is how we say it or no it&#39;s like optional true optional optional true free reload c is comment dot find 8 c dot content is equal to i&#39;m the parent comment c dot save bang okay so now c&#39;s content is i&#39;m the parent comment we come over here and refresh i&#39;m the parent comment we want this child comment to be nested underneath the parent comment so we&#39;ve got to figure out how to make sure that the comments that are showing up at the top level are those that don&#39;t have any parent id so let&#39;s make that adjustment so we&#39;ll come over here and actually let&#39;s also confirm that c dot comments okay so c.comments does have all the child comments as expected so that&#39;s great so let&#39;s come back over to our um post show and this is where we&#39;re actually running the query to pull out all the comments and i think what we want to do here is say like dot where parent id is nil or something and that should just give us just the top level comments so we should not see i&#39;m a child here anywhere and we don&#39;t and that&#39;s so that&#39;s great the next thing is that we want to go into our comment partial that we created in previous episodes here and in addition to printing out the article for this parent comment we also want to iterate over all the sub comments and sort of recursively use this partial so we can say render comment dot comments i think i don&#39;t know let&#39;s see all right so now we&#39;ve got i&#39;m a child printing out the the horizontal reference is a little weird because it&#39;s happening uh inside of that thing okay and then also like because this is the child nested under here we want some visual some sort of visual indication that this is happening underneath class or like st i don&#39;t know class it&#39;s like sub comment and we&#39;ll try to figure out how to write some css slash div all right so then if it&#39;s a sub comment then we want we want to like oh gosh you know what yeah okay so we&#39;re just going to do this and we&#39;re going to say inspect and we&#39;re going to find sub comment and we&#39;re going to say padding left is 10 pixels is that enough 20 pixels sure 20 pixels seems good um and then it&#39;s gonna get a little weird when we have multiple sub comments that are like styled relative to their parents or something i don&#39;t know we&#39;ll see how this goes so we&#39;re gonna go to comments scss so we have a style sheet here where we can write our own stuff so dot sub comment you probably haven&#39;t seen me write css because i don&#39;t i don&#39;t like css padding left is like 20 pixels so now if we refresh the page we&#39;ve got ourselves some nested comments that&#39;s pretty cool so let&#39;s figure out how we can actually add a reply button we&#39;re going to have an issue here because if we have if i&#39;m a child has it nested i think they&#39;re going to line up also to the left but we&#39;ll come back and fix that in a minute so let&#39;s add a reply button or link or something and when we click on the link we will see the form to add a comment we&#39;ve got a couple options one is we could take a naive approach and just put the form on every single comment so we could say render partial comments form locals is post comment.post um but now we have a we have the comment form on every single comment i&#39;ve seen blogs that have this it just looks exactly like this but it&#39;s not the greatest so let&#39;s go back to our form so now we have the form on every single comment i don&#39;t think that looks very good so what i want to do is actually go back over here to our comment partial and let&#39;s hide the form so a drift reply okay and then i think what we want to do is put this inside of a div that has like um class form or something like that or like comment form and we&#39;ll say comment form display none and then we&#39;ve got to figure out how to make this reply button so that when we click on it it shows the form so let&#39;s try to see if we can go in here and we can find the comment form and what we want to do as like part of our exercise is actually just uncheck that or like make it so it&#39;s displayed i&#39;m trying to figure out how we can actually make this work so class like comment form display all right so what we want to do here is when we click on the reply link it should show the comment box that is right next to it and so the first thing we want to do is grab reference to all the reply links so we can do document document.query selector all and we&#39;re going to pass in this comment form display and that&#39;s going to give us back a node list so this node list is all of the different links so that&#39;s all the different replies and with the node list we can say dot four each and we&#39;re going to call our our link maybe we&#39;ll call it el for our link tag and inside of here what we want to do is we want to add an event listener to each of the links so we&#39;re going to say el.add event listener for the click event and when the click event fires call this function that&#39;s going to take in the event object and the event object we want to say prevent the default because we don&#39;t actually want to like navigate anywhere so we&#39;re going to say ev.prevent default and then next what we want to do is we want to sort of find the next let&#39;s see we want to find the next element in the dom so here the comment form is the the tag that we want to find we want to show this comment form right now recall that it&#39;s display none so we want to set its display to block and so what we can do is we can look at the element so that&#39;s the link tags next sibling next element sibling i think and say dot style is equal to display block and i think that might actually make it so that when we click on this yeah it shows the form so now we see this display block is applied directly inline so that that is taking precedence over the class level display none so if we remove display none from the class level comment form that would show all of the the buttons but here we&#39;re just applying display block to that single elements that it&#39;s shown and here we can say like i am a child of test and when we submit it oh no this is appearing as a top level comment so let&#39;s head over to the database and see what&#39;s going on so if we say comment dot last oh gosh okay so we see parent id is nil that&#39;s because we didn&#39;t actually set the parent id when we were creating the comment so we&#39;ve got to do a couple things first of all inside of our comment form we need to pass down the parent id if there is a parent id so in this case what we might want to do is say let&#39;s create like a input type hidden or something that has the name comment parent id and the value is going to be parent.id so we&#39;re expecting that there&#39;s going to be some top level parent value that&#39;s passed into the form when the form is rendered in some cases this might not exist so what we might want to do is say if parent dot nil if if the parent id is not nil then render this so if the parent is not nil then we&#39;re going to pass its parent id otherwise we are going to omit this hidden field and then when we&#39;re on the comment partial we want to pass that parent id down as part of rendering the form here so we&#39;re going to say parent is comment because we&#39;re inside of the parent comment when we&#39;re rendering that form and this should make it so that when we show the form it displays that now what we&#39;re seeing here is there&#39;s no local variable or method parent for this action view and that&#39;s because at the top level the top level comment form that we passed in we did not specify any uh parent so back over here where we are rendering show we have this partial that we&#39;re rendering out for the form this one also needs the parent we&#39;re going to say the parent is nil if we refresh now we&#39;re back in business and if we look at this reply why isn&#39;t this working oh the javascript that we wrote we didn&#39;t actually do anything with it okay so let&#39;s copy this and let&#39;s paste it into our comment partial so we&#39;ll go down here and just say script all right just for now we&#39;re going to keep it here in practice you want to organize it nicely and stuff but we&#39;re just going to keep it quick and easy and if we refresh can we see it now okay so now it works cool so now if we say um i&#39;m a child of the child or something submit nope that didn&#39;t work okay so when we click on this this form let&#39;s inspect the form and make sure that it&#39;s actually getting the parent so it is getting the parent id here ah but what&#39;s happening is that um even though we&#39;re passing the parent id to the server as we can see in the server log i think let&#39;s find the post request so even though we&#39;re passing the parent id here nested under comment to our post request here for the comments we haven&#39;t implemented it in the controller we haven&#39;t like sanitized or permitted this parent id to be passed in in the controller so we need to go back to our comments controller and update this so that it also permits the parent id so now we can refresh this let&#39;s try again another try to make a child boom ah nice now we&#39;ve got a nesting going on here i&#39;m a child of the child another try to make the child all right let&#39;s go to another post here let&#39;s go to post 10 so we can start from fresh so this is this is the grandparent okay this is the parent that is a child of the grand parent submit okay now we&#39;re going to say this is a grandchild or this is a child of the parent and oh wow it is nested amazing okay cool so i was worried about the padding working deeply nested i&#39;m a baby or something okay cool so we&#39;ve got a parent grandparent whatever if we wanted to we can still reply i think to the grandparent this is another parent or something okay so now that one is nested underneath this is the grandparent but one thing that you&#39;ll notice uh if you&#39;re paying close attention is that number one um these comments are not ordered the same way as the top level comments so um this top level um comment will appear at the top wow i spelled so many things wrong okay and then um so this top level comment is the most recent comment it&#39;s appearing at the top but this is another parent was the most recent sub comment of the grandparent and that&#39;s happening because inside of our comment partial when we&#39;re rendering the comments here they are not order descending by id so the reason i use id descending here instead of like a created at or something is that the id field is indexed and it&#39;s faster to order by id instead of ordering by created at alright so that is how you build nested comments with ruby on rails we&#39;ve got this blog application now where you can you can create posts you can comment on other people&#39;s posts and you can thread those comments thanks so much for watching and we&#39;ll see in the next one [Music]

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