What is a has_many :through association in Ruby on Rails?

[Treehouse™: presents] 0:00 [Quick Tips: What is a has_many :through association in Ruby on Rails with Jason Seifer] 0:02 Hi! I'm Jason. 0:05 In this Treehouse Quick Tip, 0:06 we're going to talk about what a has_many through association is 0:08 in Ruby on Rails. 0:11 A has_many through association starts with a has_many association. 0:13 As an example, let's say that we're a magazine publisher 0:18 building out our website. 0:21 We have a model for magazines and for users. 0:24 We want to be able to show a user a list of magazines 0:27 that they are subscribed to. 0:30 We can do this by using something called a join table. 0:32 A join table will keep track of the first model--in this case, the user, 0:36 and the second model, which in this case is the magazine. 0:40 But we can call our join model a subscription. 0:44 Now, let's take a look at what the code looks like. 0:47 So here's a user class and a magazine class. 0:51 What we want to do is also create a subscription class 0:55 which is going to be our join table. 0:58 So let's say that we generated a subscription class. 1:01 It would look something like this. 1:03 Now, on the back end when we were creating our subscription class, 1:12 the table would look something like this. 1:17 In addition to an ID column, it would have two fields. 1:20 One for the user ID and one for the magazine ID. 1:24 So we're going to go ahead and pretend that that's how it is right now. 1:30 If we wanted to set all of that up as a relationship, 1:34 we could say that a user has many subscriptions. 1:37 And we would also say that a magazine has many subscriptions. 1:43 So then we set up the subscription class to say that it belongs to 1:52 a user and it belongs to a magazine as well. 1:56 Now that we have these associations set up, 2:05 we can say that our user has many magazines 2:08 through the subscriptions relationship. 2:15 And we can also do the same thing with our magazine class. 2:20 Well, how would you use something like this? 2:30 Well, let's say that you we're trying to find the user. 2:33 Grab the first user, 2:37 and you also grab the first magazine. 2:41 If we wanted to create a subscription, 2:44 we could say something like user.subscriptions.create. 2:46 And then we tell it that the magazine, 2:53 which is an association up here on subscription, 2:55 is this magazine that we just found. 2:59 Now what we can do, if we really want to, is call user.magazines, 3:03 and that would return our magazine. 3:10 The same thing can apply to models that refer back to themselves. 3:14 For example, let's take a site like Twitter 3:18 and apply the same model of subscriptions but to users and followers. 3:21 Followers are just another class of users, 3:26 and here's what the code for that looks like. 3:29 Now, you might be wondering what we would do 3:32 if we wanted to have a has_many through association 3:33 where the other end of the has_many through association 3:36 is the same model as the original. 3:40 Sort of like how you would do a Twitter site with users and followers. 3:43 Well, the way we would do that is like this. 3:48 Let's say we had a our user class 3:50 and then the join table that we we're going to use-- 3:53 the joining class is called followings. 3:56 So let's say we created a following class that had a user ID 4:01 and a follower ID. 4:03 Now, we could tell our user to say that it has_many followings. 4:11 And then our following class would belong to a user. 4:18 So this is just one part of the association. 4:23 In order to do the other part where we're going to have a follower as well, 4:27 we would say that it belongs to-- 4:32 we fix that right there-- 4:37 belongs to a follower, and the class name for that is user. 4:39 In order to specify that it points back to a class 4:47 other than what Rails usually infers, 4:49 you use the class_name hash key when specifying it belongs to association. 4:53 In this case, we're saying the class name is user. 4:58 So it's just going to point back to a different user. 5:02 Now, over on the user class, we say has_many followers, 5:06 which is this part of the association here, 5:14 through followings. 5:18 And that's how you do a has_many association 5:21 where the second part points back to the same kind of class. 5:24 Behind the scenes, Rails will look up all of the follower IDs at the table 5:29 and then do a separate query to the database 5:34 to bring back those users. 5:36 It's two database queries to get the information, 5:39 but it's an extremely useful technique. 5:41

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