[Dbix-class] Overloads in ResultSet and Stacktraces

[Aaron Schrab]

I'm working on a Catalyst project that uses DBIx::Class (0.08010) as the 
ORM, and the StackTrace plugin.  This combination is causing a problem.

In one section of this project I'm using a ResultSet to create a new row 
in a table as part of a transaction.  In some cases the new row will 
violate a unique constraint, so I want to catch those errors and notify 
the user.  However, the error that I get in $@ isn't due to the 
constraint violation.

Instead the error is due to trying to count the number of items in the 
ResultSet while in a failed transaction.  I've traced this to the 
Devel::StackTrace module attempting to stringify the ResultSet.  Because 
the string conversion operator isn't implemented but the numeric 
conversion operator is, the latter is used in place of the former.  This 
results in trying to do a sql count before the failed transaction is 
rolled back.

I've worked around this problem by overloading string conversion to 
return a static string, but there may be better ways to avoid this 
problem.

Beginning of actual error text:
DBI Exception: DBD::Pg::st execute failed: ERROR:  current transaction is aborted, commands ignored until end of transaction block
  [for Statement "SELECT COUNT( * ) FROM property me"] at /usr/local/share/perl/5.8.8/DBIx/Class/Schema.pm line 954

Pseudocode:

eval { $schema->txn_do( sub {
   $schema->resultset('Property')->create( \%data );
   # Violates unique constraint
}) };

if( $@ and $@ =~ /unique constraint/ ) {
   # This isn't reached
}