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We'll show you how to troubleshoot problems accessing your database. We'll also show you how to use the Rails Logger as a general debugging tool.
You can read more about the Rails Logger on the Rails Guides site.
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Suppose you had the index of
pets open in your browser and
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you clicked the link to open the show
page for one of them in a new tab.
0:03
You do some random browsing and
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then go back to the first tab and
click the delete link for that pet.
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Some more random browsing.
0:18
And then you go back to the show
page tab and click Reload.
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You'll get this error,
ActiveRecord::RecordNotFound.
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What should you do in this case?
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Possibly nothing.
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The record is missing
because it's supposed to be.
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You deleted it, after all.
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In the rare event that something
similar happens in production,
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rather than a RecordNotFound error,
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your users will see a 404 page indicating
they tried to access a nonexistent page.
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If you want finer control
over these errors,
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though, one thing you can do
is to rescue the exception.
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We can go into
app/controllers/pets_controller.rb,
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find the show action, and edit it to add
a rescue clause to rescue the exception.
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So we can say rescue
ActiveRecord::RecordNotFound,
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since that's the exception
we were getting.
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In the event that we get such an error,
we can put up a flash
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message of type notice,
saying we couldn't find that pet.
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And then we can redirect users
back to the index of all pets.
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Save that, and let's try reloading
the page with the missing pet.
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Reload it and we get redirected back
to the index of all pets with a flash
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message, we couldn't find that pet.
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You'll have to decide which is right for
your app,
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explicitly handling load errors or
just letting it show a 404 page.
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The ability to explicitly handle errors
is there if you want it, though.
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Sometimes, RecordNotFound errors
can be more serious, though.
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Suppose we were getting this error every
time we clicked an Edit link for a pet.
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ActiveRecord::RecordNotFound Couldn't
find pet with ID, and nothing.
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And if we look in the stack trace,
we see this is occurring at
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app/controllers/pets_controller.rb line
26 in the edit method.
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Let's try opening the file mentioned in
the stack trace and see what we find.
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We'll open
app/controllers/pets_controller.rb and
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we'll go to line 26.
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The problem seems to be with
that call to Pet.find but
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maybe you can't see what it is just yet.
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Let's take another look at the error.
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It says, couldn't find pet with ID, blank.
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It seems like there should
be an ID number after 'id'=.
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That's a little strange.
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Let's take a look at exactly
what we're passing to Pet.find.
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Rails apps have a logger object which
writes text messages to your terminal
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and to a file.
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All those messages you've seen regarding
GET and POST requests, errors,
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and so on were displayed by the logger.
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And you can display your own
messages using the logger too.
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You can call the logger method within
the code for any Rails controller,
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model, or mailer.
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That'll return a logger object,
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and there are several methods
that you can call on that object.
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All these methods take a string and
add it to the log.
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The difference is how
important that message is.
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Debug messages aren't very important and
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should only be included in the log
when debug mode is turned on.
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Info messages have information
about what the app is doing.
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They're probably safe to omit
from the log in most cases.
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Warn messages indicate
a potential problem.
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They should probably be
included in the log.
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Error messages
are definitely a problem and
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should be included in
the log no matter what.
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And fatal messages indicate a problem so
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serious that the app is
probably about to shut down.
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Okay, so we need to use a log message to
look at the contents of params[:pet].
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Since Rails apps turn debug messages off
by default, even in development mode,
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we're going to call the info method
to add a message to the log.
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So we'll add some code right before
the line where the error occurs.
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We'll call logger to
get the logger object.
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I'll call the info method on it and
I'll pass a string to it.
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The :pet parameter is, and
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I'll interpolate some Ruby
code into the string,
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#{params}, close the interpretation,
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close the string out,
and end the method call.
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Let's save that.
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And now let's refresh our browser page so
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that it runs the pets
controller edit method again.
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That will run the code
that calls logger.info.
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Once that's done, scroll back through
your log and you'll see your message.
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The pet parameter is nothing?
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That can't be right.
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We need to figure out what's going on.
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If you look at the log right before our
log message, you'll see these messages.
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That's the GET request our browser
made to load the edit form.
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See the parameters line there?
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That shows the contents
of the params hash.
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We see an id key, but no pet key.
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Aha, there's our problem.
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We were supposed to use params ID when
calling pet.find, not params pet.
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So let's go back into the pets
controller and fix that.
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We'll remove the log message,
save our work, and reload the page.
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Now that we're using a parameter
that actually exists,
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the pet record is found successfully and
the error is gone.
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The Rails logger can be a useful tool for
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debugging your apps in other situations,
too.
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See the teacher's notes if
you'd like more info on it.
5:45
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