--- title: Performance basics: N+1 and memoization with Ruby on Rails slug: performance-basics-n-1-and-memoization-with-ruby-on-rails published_at: 2021-08-26 13:00:09 +0000 updated_at: 2026-03-04 20:13:59 +0000 summary: description: In this episode, we'll address two of the lowest hanging fruit performance issues you'll encounter when developing a Rails application: N+1 queries and basic caching with memoization. #rails #rubyonrails tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, rails performance, performance, n+1, n+1 queries, n plus one, n plus 1, memoization, memorization] views: 3332 author: CJ Avilla url: https://www.cjav.dev/videos/performance-basics-n-1-and-memoization-with-ruby-on-rails youtube_url: https://www.youtube.com/watch?v=FInC_2qV3mc youtube_id: FInC_2qV3mc embed_url: https://www.youtube.com/embed/FInC_2qV3mc thumbnail_url: https://i.ytimg.com/vi/FInC_2qV3mc/hqdefault.jpg type: video --- # Performance basics: N+1 and memoization with Ruby on Rails *Published: August 26, 2021* *Views: 3332* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=FInC_2qV3mc) [![Performance basics: N+1 and memoization with Ruby on Rails](https://i.ytimg.com/vi/FInC_2qV3mc/hqdefault.jpg)](https://www.youtube.com/watch?v=FInC_2qV3mc) ## Description In this episode, we'll address two of the lowest hanging fruit performance issues you'll encounter when developing a Rails application: N+1 queries and basic caching with memoization. #rails #rubyonrails ## Transcript what's up welcome back in the last episode we built out a commenting system for our blog application and at the end we left it with a little bit of a cliffhanger here for a couple different performance issues that we have going on now this is one of the most common pain points that you'll see as a rails developer getting started is as you gain traction with your little applications you'll notice that you start to see tons of queries in your server logs that are all firing off for what seems to be the same exact thing and these queries start to add up especially if you have hundreds and hundreds or thousands of comments in this case these can add up and this particular issue is called an n plus one query n plus one meaning we have one query at the top where we're fetching a list of comments and then we have n queries that are happening for each one of those comments so in our case the one is really kind of like the blog post and then the many is every for every single comment on the blog post we're firing off another query sometimes this can get really bad if you have each of your partials are firing off several queries and you're not loading all the data and so the reason this is happening is if we look at our com at our comment partial and i kind of did this intentionally but if we look at our comment partial here this method where we're rendering out the comment we're calling comment.user.email and so when we say comment.user that is accessing an association and if this comments user is not already loaded then this is going to fire a database query to retrieve that user object and when it retrieves that user object then it will know its email address so like by default when we load all the comments that does not know the email address for the user because the email address is in a different table than the comment content right so in the comment table we have the id for the user but we don't have any of the other information about the user we don't have the email address the password digest or the name or any of that and so when we go to render our list of comments what we want to do is we want to print out the email address and so the way around this with rails is by doing what's called eager loading or using includes to uh to to proactively pre-load a bunch of data into memory before the view is rendered so that there are not a bunch of subsequent um requests that are fired so rails can be smart when you're here when you're actually calling this top level um request to fetch all the comments we're also going to fetch all of the users that are related to those comments so here we can say dot includes and pass in user by just adding this one tiny bit we are improving the performance immensely so let's add that in we'll go back to our view and we'll refresh this page again and now if we go back to the terminal so this was the this was the old request where we can see all of this like select users from user or select star from users where user id is and then the id is always sort of four until down here when it was nine so this was like the first five comments were for user four and then the last two were for user nine right so this is the n for our n plus one query now if we go down here we only see two that fired off um one was for current user and then one was for our for our association so here you can actually see in uh instead of saying select star from users where user id equals above we had equals here it was saying equals nine equals you know nine or four down here now we have an in statement an in sql statement where we're going to pass an array of ids so for each of the comments it's going to go through all the comments figure out which user ids we need to look up pass those as an array for a one-time query to the users table it'll like preload all that data in memory so that we can then very quickly render a bunch of comment partials so now if we come back here this view is now rendering really quickly let's do one other performance improvement and that is that when this page is loading we can actually see quite a few of these select star from users for this like current user method so here we can see this is like the query that's firing for application controller line 12 that's current user if we come down here we see the same thing application controller current user now these are like they're hitting a cache so it's loading insanely quickly but we want to like actually never hit the database more than once here so what we can do is go back to our application controller and go to line 12 now what we were doing before is we were just saying user.find session user id so we're looking up the user by the cookie right but every time we use this current user method which is several times throughout the request we are going to be firing a database query right so we're firing that here ensure current user so this is this is happening before you load the post page because you have to be logged in now then if we go to the application html erb template here we're checking if there's a current user and if there is then say hi and call current user.email yeah we're given the logout button otherwise we're not going to show anything so we're calling this current user method in a few places we can improve the performance of this using a tool called memoization so memoization basically just means like let's cache the variable here locally so that if we receive the current user method once the very first time we actually fire the database query and store it in a variable but if we receive that method again instead of going to the database we're just going to return the variable that we already have so let's look at what that looks like so here what we can do is we can say at current user is equal to user.find whatever whatever right and then at the beginning of this method what we could do is say like if there's a current user then return at current user right so at the very beginning the very first time we call current user this instance variable is not going to be defined so this will return nil and so then we should continue on down here and we'll set the current user using user.find and then that'll fire the database query the first time and then the second time this is called current user will be populated because we stored it from the last database call so current user will be populated and then we're going to return that and we won't finish the the rest of this method here so we're going to execute just that return statement returning the data that we got the very first time there's actually like a much better shortcut here that we can use and that is to say current user or equals user.fine so this is going to say like if there's a session id so if we've cookied the user then return either some value that exists in the current user instance variable so if we have something in current user then return that otherwise we're going to so the or statement is going to run and say like if the first value was falsy so if the first value was nil then we're going to execute the right hand side of the or statement in this case it has an equal sign so it's going to execute the database query store whatever the result of that is back into this instance variable and then the next time this is called we're going to return that instance variable again so this is like when you see this or equals thing you can recall that this is like memoization another way to think about it is like remembering something or um memorizing but it's memoizing i don't know it's weird that it doesn't have an r um in the word but memoizing so if we um again refresh our page here now when we come down to the request we see just a handful of database queries that are firing we have one that's firing for when we call current user method inside of application controller then we have another that's called uh for fetching all of the users that are related to the comments on a specific post so we're fetching current user we're fetching a single post we're fetching all the comments for the post that we're fetching all the users for those comments so now performance is greatly increased it's surprising how much or how expensive those database queries are especially if you're using a really big database that runs on a different server or that like lives really far away or something like that so it's important to cut down those database queries um the same way that you want to cut down like the number of like api calls or requests that are happening over the network um okay so now we have a commenting system on our blog that is pretty quick it's we've got our our posts or our comments that are in reverse chronological order we can see the title and description of our comment at the top in the next episode what i want to do is go through how you would reply to some comments so thanks for sticking around and we'll catch you the next one cheers [Music] you --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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