WEBVTT 1 00:00:00.000 --> 00:00:01.860 [Treehouse™: presents] 2 00:00:02.890 --> 00:00:05.410 [Quick Tips: What is a has_many :through association in Ruby on Rails with Jason Seifer] 3 00:00:05.410 --> 00:00:06.460 Hi! I'm Jason. 4 00:00:06.460 --> 00:00:08.270 In this Treehouse Quick Tip, 5 00:00:08.270 --> 00:00:11.810 we're going to talk about what a has_many through association is 6 00:00:11.810 --> 00:00:13.240 in Ruby on Rails. 7 00:00:13.710 --> 00:00:17.970 A has_many through association starts with a has_many association. 8 00:00:18.560 --> 00:00:21.640 As an example, let's say that we're a magazine publisher 9 00:00:21.640 --> 00:00:23.460 building out our website. 10 00:00:24.190 --> 00:00:27.080 We have a model for magazines and for users. 11 00:00:27.080 --> 00:00:30.620 We want to be able to show a user a list of magazines 12 00:00:30.620 --> 00:00:32.150 that they are subscribed to. 13 00:00:32.810 --> 00:00:35.720 We can do this by using something called a join table. 14 00:00:36.310 --> 00:00:40.650 A join table will keep track of the first model--in this case, the user, 15 00:00:40.650 --> 00:00:44.350 and the second model, which in this case is the magazine. 16 00:00:44.830 --> 00:00:47.470 But we can call our join model a subscription. 17 00:00:47.470 --> 00:00:50.340 Now, let's take a look at what the code looks like. 18 00:00:51.450 --> 00:00:54.600 So here's a user class and a magazine class. 19 00:00:55.180 --> 00:00:58.280 What we want to do is also create a subscription class 20 00:00:58.280 --> 00:01:01.130 which is going to be our join table. 21 00:01:01.800 --> 00:01:03.970 So let's say that we generated a subscription class. 22 00:01:03.970 --> 00:01:05.780 It would look something like this. 23 00:01:12.770 --> 00:01:17.140 Now, on the back end when we were creating our subscription class, 24 00:01:17.140 --> 00:01:20.180 the table would look something like this. 25 00:01:20.490 --> 00:01:24.290 In addition to an ID column, it would have two fields. 26 00:01:24.780 --> 00:01:29.600 One for the user ID and one for the magazine ID. 27 00:01:30.830 --> 00:01:33.770 So we're going to go ahead and pretend that that's how it is right now. 28 00:01:34.770 --> 00:01:37.900 If we wanted to set all of that up as a relationship, 29 00:01:37.900 --> 00:01:42.520 we could say that a user has many subscriptions. 30 00:01:43.610 --> 00:01:50.730 And we would also say that a magazine has many subscriptions. 31 00:01:52.830 --> 00:01:56.930 So then we set up the subscription class to say that it belongs to 32 00:01:56.930 --> 00:02:02.650 a user and it belongs to a magazine as well. 33 00:02:05.310 --> 00:02:08.030 Now that we have these associations set up, 34 00:02:08.520 --> 00:02:14.350 we can say that our user has many magazines 35 00:02:15.490 --> 00:02:18.830 through the subscriptions relationship. 36 00:02:20.160 --> 00:02:24.530 And we can also do the same thing with our magazine class. 37 00:02:30.420 --> 00:02:32.470 Well, how would you use something like this? 38 00:02:33.390 --> 00:02:35.560 Well, let's say that you we're trying to find the user. 39 00:02:37.240 --> 00:02:39.020 Grab the first user, 40 00:02:41.270 --> 00:02:43.250 and you also grab the first magazine. 41 00:02:44.190 --> 00:02:46.960 If we wanted to create a subscription, 42 00:02:46.960 --> 00:02:52.090 we could say something like user.subscriptions.create. 43 00:02:53.330 --> 00:02:55.080 And then we tell it that the magazine, 44 00:02:55.080 --> 00:02:59.670 which is an association up here on subscription, 45 00:02:59.670 --> 00:03:02.220 is this magazine that we just found. 46 00:03:03.330 --> 00:03:08.150 Now what we can do, if we really want to, is call user.magazines, 47 00:03:10.100 --> 00:03:13.620 and that would return our magazine. 48 00:03:14.460 --> 00:03:18.310 The same thing can apply to models that refer back to themselves. 49 00:03:18.310 --> 00:03:21.090 For example, let's take a site like Twitter 50 00:03:21.090 --> 00:03:25.800 and apply the same model of subscriptions but to users and followers. 51 00:03:26.180 --> 00:03:29.090 Followers are just another class of users, 52 00:03:29.090 --> 00:03:31.420 and here's what the code for that looks like. 53 00:03:32.050 --> 00:03:33.610 Now, you might be wondering what we would do 54 00:03:33.610 --> 00:03:36.730 if we wanted to have a has_many through association 55 00:03:36.730 --> 00:03:40.310 where the other end of the has_many through association 56 00:03:40.310 --> 00:03:43.290 is the same model as the original. 57 00:03:43.690 --> 00:03:48.250 Sort of like how you would do a Twitter site with users and followers. 58 00:03:48.910 --> 00:03:50.980 Well, the way we would do that is like this. 59 00:03:50.980 --> 00:03:52.860 Let's say we had a our user class 60 00:03:53.670 --> 00:03:56.610 and then the join table that we we're going to use-- 61 00:03:56.610 --> 00:03:59.950 the joining class is called followings. 62 00:04:01.020 --> 00:04:03.780 So let's say we created a following class that had a user ID 63 00:04:03.780 --> 00:04:05.650 and a follower ID. 64 00:04:11.840 --> 00:04:16.940 Now, we could tell our user to say that it has_many followings. 65 00:04:18.399 --> 00:04:22.640 And then our following class would belong to a user. 66 00:04:23.490 --> 00:04:26.810 So this is just one part of the association. 67 00:04:27.430 --> 00:04:31.960 In order to do the other part where we're going to have a follower as well, 68 00:04:32.830 --> 00:04:35.480 we would say that it belongs to-- 69 00:04:37.520 --> 00:04:39.070 we fix that right there-- 70 00:04:39.560 --> 00:04:46.580 belongs to a follower, and the class name for that is user. 71 00:04:47.240 --> 00:04:49.940 In order to specify that it points back to a class 72 00:04:49.940 --> 00:04:52.600 other than what Rails usually infers, 73 00:04:53.090 --> 00:04:58.350 you use the class_name hash key when specifying it belongs to association. 74 00:04:58.840 --> 00:05:02.000 In this case, we're saying the class name is user. 75 00:05:02.440 --> 00:05:05.560 So it's just going to point back to a different user. 76 00:05:06.710 --> 00:05:12.960 Now, over on the user class, we say has_many followers, 77 00:05:14.300 --> 00:05:16.320 which is this part of the association here, 78 00:05:18.240 --> 00:05:21.810 through followings. 79 00:05:21.810 --> 00:05:24.450 And that's how you do a has_many association 80 00:05:24.450 --> 00:05:28.480 where the second part points back to the same kind of class. 81 00:05:29.680 --> 00:05:34.230 Behind the scenes, Rails will look up all of the follower IDs at the table 82 00:05:34.230 --> 00:05:36.690 and then do a separate query to the database 83 00:05:36.690 --> 00:05:38.740 to bring back those users. 84 00:05:39.050 --> 00:05:41.600 It's two database queries to get the information, 85 00:05:41.600 --> 00:05:44.210 but it's an extremely useful technique.