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In our pages controller, several action
methods have code that takes the ID

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parameter, finds a page object with that
ID and stores it in an instance variable.

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We have code to do this exact same
operation at the top of the show, edit,

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update and destroy methods.

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Again, the don't repeat
yourself principle applies.

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It's usually better to
avoid repeated code.

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So let's do like we did with
the page params method and

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move the repeated code
to a separate method.

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We'll copy the page =
page.find(params id) code

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out of the show method and
move it to a set page method.

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We'll put this under the private
keyword since this method should only

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be accessed within the page's controller.

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Paste start code there.

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Then we'll go back through the show,
edit, update, and

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destroy methods and replace the repeated
code with a call to set page.

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Now that's one way to do it.

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But running a setup method at
the start of a controller action is so

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common that Rails provides
another shortcut.

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At the top of your controller class
outside of any instance methods,

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you can call the before action method.

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You can pass before action a symbol
with the name of a method, set page.

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With this set up,

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the controller will call the set page
method before running any action method.

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You can remove all of
the explicit calls to set page.

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Let's save this and go to our browser and

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see what happens if we try to
reload the list of all pages.

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We get a record not found error.

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Couldn't load page with an ID of blank.

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This happens because of
course the index new and

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create methods don't have an ID parameter.

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There's none in the URL for it to load.

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But Rails will try to run the set page
method anyway resulting in an error.

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So you may want to add the except argument
on to your call to before action.

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Except takes an array of symbols
with the names of methods.

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So we'll give it method index,
new and create.

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Rails won't run the setup method before

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any action that appears
in the except list.

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If we reload our browser,
we'll see that the index,

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new and
create methods all work correctly again.

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This is because Rails isn't calling
set page before them anymore.

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It'll still run set page before the show,
edit, update and destroy actions, though.

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If we maintain a list of the actions we
don't want to run with set page, though,

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we have to remember to add to the list
any time we add another action method.

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We might be better off maintaining
a list of the actions we

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do want to run with set page.

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We can do that by changing
the except argument to only and

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changing the list of method names
from index, show and create to show,

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edit, update and

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destroy.

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Now the before action will run
before only these methods.

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If we try all the actions
out in our browser,

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we'll see that everything
still works correctly.