Rails - Jun 2006 - Transaction syntax for > 1 table/model

If I want to wrap two account updates, the syntax is ...

Account.transaction(dennis, david) do
  dennis.withdraw(10);
  david.deposit(10);
end

.. where both dennis and david are instances of Account. 

Can someone please let me know what the syntax is for starting and ending 
a *single* database transactions which spans two *different* kinds of 
models/tables ?

Dennis Byrne
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Hi Dennis,

On Jun 26, 2006, at 6:41 PM, Dennis_Byrne@ak.blm.gov
wrote:
> If I want to wrap two account updates, the syntax is ...
>
> Account.transaction(dennis, david) do
>   dennis.withdraw(10);
>   david.deposit(10);
> end
>
> .. where both dennis and david are instances of Account.
>
> Can someone please let me know what the syntax is for starting and  
> ending a *single* database transactions which spans two *different*  
> kinds of models/tables ?
When you start a database transaction it''s in effect for that  
connection as a whole, not any particular table or row. The  
Account.transaction and dennis.transaction class and instance methods  
are simply conveniences which delegate to the current database  
connection.

   A.transaction do  # begin
     B.create!       # insert into bs (attrs) values (?)
     C.create!       # insert into cs (attrs) values (?)
   end               # commit

If you have models whose tables are in different databases and you  
need transactional guarantees, you''re out of luck - distributed  
transactions are unsupported.

In your example you use an object transaction as well. You rarely  
want this: if a database transaction rolls back, you''ll want to, say,  
display dennis.errors. But the object transaction rolls dennis back  
to his state - including dennis.errors - before the withdrawal.

Best,
jeremy

> When you start a database transaction it''s in effect for that 
> connection as a whole, not any particular table or row. The 
> Account.transaction and dennis.transaction class and instance methods 
> are simply conveniences which delegate to the current database 
Thanks Jeremy,

Yes, the transaction is scoped to the entire connection ;) , but the 
syntax led me to ask if the transaction was somehow restricted to 
accounts. 
> In your example you use an object transaction as well. You rarely 
> want this: if a database transaction rolls back, you''ll want to,
say,
> display dennis.errors. But the object transaction rolls dennis back 
> to his state - including dennis.errors - before the withdrawal.
I didn''t know this.  Thanks again.  I will assume it is better to go
with
a database transaction than an object transaction for this reason.
 
Dennis Byrne
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