--- title: const_get - Metaprogramming in Ruby slug: const_get-metaprogramming-in-ruby published_at: 2021-01-25 20:00:08 +0000 updated_at: 2026-03-04 20:14:33 +0000 summary: description: Another metaprogramming in Ruby that you'll find useful. Module.const_get allows you to pass a string or symbol value and returns the real constant or class. In this video, you'll learn how metaprogramming is used to implement the `resources` method in Rails routes. Presenter: https://twitter.com/cjav_dev Code: https://gist.github.com/cjavdev/10e280a5210cdf7297f4ecb436a73aed tags: [metaprogramming in ruby, metaprogramming, ruby metaprogramming, const_get, Module.const_get, DSL, ruby dsls, ruby, ruby on rails, Rails] views: 698 author: CJ Avilla url: https://www.cjav.dev/videos/const_get-metaprogramming-in-ruby youtube_url: https://www.youtube.com/watch?v=yXROBruo1JE youtube_id: yXROBruo1JE embed_url: https://www.youtube.com/embed/yXROBruo1JE thumbnail_url: https://i.ytimg.com/vi/yXROBruo1JE/hqdefault.jpg type: video --- # const_get - Metaprogramming in Ruby *Published: January 25, 2021* *Views: 698* ## Watch [Watch on YouTube](https://www.youtube.com/watch?v=yXROBruo1JE) [![const_get - Metaprogramming in Ruby](https://i.ytimg.com/vi/yXROBruo1JE/hqdefault.jpg)](https://www.youtube.com/watch?v=yXROBruo1JE) ## Description Another metaprogramming in Ruby that you'll find useful. Module.const_get allows you to pass a string or symbol value and returns the real constant or class. In this video, you'll learn how metaprogramming is used to implement the `resources` method in Rails routes. Presenter: https://twitter.com/cjav_dev Code: https://gist.github.com/cjavdev/10e280a5210cdf7297f4ecb436a73aed ## Transcript what's up friends i just wanted to quickly go over another meta programming concept in ruby this time we're going to talk about const get which is a way to uh just pass a string or a symbol and get back the constant value so if you had just some top level thing defined here in main like maybe we have some temperature value that is a constant if we wanted to get that further down in our program somewhere far away we didn't really have access to this variable what we could do is we could call module.const get and pass in the string value here for that variable so if we print this out we should see 32 and we do so this is just one way that we can get constants that are defined it turns out that classes that are named in ruby so if you have a class whose name is cat so we say class cat and then maybe we just give it a method here um let's actually uh again initialize with some name we'll say like at name equals name and then uh meow is the method and we'll just puts um and then their name okay so what what's cool about this is with with const get we can access the cat class just by having that string value for the name cat so if we just say you know puts uh or actually we can just call module.const get and pass in the name of the class as a string then we can call dot uh so that that will give us back like the class right so we could say like class so it's a one one common way that you might see or one thing that you might see in ruby is folks will name class if like the thing stored in the class is uh the class object itself rather than an instance of the class uh technically it's an instance of the class class but um yeah so the the cat the class cat will be stored in this class that starts with the k let's actually just call it the cat class like that so if we want to create a new cat we could say like fido is cat class dot new fido and then we could say fido.meow because now now that we have the constant right because we called const get we're getting back the instance of or the the cat class and now we can create an instance of it by newing it up just like so let's run this again and now we see hi i'm fido because that fido.meow is the method on the instance of the cat class um so one one additional thing that we might look at is um here i've been calling const get directly on module but you can you can call const get directly on the class object itself and that will sort of like start the lookup for the constant inside of the cat class so um if we had cat and it was just like uh i don't know let's say xxx equals thing i don't know so if we say uh now we can say just like puts um mod or a cat dot const get and we can put in xxx and that should print out um thing right because we're looking up thing inside of the cat class which is totally cool but if we pass in temp here um it actually it turns out that it will uh it'll also work right so we're gonna get 32 again at the bottom and that's because when we call const get directly on a class like this it will start looking inside the class and then it will start looking in the class's parent scopes so we're able to see temp if we were to create another class uh dog and if in here we had another constant called yyyy another uh thing maybe if we were to try to look up the yyyy constant from the cat class we wouldn't be able to find it because it's inside of the dog it's defined inside of the dog class so if we run this we we get a failure because this is not an initialized constant right because yyy isn't defined inside of cat and it's not it's also not defined inside of the the level that cat is defined at so um this can be super helpful and super useful when thinking about the way that rails sort of initializes controllers right given your the the routes.rb dsl or domain specific language because when we're when we're talking about rails we just say resources and then maybe we say you know posts right and this will create for us a posts route it'll create for us posts slash id it'll create uh posts right so when we call resources posts we're passing in this one symbol and it knows to create all of these different methods and it knows which controller actions to call by convention etc etc but what's interesting is when we use this resources method we're only passing this name we're not actually passing anything else so if you have like a class posts controller let's think about how we could define a method resources that takes in the symbol posts and knows to new up the posts controller when it's receiving one of these sort of actions well we might define a method like this resources and then let's just say like the type or something and then we want to say you know like let's take that string or that symbol value and we'll turn it into a string by saying like uh controller class name is equal to um and so uh in rails there's a whole bunch of stuff that comes from active support that allows you to like capitalize or title case or whatever your string values so here i'm just going to kind of like take the naive approach and say t let's see um actually is capitalize is that directly in ruby let's see pry test dot capitalize i think that's in oh it's okay cool it's in it's in ruby so we'll just say like t dot capitalize that's going to be the controller name and then we could say controller class is equal to module.const get controller class name and that should give us back an instance of the controller and then we could say like controller class dot index right or something whatever like um we would need some sort of like if statement if uh the route is like posts or something like we're just getting the the index route then we'll say you know def index puts i'm in the index okay so now let's just call uh resources posts and see what happens okay uh controller class what happened here so it says undefined local variable controller class name uh control constroller okay try again uh undefined method index for postcontroller class oh but it's here oh you know what this needs to be in dot new dot index we didn't actually initialize a new instance of it okay so now we're yeah because controller class again remember this is going to be like uh the posts controller so we still have to initialize an instance of it and call the method on it so now we get back i'm in the index so this would be helpful uh and i um yeah module.cons get is cool uh it's really powerful and i'm excited to use that when we go through and rebuild a little small light version of rails to just deeply understand exactly how things might work cool all right hope you enjoyed 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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