--- title: Has many through associations in Ruby on Rails slug: has-many-through-associations-in-ruby-on-rails published_at: 2021-09-15 16:23:37 +0000 updated_at: 2026-03-04 20:13:54 +0000 summary: description: In this episode we'll talk about how to use has many through associations with a join table instead of a has and belongs to many association. Inspired by this question on Reddit: https://www.reddit.com/r/rails/comments/pn6mgm/help_figuring_out_associations/. #rails #rubyonrails tags: [cjav_dev, web development tutorials, web development for beginners, vim, ruby, rails, javascript, habtm, Has many through associations in Ruby on Rails, ruby on rails tutorial, ruby on rails, has and belongs to many, has and belongs to many association] views: 4957 author: CJ Avilla url: https://www.cjav.dev/videos/has-many-through-associations-in-ruby-on-rails youtube_url: https://www.youtube.com/watch?v=YsgyM0qOt1U youtube_id: YsgyM0qOt1U embed_url: https://www.youtube.com/embed/YsgyM0qOt1U thumbnail_url: https://i.ytimg.com/vi/YsgyM0qOt1U/hqdefault.jpg type: video --- # Has many through associations in Ruby on Rails *Published: September 15, 2021* *Views: 4957* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=YsgyM0qOt1U) [![Has many through associations in Ruby on Rails](https://i.ytimg.com/vi/YsgyM0qOt1U/hqdefault.jpg)](https://www.youtube.com/watch?v=YsgyM0qOt1U) ## Description In this episode we'll talk about how to use has many through associations with a join table instead of a has and belongs to many association. Inspired by this question on Reddit: https://www.reddit.com/r/rails/comments/pn6mgm/help_figuring_out_associations/. #rails #rubyonrails ## Transcript what's up welcome back everyone in this episode i wanted to talk about how do you build uh has and belongs to many style associations i never actually use that association type has and belongs to many i always use an association through what we'll call a join table and so what i wanted to do was actually start by drawing out the um this like database structure given this question so i'm this this user on reddit posted you know they were looking for help figuring out these associations they've got a user table that has many listings and they've got a search and a search belongs to a listing and a user and they have many listings that have searches except a search should be able to have multiple listings so right now a search belongs to a specific listing but a search should be able to have multiple listings so what i then have to use has and belongs to many instead so here we have a user and a user can have many searches a user can also have many listings through those searches and a search can have many listings so for context the app allows the user to save a number of search queries you can save your search queries it's going to periodically hit some api with those queries and then save the matching listings to a database and so a user in this case can save multiple queries so a user is going to have multiple queries those are their searches and we're going to hit the api and it's going to return those listings and save those to a database and a each individual search could be related to many listings and each of those listings is going to be tied back to a specific search so what we want to be able to do is say current user.listings as well as search.listings and the they presented this sort of possible scenario where the user has many searches and then a search belongs to a user and a search belongs to a listing but where this falls down is we want the search to actually have many listings and so in this case we need a uh a has and belongs to many relationship between search and listings so let's draw out the tables here the way i like to draw my tables is like a open bottom rectangle where i'll draw a line across and then i will name the table so we're gonna have a user table and in the user table we'll have an id and i like to draw a line underneath the foreign key so that's like our table name that's our foreign key and then this is gonna be like any of our columns that are in the user table the reason i keep it open-ended is you kind of never know really when you start out drawing these how many columns you're going to have and then once you've really like nailed down the number of columns you can sort of finish this thing up so that you have a table to look at so we're going to start off with the user and a user is going to have an id okay and then we also need to have another object here for a search okay and the search is itself also going to have an id and we know that a search is going to belong to a specific user because a user is creating these searches and they're they're kind of managing their searches they might like add or delete searches but a search isn't really going to be shared between multiple users at least given the context that we have so inside the search table we're going to add a user id and this is going to be what's called a foreign key and we might draw a line between the foreign key in the search table that's our foreign key and this is our primary key in the user table there's a couple different ways to like show where there is like the one and where there's the many so in this case you might do like a one to one or you might do a one to many with a star i've seen that before but the way that i like to do it is called crow's foot notation it kind of looks like a crow's foot and so i'll put a line across on the side that's going to have one and then i will put like a crow's looking crow's foot looking thing on the other side where there's going to be many and this kind of like shows me that there's going to be many rows in the search table for each user okay and then we have somewhere around here we're going to have a listing okay and a listing needs its own little box and inside of there it's gonna have an id okay so a listing has an id now a listing can be in many searches and a search can be in many listings so we don't really have a way to represent that right we can't have like well we one way that you might think of doing this is like okay let's store like the search ids column and this is going to be like an array of 1 2 3 or something like this like maybe we have a column inside the listings table that is an array of ids and those ids are tied to search ids so maybe like this is an id of a search right and then maybe inside the searches table we have listing ids okay and this is also an array and this is like i don't know five six seven and these ids might tie back to the listing ids so this is like a really really naive approach to trying to figure out how we're going to associate listings and searches but that's not actually the way that we want to do it okay so this is like one of the one of the ways that we want to make sure that our database is normalized is by never having columns that have sort of lists of ids and so the way that you approach this is you'll create a separate table called a join table and this join table is going to be called listing search okay and the listing search table will itself have an id it will also have a listing id and it will have a search id okay and the listing id is going to be one too many and the search id is also going to be 1 too many okay now this this is a really really common way to build what we're calling a join table so we don't know what this is going to be like the ebay id or something i don't know what whatever ends up actually being the contents of the listing or the user of the search is kind of irrelevant for building out the association so we'll just close out our tables here and this is the sort of the database that we want to build this is the database structure that we want to build now when we do this there's a couple of like interesting things that i wanted to talk about that are part of this search table so we're going to look at this table and talk about how we want to make these ids how we want to set up these ids now technically inside of rails you can use a has and belongs to many association but the reason one of the reasons why we don't want to do that is oftentimes inside of this listing search table here you might want to add other columns like scene or like reviewed a two id and a from id when you're building on a road but then you might also have like the speed limit or the toll cost or other things that are part of that join table right and so in this case we it's it's almost always preferable to have a join table all right so let's take a look at how this is actually all right so here we're gonna i've got a rails app already started i'm gonna say uh railsg model user and this is just gonna have like a name or something doesn't really matter then we can say rails g model search and maybe we'll have like the query that they're going to type in and the user references so the search again is going to belong to a user and separately we want to say railsg model listing we want to create the listing first so that when we create the search listing it can refer back to back to this listing model we can build the references that way so now we're going to generate the listing and this might have a title or something like whatever the ebay listing is or the thing that someone is going to purchase but that's really all that it actually needs for now and finally we're going to say railsg model listing search and this is going to have a listing and a search okay so we're gonna we're gonna build this join table that represents the relationship to a listing and to a search so if we open up these migrations so we're creating the listing searches table here and we're building these two associations or this is going to build two foreign keys in the listing searches table back out to the listing table and the search table if we wanted to make it so that our we could only have one listing like the same listing per search and only one search per or like if you want to make sure that you don't have duplicate results where a listing is appearing multiple times in the same search and that same search is appearing multiple times in the same listing like you don't want to have a in the table in this table where you might have something like listing id is equal to one and search id is equal to one and then you have like another column that has the same data listing id one search id one if you want to avoid this what you can do is you can build a an um an index that has a uniqueness constraint on the pair of those two so add index for listing searches for the combination of listing id and search id and we can make this unique true so that is one one additional thing that can be really helpful when you're building out um these sort of uh join tables okay so now i think that's all we really need so rails db migrate create db migrate all right so now let's talk about those associations so if we jump actually i'm going to add um annotate to our gem file because this can be really helpful for visualizing the columns that are in the table gem annotate you can say bundle exec annotate dash dash models and that will add some comments to the top of our model so now when we open up for instance this listing search model now at the very top we have all of the different columns so we can take a look and see exactly what's going on here so the we have our listing search and that belongs to a listing now if we go to the listing we can say the listing has many searches okay and then if we go back we can go to um listing search belongs to a search and oops i'm sorry this is not list this is not searches yet so a listing has many listing searches we're going to do the like one-way associations first listing search um okay and then we'll open up search and here uh our search is gonna has many listing searches okay and then we'll open up our user model over here and we'll say a user has many searches right because the user or like a search directly belongs to a user and so a user can have many uh have many searches and because a search has many listing searches now we can say um a user has many listing searches through searches okay so now we're able to associate a user with listing searches through those searches that they've created so a user is going to be creating these searches and then we're going to periodically fire off a query to the ebay api which is going to give us back a bunch of results that's going to create listings that are as it stands um going to associate they're going to create these join tables the listing search tables every time there's a listing that responds that corresponds with a search and now the user will be able to access the join object so we can see a listing search now if we want to see which listings the user has we can say has many listings through listing searches okay so we can we can sort of chain these uh associ these has many through associations um pretty deep so right now we're saying that a user has many searches through those searches it has many listing searches and through those listing searches it has many listings so that's how we're able to get back all the way back to the listing so let's open up the rails console and just play around with this for a second so we'll create a user user.create and then we'll create a listing l equals listing.create s equals search.create for user being you and then we'll say ls is listing search dot create for the listing and the search and okay so now we should have like one of each of the objects so if we say u dot searches we should get back the active record uh collection for the search and we do so we see the one search that they have okay now if we say udot listing searches we can see those listing searches and again like we can also see the query that's being fired off right so here when we say user.searches that's doing select star from searches where searches user id is the id for that user then when we say u.listing searches now we're seeing select star from listing searches inner join searches so when we're going through an association it's actually creating an inner join with that through table on listing searches.search id equals searches.id how does it even know that it's all by convention it's all based on the underlying convention and we can actually change what those column names are if we wanted to by specifying the class name and the foreign key all right so the next thing here is we want to do udot listings right check out that sql query select star from listings interjoin listing searches on listing.id is listing searches.listing id another interjoin so two inner joins here back through searches unlisting the searches that search id searches.id where searches.userid equals the user's id and indeed we get back the one listing that should have been found so that's cool the other thing that i wanted to try was just creating one more listing search with uh with l with the same listing in the same search if we do this it should fail and it does fail because it violates the um that index the uniqueness constraint on that index that we created earlier all right that is hopefully that's helpful this is how you might create a has and belongs to many using a join table in ruby on rails thanks so much for watching and we'll see you next time [Music] --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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