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

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

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

update and destroy methods.

Again, the don't repeat yourself principle applies.

It's usually better to avoid repeated code.

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

move the repeated code to a separate method.

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

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

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

be accessed within the page's controller.

Paste start code there.

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

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

Now that's one way to do it.

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

common that Rails provides another shortcut.

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

you can call the before action method.

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

With this set up,

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

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

Let's save this and go to our browser and

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

We get a record not found error.

Couldn't load page with an ID of blank.

This happens because of course the index new and

create methods don't have an ID parameter.

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

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

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

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

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

Rails won't run the setup method before

any action that appears in the except list.

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

new and create methods all work correctly again.

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

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

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

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

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

do want to run with set page.

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

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

edit, update and

destroy.

Now the before action will run before only these methods.

If we try all the actions out in our browser,

we'll see that everything still works correctly.