Rails - Mar 2005 - Transactions

I cant seem to find much information apart from the basics on 
transactions in rails.  I have 5 nested transactions, and am performing 
the following basic operations:

@whatever1.transaction do
  @whatever2.transaction do
   ....
   @whatever2.something = "foo"
   @whatever1.something = "bar"
   @whatever1.save
   @whatever2.save
.....
  end
end

((( I had tried using if''s inside the transaction on the
save''s, and it
worked for a bit but I then discovered it wasnt fool proof.  I assumed 
that was not the correct way, so I abandoned that route. )))

Now here is where im stuck.  How do I know if the transaction committed 
or rolled back?  If it rolled back id obviously want to throw them back, 
whereas if it committed id want to show a completion page.  Something 
along the following lines:

unless @???.committed?
  render_action ''somewhere''
end

Is this how its done (idealogically) or am I on the wrong track?  If 
anyone can give me an example of doing what Im attempting id really 
appreciate it!  Thanks.

Hi Joe,

A transaction is rolled back when an exception is thrown beyond 
the .transaction block. You''ll often see:

@a = A.new ...
@b = B.new ...
@b.a = @a
begin
  transaction do
    raise SomeError unless a.save && b.save 
    notice "Save successful" # records have been saved, show success
page
    redirect_to :show ...
  end
rescue SomeError # rollback done
  notice "Save failed" # show error message
  # assuming postback, render the form again so user can fix input.
end

-- 

Nicholas Seckar aka. Ulysses

Ok, this was helpful and Ive rewritten my code in the format below.  My 
raise and rescue are not working though.  I did:
raise zipError unless @zip_code = ZipCode.find_by_zip(@user_detail.zip5)
rescue zipError 
      @user_detail.errors.add("zip5", "does not exist in our
database")

and receive: undefined local variable or method `zipError''

Im assuming I cant define my own exception, and I have to raise some 
exception that rails know about.  I cant find anything about the 
possible exceptions i could raise.  Maybe Im just way off and not even 
on the right track here, I have no clue at this point. 


Nicholas Seckar wrote:
>Hi Joe,
>
>A transaction is rolled back when an exception is thrown beyond 
>the .transaction block. You''ll often see:
>
>@a = A.new ...
>@b = B.new ...
>@b.a = @a
>begin
>  transaction do
>    raise SomeError unless a.save && b.save 
>    notice "Save successful" # records have been saved, show
success page
>    redirect_to :show ...
>  end
>rescue SomeError # rollback done
>  notice "Save failed" # show error message
>  # assuming postback, render the form again so user can fix input.
>end
>
>  
>

zipError needs to be an actual error class. You can use one of Ruby''s
build in classes, such as ArgumentError, or one of ActiveRecord''s such
as RecordNotFound error, or you can add your own.

class SaveFailed < Exception
end

begin raise SaveFailed
rescue SaveFailed
  puts "caught save failed error"
end

Alternatively, you can use a blank raise, with will raise a RuntimeError.

I was curious about how much of an operation should be wrapped in a 
transaction.
Say I had something like 

list = []
objects.each{ |o| list << o.process; o.destroy}
record.content = list.join
record.save

If I wanted to ensure that the entire process took place, would I wrap the 
whole thing in a transaction? Further, does a transaction only roll back the 
state if an exception is cast? If so, should one use
''record.save!'' in this
case?

Also, does the normal transaction just rely on the database, where as the 
object transaction depends on binding the state of the objects beforehand?

Just trying to make sure I know what''s going on before I rely on them
;)

.adam sanderson


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