So in my never-ending quest to remove conditional logic from code, I began writing my Rails actions like:
def create
@user = User.new params[:user]
@user.save!
redirect_to user_path(@user)
rescue ActiveRecord::RecordNotSaved
flash[:notice] = 'Unable to create user'
render :action => :new
end
Instead of the traditional way using conditional logic like:
def create
@user = User.new params[:user]
if @user.save
redirect_to user_path(@user)
else
flash[:notice] = 'Unable to create user'
render :action => :new
end
end
I did this until one of my co-workers saw this and said:
exceptions should not be expected
What?
When writing an application you expect invalid input from users. Since we expect invalid input we should NOT be handling it via exceptions because exceptions should only be used for unexpected situations.
Well what’s an unexpected situation?
- Losing a connection to your database
- Running out of memory
- Some obscure IO/socket error
Now I’m not going to write error handling for those unexpected circumstances because I don’t expect them to happen. If they do then I want the user to see an error page (500) and the developers to be notified via email about the exception (using exception notifier in Rails) (if i cared then yes I’d have to write error handling code but trying to recover from situations such as an out of memory is not going to be easy, if at all possible, so I want my application to fail in such a rare situation).
Let’s look at an example of reading from a file from the pickaxe.
File.open('testfile') do |file|
while line = file.gets
puts line
end
end
Now IO#gets returns a String or nil. That is correct because reaching
the end of the file is NOT an unexpected situation and should be handled with
conditional logic.
What if ruby raised some EOFError instead:
File.open('testfile') do |file|
while line = file.gets
puts line
end
rescue EOFError
return
end
Raising an EOFError would mean reaching the end of the file would be
unexpected. That’s crazy talk.
I think ActiveRecord::Base#save! and ActiveRecord::Base.update_attributes!
should be pulled from the public API. All they do is save the object and raise an exception if
any validation fails. I doubt there are any web applications out there that do
NOT expect invalid input. So there is no point in using #save! and
#update_attributes! over their true/false equivalents #save and
#update_attributes.
However, there is one case where they both have to be used. In transactions. Apparently the only way to roll back a transaction is to raise an exception. I say there should be some #rollback method so you could do something like:
User.transaction do
user.save || rollback
another_user.save || rollback
end
Following this convention we could also return the ‘!’ back to its original
meaning of ‘modifying the receiver’. Rails, with its #save! and
#update_attributes!, has pushed the ‘!’ to mean ‘something dangerous’ in order
to justify the exception raising of methods like #save! and
#update_attributes!.
Another Ruby idiom that is an abuse of exceptions is something along the lines of:
user.address.street rescue ''
# or
user.address.street rescue nil
Code like this says that an address is optional for a user. Since a user without an address is not an exceptional situation it should be handled with conditional logic instead of exceptions.
if ! user.address.nil?
user.address.street
end
I can’t stand that ‘rescue nil’ hack. Quit being lazy and write the ‘if’ statement.
