libs/Tangram/Storage.pm

# (c) Sound Object Logic 2000-2001

use strict;

package Tangram::Storage;
use DBI;
use Carp;

use vars qw( %storage_class );

BEGIN {

  eval { require 'WeakRef.pm' };

  if ($@) {
    *Tangram::weaken = sub { };
    $Tangram::no_weakrefs = 1;
  } else {
    *Tangram::weaken = \&WeakRef::weaken;
    $Tangram::no_weakrefs = 0;
  }
}

sub new
{
    my $pkg = shift;
    return bless { @_ }, $pkg;
}

sub schema
{
    shift->{schema}
}

sub export_object
  {
    my ($self, $obj) = @_;
    return $self->{export_id}->($self->{get_id}->($obj));
  }

sub split_id
  {
        carp unless wantarray;
        my ($self, $id) = @_;
        my $cid_size = $self->{cid_size};
        return ( substr($id, 0, -$cid_size), substr($id, -$cid_size) );
  }

sub combine_ids
  {
        my $self = shift;
        return $self->{layout1} ? shift : sprintf("%d%0$self->{cid_size}d", @_);
  }

sub _open
  {
    my ($self, $schema) = @_;

        my $dbh = $self->{db};

    $self->{table_top} = 0;
    $self->{free_tables} = [];

    $self->{tx} = [];

    $self->{schema} = $schema;

        {
          local $dbh->{PrintError} = 0;
          my $control = $dbh->selectall_arrayref("SELECT * FROM $schema->{control}");

          $self->{id_col} = $schema->{sql}{id_col};

          if ($control) {
                $self->{class_col} = $schema->{sql}{class_col} || 'type';
                $self->{import_id} = sub { shift() . sprintf("%0$self->{cid_size}d", shift()) };
                $self->{export_id} = sub { substr shift(), 0, -$self->{cid_size} };
          } else {
                $self->{class_col} = 'classId';
                $self->{layout1} = 1;
                $self->{import_id} = sub { shift() };
                $self->{export_id} = sub { shift() };
          }
        }

        my %id2class;

        if ($self->{layout1}) {
          # compatibility with version 1.x
          %id2class = map { @$_ } @{ $self->{db}->selectall_arrayref("SELECT classId, className FROM $schema->{class_table}") };
        } else {
          my $classes = $schema->{classes};
          %id2class = map { $classes->{$_}{id}, $_ } keys %$classes;
        }

        $self->{id2class} = \%id2class;
        @{ $self->{class2id} }{ values %id2class } = keys %id2class;

    $self->{set_id} = $schema->{set_id} ||
      sub
        {
          my ($obj, $id) = @_;

          if ($id) {
            $self->{ids}{0 + $obj} = $id;
          } else {
            delete $self->{ids}{0 + $obj};
          }
        };

    $self->{get_id} = $schema->{get_id} || sub {
          my $address = 0 + shift();
          my $id = $self->{ids}{$address};
          return undef unless $id;
          return $id if $self->{objects}{$id};
          delete $self->{ids}{$address};
          delete $self->{objects}{$id};
          return undef;
        };

    return $self;
  }

sub alloc_table
{
    my ($self) = @_;

    return @{$self->{free_tables}} > 0
        ? pop @{$self->{free_tables}}
            : ++$self->{table_top};
}

sub free_table
{
    my $self = shift;
    push @{$self->{free_tables}}, grep { $_ } @_;
}

sub open_connection
{
    # private - open a new connection to DB for read

    my $self = shift;
    DBI->connect($self->{-cs}, $self->{-user}, $self->{-pw}) or die;
}

sub close_connection
  {
    # private - close read connection to DB unless it's the default one
        
    my ($self, $conn) = @_;

        return unless $conn &&  $self->{db};
        
    if ($conn == $self->{db})
          {
                $conn->commit unless $self->{no_tx} || @{ $self->{tx} };
          }
    else
          {
                $conn->disconnect;
          }
  }

sub cursor
{
    my ($self, $class, @args) = @_;
    my $cursor = Tangram::Cursor->new($self, $class, $self->open_connection());
    $cursor->select(@args);
    return $cursor;
}

sub my_cursor
{
    my ($self, $class, @args) = @_;
    my $cursor = Tangram::Cursor->new($self, $class, $self->{db});
    $cursor->select(@args);
    return $cursor;
}

sub select_data
{
    my $self = shift;
    Tangram::Select->new(@_)->execute($self, $self->open_connection());
}

sub selectall_arrayref
{
    shift->select_data(@_)->fetchall_arrayref();
}

sub my_select_data
{
    my $self = shift;
    Tangram::Select->new(@_)->execute($self, $self->{db});
}

my $psi = 1;

sub prepare
  {
        my ($self, $sql) = @_;
        
        print $Tangram::TRACE "preparing [@{[ $psi++ ]}] $sql\n" if $Tangram::TRACE;
        $self->{db}->prepare($sql);
  }

*prepare_insert = \&prepare;
*prepare_update = \&prepare;
*prepare_select = \&prepare;

sub make_id
  {
    my ($self, $class_id) = @_;
        
        unless ($self->{layout1}) {
          my $id;
          
          if (exists $self->{mark}) {
                $id = $self->{mark}++;
                $self->{set_mark} = 1;  # cleared by tx_start
          } else {
                $id = $self->make_1st_id_in_tx();
          }
          
          return sprintf "%d%0$self->{cid_size}d", $id, $class_id;
        }

        # ------------------------------
        # compatibility with version 1.x

    my $alloc_id = $self->{alloc_id} ||= {};
    
    my $id = $alloc_id->{$class_id};
    
    if ($id)      {
                $id = -$id if $id < 0;
                $alloc_id->{$class_id} = ++$id;
      } else {
                my $table = $self->{schema}{class_table};
                $self->sql_do("UPDATE $table SET lastObjectId = lastObjectId + 1 WHERE classId = $class_id");
                $id = $self
                  ->sql_selectall_arrayref("SELECT lastObjectId from $table WHERE classId = $class_id")->[0][0];
                $alloc_id->{$class_id} = -$id;
      }
    
    return sprintf "%d%0$self->{cid_size}d", $id, $class_id;
  }


# create global unique identifers using Data::UUID
sub make_guid 
  {
    my $self = shift;

    eval ("use Data::UUID;");
    my $ug = Data::UUID->new();
    my $guid = $ug->create_str();
    
    return $guid;
  }

sub make_1st_id_in_tx
  {
    my ($self) = @_;
    
        unless ($self->{make_id}) {
          my $table = $self->{schema}{control};
          my $dbh = $self->{db};
          $self->{make_id}{inc} = $self->prepare("UPDATE $table SET mark = mark + 1");
          $self->{make_id}{set} = $self->prepare("UPDATE $table SET mark = ?");
          $self->{make_id}{get} = $self->prepare("SELECT mark from $table");
        }
        
        my $sth;
        
        $sth = $self->{make_id}{inc};
        $sth->execute();
        $sth->finish();
        
        $sth = $self->{make_id}{get};
        $sth->execute();
        my $id = $sth->fetchrow_arrayref()->[0];
        $sth->finish();

        return $id;
  }

sub update_id_in_tx
  {
        my ($self, $mark) = @_;
        my $sth = $self->{make_id}{set};
        $sth->execute($mark);
        $sth->finish();
  }

sub unknown_classid
{
    my $class = shift;
    confess "class '$class' doesn't exist in this storage"
}

sub class_id
{
    my ($self, $class) = @_;
    $self->{class2id}{$class} or unknown_classid $class;
}

#############################################################################
# Transaction

my $error_no_transaction = 'no transaction is currently active';

sub tx_start
{
    my $self = shift;

        unless (@{ $self->{tx} }) {
          delete $self->{set_mark};
          delete $self->{mark};
        }

    push @{ $self->{tx} }, [];
}

sub tx_commit
  {
    # public - commit current transaction
    
    my $self = shift;
    
    carp $error_no_transaction unless @{ $self->{tx} };
    
    # update lastObjectId's
    
    if ($self->{set_mark}) {
          $self->update_id_in_tx($self->{mark});
        }

        # ------------------------------
        # compatibility with version 1.x

    if (my $alloc_id = $self->{alloc_id}) {
          my $table = $self->{schema}{class_table};
        
          for my $class_id (keys %$alloc_id)
                {
                  my $id = $alloc_id->{$class_id};
                  next if $id < 0;
                  $self->sql_do("UPDATE $table SET lastObjectId = $id WHERE classId = $class_id");
                }
          
          delete $self->{alloc_id};
        }
        
        # compatibility with version 1.x
        # ------------------------------
    
    unless ($self->{no_tx} || @{ $self->{tx} } > 1) {
          # committing outer tx: commit to db
          $self->{db}->commit;
        }
        
    pop @{ $self->{tx} };               # drop rollback subs
  }

sub tx_rollback
  {
    my $self = shift;
    
    carp $error_no_transaction unless @{ $self->{tx} };
    
    if ($self->{no_tx})
      {
                pop @{ $self->{tx} };
      }
    else
      {
                $self->{db}->rollback if @{ $self->{tx} } == 1; # don't rollback db if nested tx
                
                # execute rollback subs in reverse order
                
                foreach my $rollback ( @{ pop @{ $self->{tx} } } )
                  {
                        $rollback->($self);
                  }
          }
}

sub tx_do
{
    # public - execute subroutine inside tx

    my ($self, $sub, @params) = @_;

    $self->tx_start();

    my ($results, @results);
    my $wantarray = wantarray();

    eval
    {
                if ($wantarray)
                {
                        @results = $sub->(@params);
                }
                else
                {
                        $results = $sub->(@params);
                }
    };

    if ($@)
    {
                $self->tx_rollback();
                die $@;
    }
    else
    {
                $self->tx_commit();
    }

    return wantarray ? @results : $results;
}

sub tx_on_rollback
{
    # private - register a sub that will be called if/when the tx is rolled back

    my ($self, $rollback) = @_;
    carp $error_no_transaction if $^W && !@{ $self->{tx} };
    unshift @{ $self->{tx}[0] }, $rollback; # rollback subs are executed in reverse order
}

#############################################################################
# insertion

sub insert
{
    # public - insert objects into storage; return their assigned ids

    my ($self, @objs) = @_;

    my @ids = $self->tx_do(
           sub
           {
                   my ($self, @objs) = @_;
                   map
                   {
                           local $self->{defered} = [];
                           my $id = $self->_insert($_, Set::Object->new() );
                           $self->do_defered;
                           $id;
                   } @objs;
           }, $self, @objs );

    return wantarray ? @ids : shift @ids;
}

sub _insert
{
    my ($self, $obj, $saving) = @_;

        die unless $saving;

    my $schema = $self->{schema};

    return $self->id($obj)
      if $self->id($obj);

        # insert global unique identifier in object to persist across re-deploys 
        $obj->{guid} = $self->make_guid();

  # debug
  use Data::Dumper;
        print Dumper($obj);
        
    $saving->insert($obj);

    my $class_name = ref $obj;
    my $classId = $self->{class2id}{$class_name} or unknown_classid $class_name;
        my $class = $self->{schema}->classdef($class_name);

    my $id = $self->make_id($classId);

    $self->welcome($obj, $id);
    $self->tx_on_rollback( sub { $self->goodbye($obj, $id) } );

        my $dbh = $self->{db};
        my $engine = $self->{engine};
        my $cache = $engine->get_save_cache($class);

        my $sths = $self->{INSERT_STHS}{$class_name} ||=
          [ map { $self->prepare($_) } @{ $cache->{INSERTS} } ];

        my $context = { storage => $self, dbh => $dbh, id => $id, SAVING => $saving };
        my @state = ( $self->{export_id}->($id), $classId, $cache->{EXPORTER}->($obj, $context) );

        my $fields = $cache->{INSERT_FIELDS};

        use integer;

        for my $i (0..$#$sths) {

          if ($Tangram::TRACE) {
                printf $Tangram::TRACE "executing %s with (%s)\n",
                $cache->{INSERTS}[$i], 
                join(', ', map { $_ || 'NULL' } @state[ @{ $fields->[$i] } ] )
          }

          my $sth = $sths->[$i];
          $sth->execute(@state[ @{ $fields->[$i] } ]);
          $sth->finish();
        }

    return $id;
  }

#############################################################################
# update

sub update
{
    # public - write objects to storage

    my ($self, @objs) = @_;

    $self->tx_do(
                 sub
                 {
                     my ($self, @objs) = @_;
                     foreach my $obj (@objs)
                     {
                           local $self->{defered} = [];

                           $self->_update($obj, Set::Object->new() );
                           $self->do_defered;
                     }
                   }, $self, @objs);
  }

sub _update
  {
    my ($self, $obj, $saving) = @_;

        die unless $saving;

    my $id = $self->id($obj) or confess "$obj must be persistent";

    $saving->insert($obj);

    my $class = $self->{schema}->classdef(ref $obj);
        my $dbh = $self->{db};
        my $context = { storage => $self, dbh => $dbh, id => $id, SAVING => $saving };

        my $cache = $self->{engine}->get_save_cache($class);
        my @state = ( $self->{export_id}->($id), substr($id, -$self->{cid_size}), $cache->{EXPORTER}->($obj, $context) );

        my $fields = $cache->{UPDATE_FIELDS};

        my $sths = $self->{UPDATE_STHS}{$class->{name}} ||=
          [ map {
                print $Tangram::TRACE "preparing $_\n" if $Tangram::TRACE;
                $self->prepare($_)
          } @{ $cache->{UPDATES} } ];

        use integer;

        for my $i (0..$#$sths) {

          if ($Tangram::TRACE) {
                printf $Tangram::TRACE "executing %s with (%s)\n",
                $cache->{UPDATES}[$i],
                join(', ', map { $_ || 'NULL' } @state[ @{ $fields->[$i] } ] )
          }

          my $sth = $sths->[$i];
          $sth->execute(@state[ @{ $fields->[$i] } ]);
          $sth->finish();
        }
  }

#############################################################################
# save

sub save
  {
    my $self = shift;
        
    foreach my $obj (@_) {
          if ($self->id($obj)) {
            $self->update($obj)
          }     else {
            $self->insert($obj)
          }
    }
  }

sub _save
  {
        my ($self, $obj, $saving) = @_;
        
        if ($self->id($obj)) {
          $self->_update($obj, $saving)
        } else {
          $self->_insert($obj, $saving)
        }
  }


#############################################################################
# erase

sub erase
  {
    my ($self, @objs) = @_;

    $self->tx_do(
                 sub
                 {
                   my ($self, @objs) = @_;
                   my $schema = $self->{schema};
                   my $classes = $self->{schema}{classes};

                   foreach my $obj (@objs)
                     {
                       my $id = $self->id($obj) or confess "object $obj is not persistent";
                           my $class = $schema->classdef(ref $obj);

                       local $self->{defered} = [];
                           
                       $schema->visit_down(ref($obj),
                                           sub
                                           {
                                             my $class = shift;
                                             my $classdef = $classes->{$class};

                                             foreach my $typetag (keys %{$classdef->{members}}) {
                                               my $members = $classdef->{members}{$typetag};
                                               my $type = $schema->{types}{$typetag};
                                               $type->erase($self, $obj, $members, $id);
                                             }
                                           } );

                           my $sths = $self->{DELETE_STHS}{$class->{name}} ||=
                                 [ map { $self->prepare($_) } @{ $self->{engine}->get_deletes($class) } ];
                   
                       my $eid = $self->{export_id}->($id);

                           for my $sth (@$sths) {
                                 $sth->execute($eid);
                                 $sth->finish();
                           }

                       $self->do_defered;

                       $self->goodbye($obj, $id);
                       $self->tx_on_rollback( sub { $self->welcome($obj, $id) } );
                     }
                 }, $self, @objs );
  }

sub do_defered
{
    my ($self) = @_;

    foreach my $defered (@{$self->{defered}})
    {
                $defered->($self);
    }

    $self->{defered} = [];
}

sub defer
{
    my ($self, $action) = @_;
    push @{$self->{defered}}, $action;
}

sub load
{
    my $self = shift;

    return map { scalar $self->load( $_ ) } @_ if wantarray;

    my $id = shift;
    die if @_;

    return $self->{objects}{$id}
      if exists $self->{objects}{$id} && defined $self->{objects}{$id};

    my $class = $self->{schema}->classdef( $self->{id2class}{ int(substr($id, -$self->{cid_size})) } );

        my $row = _fetch_object_state($self, $id, $class);

    my $obj = $self->read_object($id, $class->{name}, $row);

    # ??? $self->{-residue} = \@row;

    return $obj;
}

sub reload
{
    my $self = shift;

    return map { scalar $self->load( $_ ) } @_ if wantarray;

        my $obj = shift;
    my $id = $self->id($obj) or die "'$obj' is not persistent";
    my $class = $self->{schema}->classdef( $self->{id2class}{ int(substr($id, -$self->{cid_size})) } );

        my $row = _fetch_object_state($self, $id, $class);
    _row_to_object($self, $obj, $id, $class->{name}, $row);

    return $obj;
}

sub welcome
  {
    my ($self, $obj, $id) = @_;
    $self->{set_id}->($obj, $id);
    Tangram::weaken( $self->{objects}{$id} = $obj );
  }

sub goodbye
  {
    my ($self, $obj, $id) = @_;
    $self->{set_id}->($obj, undef) if $obj;
    delete $self->{objects}{$id};
    delete $self->{PREFETCH}{$id};
  }

sub shrink
  {
    my ($self) = @_;

    my $objects = $self->{objects};
    my $prefetch = $self->{prefetch};

    for my $id (keys %$objects)
      {
        next if $objects->{$id};
        delete $objects->{$id};
        delete $prefetch->{$id};
      }
  }

sub read_object
  {
    my ($self, $id, $class, $row, @parts) = @_;

    my $schema = $self->{schema};

    my $obj = $schema->{make_object}->($class);

    unless (exists $self->{objects}{$id} && defined $self->{objects}{$id}) {
      # do this only if object is not loaded yet
      # otherwise we're just skipping columns in $row
      $self->welcome($obj, $id);
    }

    _row_to_object($self, $obj, $id, $class, $row, @parts);

    return $obj;
  }

sub _row_to_object
  {
    my ($self, $obj, $id, $class, $row) = @_;
        $self->{engine}->get_import_cache($self->{schema}->classdef($class))
          ->($obj, $row, { storage => $self, id => $id, layout1 => $self->{layout1} });
        return $obj;
}

sub _fetch_object_state
{
    my ($self, $id, $class) = @_;

        my $sth = $self->{LOAD_STH}{$class->{name}} ||=
          $self->prepare($self->{engine}->get_instance_select($class));

        $sth->execute($self->{export_id}->($id));
        my $state = [ $sth->fetchrow_array() ];
        $sth->finish();

        return $state;
}

sub get_polymorphic_select
  {
        my ($self, $class) = @_;
        return $self->{engine}->get_polymorphic_select($self->{schema}->classdef($class), $self);
  }

sub select {
  croak "valid only in list context" unless wantarray;
  
  my ($self, $target, @args) = @_;
  
  unless (ref($target) eq 'ARRAY') {
        my $cursor = Tangram::Cursor->new($self, $target, $self->{db});
        return $cursor->select(@args);
  }
  
  my ($first, @others) = @$target;
  
  my @cache = map { $self->select( $_, @args ) } @others;
  
  my $cursor = Tangram::Cursor->new($self, $first, $self->{db});
  $cursor->retrieve( map { $_->{_IID_}, $_->{_TYPE_ } } @others );
  
  my $obj = $cursor->select( @args );
  my @results;
  
  while ($obj) {
        my @tuple = $obj;
        my @residue = $cursor->residue;
        
        while (my $id = shift @residue) {
          push @tuple, $self->load($self->combine_ids($id, shift @residue));
        }
        
        push @results, \@tuple;
        $obj = $cursor->next;
  }
  
  return @results;
}

sub cursor_object
  {
    my ($self, $class) = @_;
    $self->{IMPLICIT}{$class} ||= Tangram::RDBObject->new($self, $class)
}

sub query_objects
{
    my ($self, @classes) = @_;
    map { Tangram::QueryObject->new(Tangram::RDBObject->new($self, $_)) } @classes;
}

sub remote
{
    my ($self, @classes) = @_;
    wantarray ? $self->query_objects(@classes) : (&remote)[0]
}

sub expr
  {
    my $self = shift;
    return shift->expr( @_ );
  }

sub object
{
    carp "cannot be called in list context; use objects instead" if wantarray;
    my $self = shift;
    my ($obj) = $self->query_objects(@_);
    $obj;
}

sub count
{
    my $self = shift;

    my ($target, $filter);
    my $objects = Set::Object->new;

    if (@_ == 1)
    {
        $target = '*';
        $filter = shift;
    }
    else
    {
        my $expr = shift;
        $target = $expr->{expr};
        $objects->insert($expr->objects);
        $filter = shift;
    }

    my @filter_expr;

    if ($filter)
    {
        $objects->insert($filter->objects);
        @filter_expr = ( "($filter->{expr})" );
    }

    my $sql = "SELECT COUNT($target) FROM " . join(', ', map { $_->from } $objects->members);
   
    $sql .= "\nWHERE " . join(' AND ', @filter_expr, map { $_->where } $objects->members);

    print $Tangram::TRACE "$sql\n" if $Tangram::TRACE;

    return ($self->{db}->selectrow_array($sql))[0];
}

sub sum
{
    my ($self, $expr, $filter) = @_;

    my $objects = Set::Object->new($expr->objects);

    my @filter_expr;

    if ($filter)
    {
        $objects->insert($filter->objects);
        @filter_expr = ( "($filter->{expr})" );
    }

    my $sql = "SELECT SUM($expr->{expr}) FROM " . join(', ', map { $_->from } $objects->members);

    $sql .= "\nWHERE " . join(' AND ', @filter_expr, map { $_->where } $objects->members);

    print $Tangram::TRACE "$sql\n" if $Tangram::TRACE;

    return ($self->{db}->selectrow_array($sql))[0];
}

sub id
{
    my ($self, $obj) = @_;
    return $self->{get_id}->($obj);
}

sub disconnect
{
    my ($self) = @_;

    unless ($self->{no_tx})
    {   
        if (@{ $self->{tx} })
        {
            $self->{db}->rollback;
        }
        else
        {
            $self->{db}->commit;
        }
    }
   
    $self->{db}->disconnect;

    %$self = ();
}

sub _kind_class_ids
{
    my ($self, $class) = @_;

    my $schema = $self->{schema};
    my $classes = $self->{schema}{classes};
    my $class2id = $self->{class2id};

    my @ids;

    push @ids, $self->class_id($class) unless $classes->{$class}{abstract};

    $schema->for_each_spec($class,
                           sub { my $spec = shift; push @ids, $class2id->{$spec} unless $classes->{$spec}{abstract} } );

    return @ids;
}

sub is_persistent
{
    my ($self, $obj) = @_;
    return $self->{schema}->is_persistent($obj) && $self->id($obj);
}

sub prefetch
{
        my ($self, $remote, $member, $filter) = @_;

        my $class;

        if (ref $remote)
        {
                $class = $remote->class();
        }
        else
        {
                $class = $remote;
                $remote = $self->remote($class);
        }

        my $schema = $self->{schema};

        my $member_class = $schema->find_member_class($class, $member)
                or die "no member '$member' in class '$class'";

        my $classdef = $schema->{classes}{$member_class};
        my $type = $classdef->{member_type}{$member};
        my $memdef = $classdef->{MEMDEFS}{$member};

        $type->prefetch($self, $memdef, $remote, $class, $member, $filter);
}

sub connect
{
    my ($pkg, $schema, $cs, $user, $pw, $opts) = @_;

    my $self = $pkg->new;

        $opts ||= {};

    my $db = $opts->{dbh} || DBI->connect($cs, $user, $pw);

    eval { $db->{AutoCommit} = 0 };

    $self->{no_tx} = $db->{AutoCommit};

    $self->{db} = $db;

    @$self{ -cs, -user, -pw } = ($cs, $user, $pw);

    $self->{cid_size} = $schema->{sql}{cid_size};
        
    $self->_open($schema);

        $self->{engine} = Tangram::Relational::Engine->new($schema, layout1 => $self->{layout1});

    return $self;
}

sub sql_do
{
    my ($self, $sql) = @_;
    print $Tangram::TRACE "$sql\n" if $Tangram::TRACE;
        my $rows_affected = $self->{db}->do($sql);
    return defined($rows_affected) ? $rows_affected
          : croak $DBI::errstr;
}

sub sql_selectall_arrayref
{
    my ($self, $sql, $dbh) = @_;
    print $Tangram::TRACE "$sql\n" if $Tangram::TRACE;
        ($dbh || $self->{db})->selectall_arrayref($sql);
}

sub sql_prepare
{
    my ($self, $sql, $connection) = @_;
    confess unless $connection;
    print $Tangram::TRACE "$sql\n" if $Tangram::TRACE;
    return $connection->prepare($sql) or die;
}

sub sql_cursor
{
    my ($self, $sql, $connection) = @_;

    confess unless $connection;

    print $Tangram::TRACE "$sql\n" if $Tangram::TRACE;

    my $sth = $connection->prepare($sql) or die;
    $sth->execute() or confess;

    Tangram::Storage::Statement->new( statement => $sth, storage => $self,
                                     connection => $connection );
}

sub unload
  {
    my $self = shift;
    my $objects = $self->{objects};

    if (@_) {
      for my $item (@_) {
        if (ref $item) {
          $self->goodbye($item, $self->{get_id}->($item));
        } else {
          $self->goodbye($objects->{$item}, $item);
        }
      }
    } else {
      for my $id (keys %$objects) {
        $self->goodbye($objects->{$id}, $id);
      }
    }
  }

*reset = \&unload; # deprecated, use unload() instead

sub DESTROY
{
    my $self = shift;
    $self->{db}->disconnect if $self->{db};
}

package Tangram::Storage::Statement;

sub new
{
    my $class = shift;
    bless { @_ }, $class;
}

sub fetchrow
{
    return shift->{statement}->fetchrow;
}

sub close
{
    my $self = shift;

    if ($self->{storage})
    {
        $self->{statement}->finish;
        $self->{storage}->close_connection($self->{connection});
        %$self = ();
    }
}

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