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Rails Routes and Resources

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3:41 with Jay McGavren

At the top of your controller class, outside of any instance methods, you can call the before_action method, and pass it a symbol with the name of a method. With this set up, the controller will call that method before running any action method.

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    In our pages controller, several action methods have code that takes the ID 0:00
    parameter, finds a page object with that ID and stores it in an instance variable. 0:04
    We have code to do this exact same operation at the top of the show, edit, 0:10
    update and destroy methods. 0:15
    Again, the don't repeat yourself principle applies. 0:17
    It's usually better to avoid repeated code. 0:19
    So let's do like we did with the page_params method and 0:22
    move the repeated code to a separate method. 0:25
    We'll copy the page = page.find(params[:id]) code 0:28
    out of the show method and move it to a set_page method. 0:32
    We'll put this under the private keyword since this method should only 0:37
    be accessed within the page's controller. 0:41
    Paste start code there. 0:49
    Then we'll go back through the show, edit, update, and 0:52
    destroy methods and replace the repeated code with a call to set_page. 0:55
    Now that's one way to do it. 1:07
    But running a setup method at the start of a controller action is so 1:09
    common that Rails provides another shortcut. 1:12
    At the top of your controller class outside of any instance methods, 1:15
    you can call the before_action method. 1:19
    You can pass before_action a symbol with the name of a method, set_page. 1:24
    With this set up, 1:30
    the controller will call the set_page method before running any action method. 1:30
    You can remove all of the explicit calls to set_page. 1:35
    Let's save this and go to our browser and 1:44
    see what happens if we try to reload the list of all pages. 1:46
    We get a record not found error. 1:50
    Couldn't load page with an ID of blank. 1:51
    This happens because of course the index new and 1:54
    create methods don't have an ID parameter. 1:56
    There's none in the URL for it to load. 1:59
    But Rails will try to run the set page method anyway resulting in an error. 2:02
    So you may want to add the except argument on to your call to before action. 2:06
    Except takes an array of symbols with the names of methods. 2:13
    So we'll give it method index, new and create. 2:16
    Rails won't run the setup method before 2:24
    any action that appears in the except list. 2:26
    If we reload our browser, we'll see that the index, 2:29
    new and create methods all work correctly again. 2:34
    This is because Rails isn't calling set_page before them anymore. 2:39
    It'll still run set_page before the show, edit, update and destroy actions, though. 2:43
    If we maintain a list of the actions we don't want to run with set_page, though, 2:50
    we have to remember to add to the list any time we add another action method. 2:54
    We might be better off maintaining a list of the actions we 2:59
    do want to run with set_page. 3:01
    We can do that by changing the except argument to only and 3:03
    changing the list of method names from :index, :new, and :create to :show, 3:09
    :edit, :update and 3:15
    :destroy. 3:21
    Now the before action will run before only these methods. 3:24
    If we try all the actions out in our browser, 3:28
    we'll see that everything still works correctly. 3:30

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