---
title: Polymorphic associations with Ruby on Rails - like all the things
slug: polymorphic-associations-with-ruby-on-rails-like-all-the-things
published_at: 2021-09-01 14:00:07 +0000
updated_at: 2026-03-04 20:13:58 +0000
summary: 
description: In this episode, you'll learn how to migrate from a one to many association to a polymorphic association. Imagine you've implemented a like button for a blog post. You're tracking whether a user has liked a post or not using a Like model related directly to the post. Now you want to also support \"liking\" other objects: comments, images, friends, etc. In this episode, you'll see how to modify the database to support these polymorphic associations between a Like model and any other \"likeable\" model. #rails #rubyonrails
tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, ruby on rails, ruby on rails tutorial, poly, polymorphic, polymorphic association, polymorphic associations, polymorphic rails, polymorphic associations rails, polymorpic associations with ruby on rails, likes, adding like buttons]
views: 5972
author: CJ Avilla
url: https://www.cjav.dev/videos/polymorphic-associations-with-ruby-on-rails-like-all-the-things
youtube_url: https://www.youtube.com/watch?v=5EidzTqsw-E
youtube_id: 5EidzTqsw-E
embed_url: https://www.youtube.com/embed/5EidzTqsw-E
thumbnail_url: https://i.ytimg.com/vi/5EidzTqsw-E/hqdefault.jpg
type: video
---

# Polymorphic associations with Ruby on Rails - like all the things

*Published: September 01, 2021*
*Views: 5972*

## Watch

[Watch on YouTube](https://www.youtube.com/watch?v=5EidzTqsw-E)

[![Polymorphic associations with Ruby on Rails - like all the things](https://i.ytimg.com/vi/5EidzTqsw-E/hqdefault.jpg)](https://www.youtube.com/watch?v=5EidzTqsw-E)

## Description

In this episode, you'll learn how to migrate from a one to many association to a polymorphic association. Imagine you've implemented a like button for a blog post. You're tracking whether a user has liked a post or not using a Like model related directly to the post. Now you want to also support "liking" other objects: comments, images, friends, etc. In this episode, you'll see how to modify the database to support these polymorphic associations between a Like model and any other "likeable" model.
#rails #rubyonrails

## Transcript

hey what's up welcome back in this episode we're going to talk a little bit about polymorphic associations and that's a super fancy word for saying we're going to make objects that can be related to lots of different kinds of objects instead of just one like sort of a one-to-one relationship between class and another class so in this episode we're talking about how we can extend our likeability so we have a this this application here we have this blog app where we can like and unlike the blog post and we're going to sort of track the number of likes but we want to also be able to like a comment or like a reply to another comment right and so we want to be able to extend the capabilities of a like to apply that to other objects in our system maybe we want to be able to like a photo or we want to also be able to like a comment in like a blog post and so one way that you might do this is with polymorphic associations and so let's talk about that for a second and actually let's look at our our schema our database schema right now so right now we have this like model and inside of our table for a like we have a relationship to the user who liked the object and then we have a direct relationship to the id of a post now this is a specific kind of thing that we could like right and in practice what you might do is go like if you if you wanted to add a feature for liking a comment one thing you might do is go create a separate table called like a comment likes a comment likes table or something and that would have relationship between a user id and a comment and then you might go create another table that's for images and it's called like image likes and it has relationship between a user id and an image well if you you know as your application starts to grow if you have features around a like so maybe you like is carrying some weight with it or you have some methods that are used to calculate different things about a like now you have to start to copy all of that logic across your like model your comment-like model and your image-like model and it makes it so that you're you're really kind of complicating the system when in reality the same action and same behavior and the same sort of like data that we're storing under the hood is really the relationship of a user id and the thing that they're liking now if we were to just store all of our likes for all the different objects in one database table like maybe if we consider changing this this column to like the thing that we liked uh the thing that we liked id right so maybe if we have a post with id 11 and the user like that will have you know the user's id and 11 in the database and we'll know that the user id liked post 11. now if we have an image and its id is 44 we might have the user id again and then the number 44. and if you're jumping ahead and you have the intuition right you might realize that it's possible that you have an image object whose id is 44 and later you have a post object whose id is also 44. and so in that case you would run into a conflict where when you try to ask what this like is related to you wouldn't be able to determine determine is it liking the post or is it liking the image or is it liking the comment with that same id right and so what we do is we're going to add another field to the database here so it's going to be another uh it's actually going to be a string and it's going to be called thing that we liked type so we want to track the type of the thing that we like so this is going to be a string value that's going to be the class name for the thing we like so it might be a post or it might be an image or it might be a comment okay so the thing that we liked it's we need to track both the id for the thing and the type of it and by doing this we know that uh if we're storing the current user's id and the id of the post so let's say it's 11 and the type as post then we know that we can query the posts table where the post id is 11 and we would get back this like for uh for that post so this this gives us the ability to uh track multiple different objects that might have the same id but we know which type we're talking about here so by adding these two fields here we can create polymorphism now in practice we don't actually we don't actually call these thing that we liked underscore id or thing that we like to underscore type instead there's a convention that we're going to call this likable type and likable id likable id now you also might notice that down here under indexes we have an index on the post id we have an index on the user id and post id being unique that was so that we could only like the post one time we also have an index on the user id so we actually need to go in and change this index because if we start adding uh ids or if we like just change the name of post id everywhere to likable id if you imagine like if we um retain this same uniqueness constraint and index on this pair then we're going to run into an issue because we won't be able to like an object of a different type that has the same idea of something that we've already liked so for instance if this was likable id and we've already liked a post with id9 and then we went to try to like a comment with id9 it would fail because we actually need this to now be a um a throuple or like a three things with likable type and likable id here or yeah likable id exactly and so we want to have the uniqueness constraint now on all three of those so that we can't re-like the same thing multiple times whatever this is super long-winded to just sort of like set you up and prep you to think about what changes we're going to make to the database here with the migration so we're going to say rails g migration make likes polymorphic and we'll talk about all the different rails things that we need at the model layer but i first want to get the database right okay so let's jump in here to this migration the first thing we want to do is rename that post id column on the like on the likes table so we're going to say rename column on the likes table from post id to likable id that's going to be the very first change that we make the next thing that we want to do is we want to remove the index on likes where we had a pair of user id to to post id but it's going to be renamed to likable likeable id so we're going to remove that index we also now need to add a new column so we're going to say add column to likes for the likable type right and this is going to be a string we also want to now re-add a new uniqueness constraint index for the combination of all three of those so we're going to say add index to likes for user id likable id and likable likable type and this is going to be unique true now one other thing that we might do a lot is look up like count all the likes for all posts or count all the likes for all images or comments or something so we also let's also add an index for uh likes for the combination of likable and likable type likeable id and likable type and so yeah that should be helpful we don't need that to be unique but we do need to probably have that in there for for speed for speed okay so let's see if this works so rails db migrate very cool okay so we've got we've got a bunch of changes that we made to our schema now let's go into that like object itself and make the changes that we need to here so everywhere that we're talking about posts that now needs to turn into likable something about likable so the very first thing that we have here is a validation that validates that the user id is unique in the scope of the post id which is now going to be likable id and we also need to make this in scope of the likable type so we can we can make that pair here so it's like likable id and likable type and then instead of belonging to a post here we're going to say it belongs to a likable polymorphic true so on the belongs to side of the association this is the trick to getting this to work so we have a belongs to likable which is polymorphic true that is that is part of the trick to get this part working now on the other side of the association anywhere that we were already using like so in this case on the post object we need to go over to the post model and here we're going to say has many likes but instead of just saying like uh just leaving it as just likes we now need to say as uh likable so that it knows which association on the other side is coming back to this one and that it's polymorphic i think that should give us something to play with here so if we open up the rails console um we should be able to say uh like that last i don't know let's see if this has uh so it has a likable id let's actually do like dot update no like dot destroy all we'll just remove all the likes from the database and then um let's do uh like dot create so we're gonna create a new like with um p equals post.last and we'll say like.create for likable is the post and user is you and that created okay so here we go we're saying like dot create and we're passing in the likable instead of passing in post now we need to say likable because now it's just like a generic thing that we can add a like to and in this case it happens to be a post object and then we also need to pass in the user and here uh down below we'll see the actual like that came out and we can see the likable id and so if we look at the p's id is 11 so we've got the id correct and now look at the likable type that is like the string representation of the class that uh that is liked so if we go to like dot last um that's what we're gonna find we can say like like dot last dot likable and that gives us back the post object let's also try to go like p dot likes and that should give us back the like that we just created and that looks that looks correct oh let's also confirm that our validation is working so we'll try to create another one with the same post in the same user and this should fail and it did and it rolled back and that's great so let's actually go like dot new um like dot new and we'll say l dot errors dot full messages and what l dot valid and then errors full messages user has already been taken again this error message whatever we can go fix it later sometime okay so i think we've got our database layer we've got our model layer let's move up to the controller and keep working our way towards the view so in the controller for the likes controller here before we were using post in a bunch of places we don't want to use post anymore now we want to use likable what i'm going to do is actually just do a find and replace for post and i'm going to convert that to likable and that should convert a bunch of stuff for us but now we have to go through sort of the fine tooth comb here so we're going to create the the new like using like params which right now we're just passing in the likable id but we also need the likable type in order to to make that successful um because i think if you think about it right like when we're receiving the post request to create the like we don't actually have reference to the comment or the post or the whatever we just have the id it that's like serialized it's just that's it's gonna be a string value for the integer id that is um the object that we're trying to like and we'll also need to pass in the type so that we can make sure that we're liking the post and not the comment when we pass in the number one or something okay and then we can redirect to likable that makes sense we're going to find the like and destroy it and we're going to redirect back to the likable great i think all this looks good let's go into the view now so we're making our way all the way to the front end and we'll go to our post show view and here we had a couple of forms and we also talked about button two and why my preference is to use a form like this so here we're going to say that we need to pass the likable the likable id and this in this case it is still going to be at post.id because that is the instance variable for the post object and that is the id that we do indeed want to send as the likable id but we need to add another hidden field that is the likable type and the value for the likable type is going to be post just like that okay and then our delete method should work exactly the same i think um i don't know we'll see uh the other thing we need to change here is i guess okay so uh when we're looking up the like and we're saying currentuser.likes.findby instead of post here i think we need likable right yeah we need to pass in the likable so we can look up the actual the actual like object all right let's restart our server all right we've got an unlike button we can unlike it and then we can like it again we can unlike it that looks so good i am i'm really pumped about that let's check too if we like it uh from one user and we log out and we say wave and we log back in as another user and we go to slash i think it was post 11. yeah so there's one like here already if we like it again now there's two likes so that's all working totally as expected and um yeah so that's how you set up polymorphic associations that's how we migrated our like object to a polymorphic association so again we'll look at the schema and talk about all the different changes here real quick so now we have that likable id and we have a likable type instead of just having the post's id all right so now we've got it set up we've sort of refactored to have this polymorphic association between posts and likes now let's try to apply this to comments so let's see if we can just take our take our likable concept here and move it directly into comments so let's do a little bit of refactoring here what i think i want to do is actually move this like button into a partial so we'll go to our post show again and let's grab all of this content about the likable liking and unliking and we'll move this out into a new view inside of the likes controller or sorry the likes view so we're going to say likes and in here we're going to add i don't know button.html.erb and we'll grab all of this content for that button and we'll paste it in here now one thing that we're going to need to change is instead of passing in at post or uh you know the the post specific type here we're gonna need to receive those as arguments into this partial so that we can re-render the partial uh as part of other types so here we're gonna just we'll just make this uh likable and then down here we can make this likable dot id and i think here we can do um whoops likable.class dot 2s i think that should work okay so now we need to pass in the likable when we are when we're rendering this partial so here we're going to say render partial likes button and locals is going to be likable at post okay so let's see if that continues to work as expected okay undefined local variable or method likable that should have worked oh i didn't spell it right okay so now we've got unlike and we've got like and those are still working let's also move this likes count inside of the partial so right above the button we will just put in the the count of the likes so we're going to say likable.likes.count and then we can remove that from there all right so now we've got this sort of self-contained button that has a counter right above it and now what we want to do is sort of apply this same control to each comment so we can track the likes on a comment so it should be as simple as taking this partial and moving it into the comment into this comment partial and maybe right after the content we have this reply thing and then maybe we also add a button to like and unlike just right after that so let's just drop this down here and then refresh the page and instead of passing the likable in as the post we want to pass it in as the comment okay so we're going to refresh and right now it's saying undefined method likes for comment that's because we haven't set up the associations so we can go into our comment model here and say this has many likes uh as likable now if we refresh again now we have zero likes and we see this like button and we can or we should be able to like the uh we should be able to like the button let's see um okay so foreign key constraint likable id is not present in table post so we've got a foreign key constraint between the likes table and the posts uh table from an old association that we set up so now if we go into our um to our schema here and take a look at our likes table somewhere in here we still have a foreign key okay so here we have a foreign key between the likes and the posts so we actually need to remove this foreign key so that we don't uh so that we are not breaking referential integrity so let's let's do a remove foreign key in a migration so rails g migration remove foreign key from likes to post and i believe it should just be remove foreign key from likes to post maybe posts let's see for maybe i don't know if i have to specify the column or not let's see ray railsdb migrate okay and then let's refresh the page oh okay so here we're trying to redirect back to the likable but this likable in this case undefined method comment url for likes controller so uh so we're gonna go into our likes controller and let's actually just say redirect to our redirect back redirect back and we will say fallback i think it's fallback let's look this up redirect back rails um it used to be redirect to back but now it's redirect back fallback location is going to be like posts url or something just so that we have some sensible fallback url okay great so now we are able to like this comment we can unlike the comment again we need to redirect back to something that's not just exactly that okay okay so we're going to go back to posts 11 refresh the page so this has a like we're going to like it again it has two likes we're going to like this top comment now it has one like we can unlike it now it has zero likes and now we can like each comment separately um we can unlike the post so now this is our now our like functionality is working and it's applicable to both posts and to comments so hopefully that was helpful cool so that's uh polymorphic associations thank you so much for watching i really appreciate your time and attention if uh if you liked this this whole episode about likes give me a like i'd really appreciate it hit that thumbs up and then yeah we'll see you in the next one cheers [Music]

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