--- title: Learn Active Record associations with Starwars slug: learn-active-record-associations-with-starwars published_at: 2021-04-16 22:05:46 +0000 updated_at: 2026-03-04 20:14:17 +0000 summary: description: Experiment with Active Record associations by running some rspec tests. Kata: https://github.com/cjavdev/st-ar-wars Original Reddit Post: https://www.reddit.com/r/rails/comments/mocwsa/how_did_you_learn_active_record_associations/ #rails #rubyonrails tags: [] views: 7319 author: CJ Avilla url: https://www.cjav.dev/videos/learn-active-record-associations-with-starwars youtube_url: https://www.youtube.com/watch?v=E2jk4h1gQ2I youtube_id: E2jk4h1gQ2I embed_url: https://www.youtube.com/embed/E2jk4h1gQ2I thumbnail_url: https://i.ytimg.com/vi/E2jk4h1gQ2I/hqdefault.jpg type: video --- # Learn Active Record associations with Starwars *Published: April 16, 2021* *Views: 7319* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=E2jk4h1gQ2I) [![Learn Active Record associations with Starwars](https://i.ytimg.com/vi/E2jk4h1gQ2I/hqdefault.jpg)](https://www.youtube.com/watch?v=E2jk4h1gQ2I) ## Description Experiment with Active Record associations by running some rspec tests. Kata: https://github.com/cjavdev/st-ar-wars Original Reddit Post: https://www.reddit.com/r/rails/comments/mocwsa/how_did_you_learn_active_record_associations/ #rails #rubyonrails ## Transcript all right what's up everyone welcome back to another video episode this one is going to be uh some more about rails this time a little bit more fundamental i saw this post a couple days ago from zilton7000 about how uh asking how folks learned active record associations and uh yeah it's it's they're kind of confusing and all the articles are great but maybe they just like uh get get even more confusing and then there's a bunch of comments about like yeah like here's a couple different ways that i modeled it i'm using the odin project or you know i'm using like a user in posts um these are really sort of like the the basic ones um but i thought that it might be helpful to just create just a super short video and then like a little exercise or like a series of exercises that folks could try out so i've created this repository called star s-t-s-t-r wars so it's like supposed to be like star but like ar is active record i know super dorky whatever but like um if you go down here the idea is that i've created a little bit of a kata or like uh some like exercises that you can try out so there are tests that are already built into um into this repository that you can run and they'll fail if the associations are not correctly set up so um we're going to go through the process of setting up the associations between these different resources we have a film a person a person film a planet and a species just a couple different models from a star wars sort of like a star wars modeling and the idea is that this should push us into learning a little bit about the most fundamental associations we're not going to talk about single table inheritance we're not going to talk about polymorphic associations in this specific video but we'll get into that in in other future videos this is really just meant to be sort of a simple phase one association caught up so i've downloaded and installed um actually let me refresh the readme that i just updated so here's how you can set it up you can follow these instructions and then here's how you run this the tests i'm using r spec um so if you run our spec spec while you're in the the um the project repo you'll see all the failing tests here so we're using shoulda matchers which is a really handy tool from thoughtbot for asserting that certain associations exist we're not going to look at too much into the actual tests but we're going to just look at these failure these test failures so a film is expected to have many people through person films so let's actually go through the readme and go line by line in the readme and talk about these associations let's also look at the code here so if we go into app models i've already created the models for you and the database migrations are all set up so we're not going to talk about how to like do the migrations we're just going to talk about the actual associations today so if you look at a film you'll see that the schema information is available at the top you can also if you don't have this annotation so i'm using a gem called annotate which allows me to like add the columns to the top of a class file with a comment but if you're not using this then one thing you could do is go into the schema dot rb file and you can also see what the database schema looks like here so i'm using postgres so when you download and run it you'll see that but here's like the name of the table and then here's all the columns that are in the table so in the films column we have a title created at updated at again these create table calls are going to add for us the id by default um we have a films people and here's kind of like what those look like so let's go back and take a look at the um at the readme here so the very first one we want to do is say that like a film so for a film you should be able to list the people in a given film now one way that i can test this a little bit easier is i'm going to pull up my film spec and have that sort of side by side my film model so the film models here on the left this is the active record model and then on the right i have my test that's that's sitting there and with vim i have i'm using like vim test i think so i can just use like leader h to run a specific test so we can just like narrow it down to that one test so a film is expected to have many people through person films okay so that means that uh in order for us to have uh to say like have or like a film has many people right this is going to say when i call film on an instance of film if i call dot people i should get back a list of person objects for people who have starred in this film now in order to get back the people that have started in this film i need to make a query a database query that says select all of the people or like select star from the person table where that person id exists in some join table between film and people and so we haven't actually set that up in fact like let's let's actually go to person film first so one of the ways that you can start out by like adding all of your associations is looking for where there is a column that um that's that ends in underscore id and just try to think to yourself like is there a related model for this column that has an id and if so we want to add it belongs to association so anytime there's a a column in the database that ends in underscore id oftentimes we'll refer to this as a foreign key to another table so for the person films table this is a join table between a person object and a film object it has two columns a person id column and a film id column the person id column will be the id of a person in the person table so here we can say belongs to person and we can say belongs to film let's actually do this for all for all models that have uh foreign key relationships so in this case we have two we have a species id and a home planet id so the species id we're gonna have a belongs to species right for the home planet this is this is going to get a little interesting here so we can say belongs to home planet now when active record is setting up these associations it's doing a lot of like conventional things so um when we started out in this person film person film model right and we say a person film belongs to a person rails knows that um because the name of the association here is person it can derive that the class name should also be person that the foreign key is going to be person underscore id okay so it knows that just because we named this person that it's going to like automatically look for a class called person and it's going to use this person id to associate to the id in the people table now rails is automatically doing this logic for like this magic for us converting between person and people so the name of the table is always plural right person films people in and rails has some like magic inside of um active support i think it's inflection maybe that does uh the conversion between like plural and singular which is pretty cool um so when we're looking at person film right we're able to just say belongs to person and belongs to film because film is also similarly right film is also a class name okay but when we get to home planet things are going to get a little weird because we don't have a class called home planet right like if we look in our models table we have a class called planet but we don't have a class called home planet so when we're looking at the planet model and it has planets right this the name of this association is home planet so that's going to look for a class called home planet but it doesn't exist and so if we're creating a belongs to association where we have the foreign key in this table that means we're we're creating the correct type of association to a home planet but if the name of the association is different from the name of the class that we're associated to then we need to uh pass a another argument to belongs to that specifies which class we want to use we're going to say class name is planet okay and we put this in strings because or like in quotes because as rails is loading up all of the different models it doesn't it's going to constantize this and look up the class based on the string name of the model if we were to just pass the class directly like this that would work as long as every single class was ordered like loaded in the correct order and there were no cyclical dependencies between the associated classes tons of stuff you don't really need to worry about just know that you have to pass classname is planet here okay so this is going to give us a person belongs to a species and it belongs to a home planet that has the class name planet great so uh let's keep going so we've done we've done film person we've done person film remember we're just going back and looking at the columns that have id and we're okay so now we're to plan it and here we have something that ends in underscore id and that's for a senator so we can say belongs to senodor now there is no class name called senator right so instead we want the senator to be a person so here again we can say class underscore name is person okay there's no other id columns in the planet so we're going to move on to species so species doesn't have any underscore id foreign keys that point to any other columns so we're done with our first pass which is like go through add all of your belongs to associations let's just take a step back and run our i'll run our specs so our spec spec and we should see some of the tests have passed here okay so we've got 12 12 examples and seven are failings we've got five tests that are passing so that's uh uh move in the right direction okay let's keep going so the next thing we want to do is add our has many association so person film here um so one way that you can do this is like by looking everywhere that you have a belongs to if you go to the other class you will either have you will almost always have a has many or sometimes a has one that points back at this table so um whoa that was weird um okay so for when we're looking at person film we see that it belongs to person and it belongs to film so we can go to the person object person and now we can add a has many has many that points back at person films so person films and when we're using a has many the the name of the association is going to be plural and that's how rails will know how rails will know to make it singular and then look up the class name and the class name in this case should be person film so if we wanted to rename this association to like people films or something like that we could now it's important to note that when we're when we're talking about this has many here that goes to this join table for person film um uh the person films table doesn't actually have anything about the films it doesn't have anything about the people it just has their ids to join them together a person film represents a specific person starring in a specific film and so if uh like luke skywalker he was in all of whatever he was in like several films right so you would have entries in the person film table where it's like okay a new hope luke skywalker's id so um a new hope id for the film id and then luke skywalker's id for uh for um the person and then you would see like the next the next episode five episode and then uh luke skywalker episode six look skywalker episode seven luke skywalker et cetera et cetera until you have like the correct data in the join table but when we call person. or if we have an instance of a person right so we have a luke here which is a person find x whatever like if we say luke dot person films that doesn't tell us anything about the films so what we want to do is have luke.films return the list of films like the movies that luke skywalker was in so there is a couple different approaches to do this one we could just not have this model and if we if we just had this table built exactly the same with the same columns over here we could say like has and belongs to many and then we can say films but i recommend uh never actually using has and belongs to many don't ever use it um and the reason is if you uh there's a there's an alternative and the alternative is better we'll go over the the differences here but i wanted to just call out has and belongs to many its purpose is to create a many-to-many relationship between two different models um where you where you would not need to worry about this this intermediate model class but you almost never never use that um okay so instead what we want to do is we want to have a has many through so here we can say has many films through uh person films uh person films and what we're doing here is we're saying like we're going to create a new association called films and this association is built through an existing association on the same model so this through what we're passing here for through this uh this symbol here must exist as an association on the model already so we have to have person films already here defined as an association on the model so that we can call through and then say person films sometimes you might even want to like pass you know the class name here so if we wanted to name the has many like act acted in films or something like that then we would have to pass class name uh film right because we're following the convention where we've named our our association films it knows again to singularize uh constantize look up the class name for the film um and then it knows how to do the query here so when we call person.film so luke dot films this will result in a query that's like select star from films um films films uh where let's see where uh uh person id wait no yeah so select star from films join person films on films dot id is equal to person films dot film id where uh person films dot person id is equal to like this the luke's id right so like that's kind of the the sql query that will be executed in a future episode i'd love to go through and just like kind of rebuild some of the basic associations for active record so that we can see how you might actually implement this stuff from scratch but for now we're just going to keep talking about these associations so if we run um our spec well let's let's go back and run our spec again so person spec um and we'll keep going okay so we've got two failures still so a a person is expected to have one senator through the home plate okay we'll come back we'll come back to that in a bit let's keep let's keep adding our has many associations so a person so in a film a film should have many uh person films and again we're gonna do a has or has many has many has many uh people through um person films okay so has many people through person films people is not a class name but it is the plural of a class name and rails knows how to again singularize people and look up the class name for that so this should work i believe so if we go to let's go to the film spec and let's run this i think we should be good to go for the film spec okay we've got a test passing for film spec so we are we're done with film so film has the right associations now we're able to look up all the people who have starred in a given film um and that that's that's pretty cool and helpful all right so now let's go into person um okay so we've got belongs to species so now we should be able to go to the species and say has many people so here this one's going to be a simple has many people right because a person belongs directly to a species so in this case the the species id directly relates to the idea of a species species happens to be uh the same singular and plural of this instance of the class so that should totally work and should return the species to us next let's go to belongs to home planet okay so we can go to our planet planet now what's what's odd about this one planet spec okay so a planet should belong to a senator so this test should pass it does um and it should have many citizens so this is another wrench that we've sort of thrown in here so um a person should have many citizens and citizens are instances of people or instances of a person so a planet i mean we could have just made it people but that wouldn't be very interesting and i wanted to show like um sometimes how you need to use this this one feature so um we're gonna say a planet has many citizens and now we can again specify class name is person right now this is interesting because when we look at uh when we look at a person right a person belongs to a home planet now when we're when we're calling planet.citizens here um we're we know that we want to use the class name person so we can look at the correct table however when we look at the people table we don't see planet id we still don't see planet id right so if we were to attempt to query for the citizens of a given planet it wouldn't know exactly which foreign key on the person is relating to the planet that the citizens belong to so in this specific case we need to tell the planet class which foreign key it needs to use when it's looking up the citizens of the planet okay so in this case the the foreign key is home planet id and so what we can do is um let's actually run the the test to see how it fails so planet spec right so if we run this one um we okay so it says uh planet is expected to have many citizens so expect the planet to have to have a has many association called citizens and it does now it says a person does not have a planet id foreign key so it doesn't you're right because it has a home underscore planet id foreign key so in this case what we need to do is go into this has many citizens association and add a third argument and this third argument is foreign key i always mix up the i and the e here so foreign key home planet id so we're going to explicitly tell this association which foreign key comes back to um comes back to planet so now if we go back to our spec planet spec and run the tests here uh we have two passing tests so we're we're looking good for planet um all right let's let's keep going oh gosh i jumped all the way out so app models um species okay that one should be good planet should be good person film should be good person should be almost good film should be good okay so let's let's run our specs again our spec spec to run all of the tests okay so we've got only two failing tests and they're both they're both related to person um so let's go let's go take a look at uh look at those so we'll jump into person here okay so um a person uh senator okay so a person should have a senator based on their home planet so a person is from naboo um that's their home planet they should have a senator and their senator should be whatever princess amidala or whatever senator amidala so we can come over here we can say like this person should have one senator and the the way that they're gonna have that senator is through their home planet right so in this case we're gonna do a have one now i'll have one we would the difference between a belongs to and a have one or a has has one uh has one the difference is that when we're doing belongs to the foreign key is in this table so the foreign key for species is in this table the foreign key for a has one is going to be in the other table coming back to us so in this case when we say has one senator right the foreign key in this case is actually in the home planet table because a planet again let's let's look planet so a planet has a senator id so this senator id is what actually relates back to the person through the the home planet so we can come back over here and say through home planet okay so now a person should have a senator through the home planet okay so let's jump back over here and run this test and it works okay so then there was one other here ah citizens okay this is great so a person should have many citizens through its home plan this is like um has many uh i guess it's like uh has many neighbors or something or has many uh citizens from the same planet or whatever but we'll just call it citizens uh just so we can keep it easy through um home planet and now we should be able to run these tests and they all pass now we can jump out and say our spec spec and they all pass so we are good to go and so that's kind of like the whole rundown for the associations that i want to show another way that i've seen this done before is with harry potter so you can have wizards wizards have wands um wizards belong to houses there's also like you know you can have quidditch matches that are between different houses and you can have the headmaster which is a you know um maybe like a type of wizard so there's there's using star wars or harry potter or whatever sort of world that you're coming from one way to practice associations is to try to like make very um idiomatic association names that actually mean something rather than just naming them the conventional oh post and comment and tweet and whatever those are really really good patterns to follow when you're working with rails is trying to stick to the conventions so you don't have to add class name foreign key et cetera et cetera however sometimes it can be really nice to have idiomatic nice associations that make sense and return the types that you expect so um hopefully this is useful if it was i would really appreciate a quick like uh and yeah i'm going to continue making content like this so if you find it helpful and you want to subscribe and turn on those notifications uh please do also like super spammy right to say this kind of stuff at the end but it turns out that like people who have uh who ask for subscriptions end up having like twice as many subscribers so it's like super interesting but um i really want to like create content help folks spin up and become web dev so yeah really really appreciate all of the all the positive comments so far and uh yeah appreciate your time and attention so hopefully this is useful if it is let me know in the comments if you have other worlds that you're thinking of that might be interesting to experiment with drop those in the comments totally open to pr's if you wanted to add some more features or some more uh suggestions in uh in the specs here on this repository so go out and associate different different models and we'll see you next time --- [Back to all videos](https://www.cjav.dev/videos) --- ## Hey Agent, thanks for visiting! 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