Include and Extend

[MUSIC] Hi. 0:04 I'm Jason, your Ruby teacher here at Treehouse. 0:06 In this workshop we're going to be going over the differences between extend and 0:08 include when working with modules in Ruby. 0:13 There are some finer points when working with each of these methods and 0:15 we're gonna go over them now. 0:19 So, let's go ahead and get started. 0:20 Okay, let's get into some of the differences between include and 0:23 extend when working with modules in Ruby. 0:28 Now, I'm going to create a very simple module here called the Shout module. 0:32 Now, this module is going to contain one method and it's going to say introduce. 0:37 And the introduce method is going 0:44 to return a string that says, Hello! 0:49 I'm here! 0:54 Now let's create a class that works with the Shout module. 0:55 So a person can Shout. 1:02 Now let's go ahead and see what happens when we run this. 1:06 I'm gonna have a little statement right here, we can say Person.new.introduce. 1:10 So if I click down into the console here in my workspace and 1:17 type ruby include_extend.rb we see that 1:21 this method was run that says introduce and it says, Hello! 1:25 I'm here! 1:29 Now if we tried to call this on the Person class, 1:30 we would get a no method error saying that the introduced method didn't exist. 1:36 Undefined method introduce for the person class. 1:42 And that makes sense because when we include a module, 1:47 we get all of the different methods in the instances of our class. 1:50 However, what if we had another class? 1:56 So we have a Person and let's also say we have a Dog class. 2:00 When we use the extend keyword with a module 2:05 the extend keyword will operate on the class, rather than on the instance. 2:13 So if we Dog.new.introduce. 2:18 I'm gonna take these out of here so that we can see. 2:22 We'll get the no method error in there. 2:26 However, when we say Dog.introduce, we do not get that error. 2:31 So here we go, undefined method introduced for dog and then the program stops. 2:36 So if we take that out and then just do Dog.introduce, it says, Hello! 2:41 I'm here! 2:46 So extend will operate at the class level and 2:48 include operates on the instance level. 2:52 Any instance of the class gets the methods inside of the module. 2:55 Now, practically speaking, we could include and extend something in a class. 3:00 So we could say extend the shout module on both Person and Dog. 3:07 And this could work for all sorts of different modules, and classes. 3:17 So if we run this again, both the class and 3:24 the instance would get these different module methods. 3:29 Now, this isn't very practical, so I've prepared a couple 3:34 modules here in the attribute_maker.rb file. 3:39 Now, let's go ahead and I'm just gonna scroll through this for just a moment and 3:43 introduce everything that's going on. 3:48 First, I'm gonna load this up into an irb session. 3:50 I'm gonna say, okay, load "./attribute_maker.rb", 3:55 and that prints a couple of things to the screen. 4:00 I'm gonna clear my screen here and what this does, just to give you an idea, 4:03 this allows you to create users with the attributes of first name and last name. 4:08 So we can say, create(first_name: "Jason", last_name: "Seifer"). 4:14 That will return a user instance with those attributes set. 4:21 And we'll do another one, (first_name: "Nick", last_name: "Pettit"). 4:27 So now we have two user instances. 4:35 And we can say, find_by(:first_name, "Nick"). 4:39 And it would return an array with all the users that met those 4:46 different qualifications found. 4:49 So, how do we do this? 4:53 All we did was write this include method to include the AttributeMaker module. 4:54 But, we're getting these class methods when that's included. 5:01 And one of those class methods is called attributes, which we pass 5:06 an array of symbols to, and thenwe can set those when we create a user. 5:10 Now, how in the world does all of this work? 5:15 Well it works through the power of modules. 5:18 So I'm gonna exit that for just a moment, and 5:20 then walk through how this was all done. 5:23 We have this module, AttributeMaker. 5:26 That's the core module that we sent in. 5:28 Ignore this line for a second right here. 5:31 We also have this ClassMethods module. 5:34 Now the ClassMethods module contains our create and 5:38 find_by methods as well as this method called attributes. 5:43 Now we know that these work, and 5:49 we're gonna walk through them a little bit more in just a moment. 5:52 Then our instances also have a method called attributes, 5:55 a two-string method, and our module overrides the initialize method. 6:00 So, now, let's go back and see how all of this works. 6:06 This is a certain pattern. 6:10 When we include the attribute maker module, in our user class, 6:13 there's a method inside of modules that gets run, automatically called included. 6:18 The included method runs every time you type include and 6:24 and then the name of a module. 6:29 So as soon as the include method is fired in the AttributeMaker module it gets run. 6:31 It takes one argument which is the class which is including the module. 6:36 Now since class is a reserved word in Ruby, 6:42 we misspell it inside of this argument to get into the method. 6:46 And it could be something besides class if we really want it to. 6:51 Other common names are base and klass and things like that. 6:55 Klass is kinda one of the more common misspellings. 7:00 So as soon as this is included, we extend the class with our ClassMethods module, 7:03 which is defined deeper inside of the AttributeMaker module. 7:12 Here's our ClassMethods module. 7:17 And that will get us all of the behavior at the class level and the instance level. 7:19 So here's where it gets a little bit tricky. 7:26 We define a class method called attributes inside of this ClassMethods module. 7:29 And this is going to take an array of arguments. 7:34 Now what we're passing in symbols, and we could write some more checks to make sure 7:38 that everything we pass in is a symbol. 7:42 Now this splat operator makes sure that anything sent in is an array. 7:45 Now we call it args to be short for arguments. 7:50 So we take this and flatten it in case there are multiple levels in the array. 7:52 Then we iterate over each item and 7:56 get this attribute block level instance variable. 8:00 And then just for debugging purposes, we've printed it out to the screen. 8:05 Now we can do something really fun. 8:10 We define a method using the method called define_method. 8:12 What the define_method method does is, 8:17 as you can imagine based on the name, creates a method by that name. 8:22 Now we're doing that at the class level, so 8:26 you can kind of think of this as saying, as soon as this method is included it 8:30 would be the same as writing defined method inside of the user class. 8:36 The first argument that we would get is first_name, so 8:43 it would run define_method with the symbol of first_name, 8:48 then we pass a block to it and this block is run. 8:52 What it does is return the value inside of the attributes hash, 8:56 with the argument, which at this point is first_name. 9:03 Essentially it would be the same as writing def 9:07 first_name, attributes[:first_name]. 9:12 But this is all done for us. 9:17 Now you might wonder where attributes is coming from. 9:19 That happens later on in this module. 9:23 We have this method definition called attributes which either initializes and 9:26 sets the attribute's instances variable to an empty hash, or returns it if it exists. 9:32 Now the attributes method is inside of the AttributeMaker module. 9:40 But, since this defined method is operating essentially 9:46 at the instance level, we have access to the attributes method. 9:51 The second part here defines another method where we're setting that attribute. 9:56 We can pass define_method either a string or a symbol. 10:02 Here we're passing it a string which will define the first_name_equals 10:07 method which will set that as the value inside our attributes hash. 10:13 Now, these methods are created automatically. 10:19 So, our user class has no idea, when it's created, 10:22 that it's going to have these methods for these different attributes. 10:27 Next, we have the create method. 10:33 Now, create initializes a new instance and this is still 10:35 in the ClassMethods module, so this is still operating at the class level. 10:39 Then what this does is initialize a new instance 10:45 with the arguments that are sent in as a hash. 10:48 It then appends that instance to an instances array and returns it. 10:51 The instances array is just another method inside of our ClassMethods 10:59 module that either initializes or returns an empty array. 11:05 Or, if the array exists, it returns the instances array and 11:10 this is at the class level. 11:15 So we would have access to user.instances. 11:17 Finally, our last method that we have at the class level is the find_by method. 11:21 And we pass in an attribute and a value. 11:26 We initialize an empty array that we call found. 11:30 We then iterate over the instances at the class level and, 11:33 if this instance has an attribute with that value, 11:38 we append that to the found array and then return that found array. 11:43 After that, we have that instances method which we went over earlier. 11:49 And then here are our actual instance methods that get included from 11:53 the AttributeMaker module. 11:57 We already went over the attributes method, and 11:59 we redefined the to_s method, which prints out the class and the attributes. 12:03 And finally we overrode the initialize method to pass in a hash for 12:09 the different attributes. 12:15 We then iterate through them and for each key in value we set that to 12:17 the internal attributes hash at the given key, with the given value. 12:22 So once we do all that, 12:30 we can do all sorts of fun things that we saw in our IRB console. 12:31 Let's go ahead and check that out one more time. 12:36 I'm gonna load this in. 12:39 And you can see as soon as I type load, it says attribute: first_name and 12:44 attribute: last_name. 12:48 If we scroll up, we can see that it got to this point in the code 12:49 because included was run and then this attributes method was run. 12:54 So now, let's go ahead and create a user, 13:00 first_name: "Jason", last_name: "Seifer". 13:06 And we'll create another one. 13:13 User.create(first_name: "Bob", 13:15 last_name: "Seifer"). 13:20 Now at this point, we should have access to the user.instances method. 13:23 And we do. And f I were to create another one, 13:29 and then I rerun this instances method, we can see that that was appended as well. 13:40 So now when we clear this If I say User.find_by, 13:45 pass in the symbol, firs_name: "nick", 13:50 we get back an array with one user in it. 13:55 If I do find_by(:last_name, 13:58 we get back an array with two items in it, just like we expected. 14:02 And, if I wanted to say user as an instance, 14:09 User.find_by(:last_name, "Seifer"), 14:13 and I will call the first item in there and 14:20 see that I can go to user.attributes, and it returns a hash of the attributes. 14:24 I can say user.first_name and that just returns the user's first name, 14:31 user.last_name "Seifer", 14:36 and then I could do user.first_name = "Also Jason" and 14:39 if I checked out the user, we can see that the attributes hash was also updated. 14:45 Now what we see here with extending 14:53 ClassMethods when including a class is pretty common. 14:57 This is a very common Ruby idiom that you'll see 15:01 if you start reading a lot of code. 15:04 Go ahead and read over the different ClassMethods and 15:06 the module, and even try including it on your own. 15:10 Go ahead and play around with it. 15:13 And these are the basics of using include and extend. 15:15 By combining both of them you get a lot of power and 15:19 flexibility when writing your Ruby programs. 15:22