=head3 Using specific columns
-When you only want selected columns from a table, you can use C<cols> to
-specify which ones you need:
+When you only want specific columns from a table, you can use
+C<columns> to specify which ones you need. This is useful to avoid
+loading columns with large amounts of data that you aren't about to
+use anyway:
my $rs = $schema->resultset('Artist')->search(
undef,
# Equivalent SQL:
# SELECT artist.name FROM artist
+This is a shortcut for C<select> and C<as>, see below. C<columns>
+cannot be used together with C<select> and C<as>.
+
=head3 Using database functions or stored procedures
The combination of C<select> and C<as> can be used to return the result of a
to access the returned value:
my $rs = $schema->resultset('Artist')->search(
- undef,
+ {},
{
select => [ 'name', { LENGTH => 'name' } ],
as => [qw/ name name_length /],
);
# Equivalent SQL:
- # SELECT name name, LENGTH( name ) name_length
+ # SELECT name name, LENGTH( name )
# FROM artist
-If your alias exists as a column in your base class (i.e. it was added with
-C<add_columns>), you just access it as normal. Our C<Artist> class has a C<name>
-column, so we just use the C<name> accessor:
+Note that the C< as > attribute has absolutely nothing to with the sql
+syntax C< SELECT foo AS bar > (see the documentation in
+L<DBIx::Class::ResultSet/ATTRIBUTES>). If your alias exists as a
+column in your base class (i.e. it was added with C<add_columns>), you
+just access it as normal. Our C<Artist> class has a C<name> column, so
+we just use the C<name> accessor:
my $artist = $rs->first();
my $name = $artist->name();
If on the other hand the alias does not correspond to an existing column, you
-can get the value using the C<get_column> accessor:
+have to fetch the value using the C<get_column> accessor:
my $name_length = $artist->get_column('name_length');
=head3 SELECT DISTINCT with multiple columns
my $rs = $schema->resultset('Foo')->search(
- undef,
+ {},
{
select => [
{ distinct => [ $source->columns ] }
],
- as => [ $source->columns ]
+ as => [ $source->columns ] # remember 'as' is not the same as SQL AS :-)
}
);
=head3 SELECT COUNT(DISTINCT colname)
my $rs = $schema->resultset('Foo')->search(
- undef,
+ {},
{
select => [
{ count => { distinct => 'colname' } }
L<DBIx::Class> supports C<GROUP BY> as follows:
my $rs = $schema->resultset('Artist')->search(
- undef,
+ {},
{
join => [qw/ cds /],
select => [ 'name', { count => 'cds.cdid' } ],
# LEFT JOIN cd cds ON ( cds.artist = me.artistid )
# GROUP BY name
+Please see L<DBIx::Class::ResultSet/ATTRIBUTES> documentation if you
+are in any way unsure about the use of the attributes above (C< join
+>, C< select >, C< as > and C< group_by >).
+
=head3 Predefined searches
-You can write your own DBIx::Class::ResultSet class by inheriting from it
+You can write your own L<DBIx::Class::ResultSet> class by inheriting from it
and define often used searches as methods:
package My::DBIC::ResultSet::CD;
deep using the same syntax as a multi-step join:
my $rs = $schema->resultset('Tag')->search(
- undef,
+ {},
{
prefetch => {
cd => 'artist'
my $tag = $rs->first;
print $tag->cd->artist->name;
+=head2 Columns of data
+
+If you want to find the sum of a particular column there are several
+ways, the obvious one is to use search:
+
+ my $rs = $schema->resultset('Items')->search(
+ {},
+ {
+ select => [ { sum => 'Cost' } ],
+ as => [ 'total_cost' ], # remember this 'as' is for DBIx::Class::ResultSet not SQL
+ }
+ );
+ my $tc = $rs->first->get_column('total_cost');
+
+Or, you can use the L<DBIx::Class::ResultSetColumn>, which gets
+returned when you ask the C<ResultSet> for a column using
+C<get_column>:
+
+ my $cost = $schema->resultset('Items')->get_column('Cost');
+ my $tc = $cost->sum;
+
+With this you can also do:
+
+ my $minvalue = $cost->min;
+ my $maxvalue = $cost->max;
+
+Or just iterate through the values of this column only:
+
+ while ( my $c = $cost->next ) {
+ print $c;
+ }
+
+ foreach my $c ($cost->all) {
+ print $c;
+ }
+
+C<ResultSetColumn> only has a limited number of built-in functions, if
+you need one that it doesn't have, then you can use the C<func> method
+instead:
+
+ my $avg = $cost->func('AVERAGE');
+
+This will cause the following SQL statement to be run:
+
+ SELECT AVERAGE(Cost) FROM Items me
+
+Which will of course only work if your database supports this function.
+See L<DBIx::Class::ResultSetColumn> for more documentation.
+
=head2 Using relationships
=head3 Create a new row in a related table
=head2 Many-to-many relationships
-This is straightforward using L<DBIx::Class::Relationship::ManyToMany>:
+This is straightforward using L<ManyToMany|DBIx::Class::Relationship/many_to_many>:
package My::DB;
# ... set up connection ...
C<next::method>.
sub new {
- my ( $class, $attrs ) = @_;
+ my ( $self, $attrs ) = @_;
$attrs->{foo} = 'bar' unless defined $attrs->{foo};
- $class->next::method($attrs);
+ $self->next::method($attrs);
+
+ return $self;
}
For more information about C<next::method>, look in the L<Class::C3>
To make an object stringify itself as a single column, use something
like this (replace C<foo> with the column/method of your choice):
- use overload '""' => 'foo', fallback => 1;
+ use overload '""' => sub { shift->name}, fallback => 1;
For more complex stringification, you can use an anonymous subroutine:
use overload '""' => sub { $_[0]->name . ", " .
$_[0]->address }, fallback => 1;
-=head3 Stringifcation Example
+=head3 Stringification Example
Suppose we have two tables: C<Product> and C<Category>. The table
specifications are:
To create a MySQL database from an existing L<DBIx::Class> schema, convert the
schema to MySQL's dialect of SQL:
- sqlt --from DBIx::Class --to MySQL --DBIx::Class "MySchema.pm" > Schema1.sql
+ sqlt --from SQL::Translator::Parser::DBIx::Class
+ --to MySQL
+ --DBIx::Class "MySchema.pm" > Schema1.sql
And import using the mysql client:
Use C<sqlt> to transform your schema into an SQL script suitable for your
customer's database. E.g. for MySQL:
- sqlt --from DBIx::Class
+ sqlt --from SQL::Translator::Parser::DBIx::Class
--to MySQL
--DBIx::Class "MySchema.pm" > Schema1.mysql.sql
If you need to target databases from multiple vendors, just generate an SQL
script suitable for each. To support PostgreSQL too:
- sqlt --from DBIx::Class
+ sqlt --from SQL::Translator::DBIx::Class
--to PostgreSQL
--DBIx::Class "MySchema.pm" > Schema1.pgsql.sql
change functionality. Once the changes are made to your schema in DBIx::Class,
export the modified schema as before, taking care not to overwrite the original:
- sqlt --from DBIx::Class
+ sqlt --from SQL::Translator::DBIx::Class
--to MySQL
--DBIx::Class "Anything.pm" > Schema2.mysql.sql
=head2 Setting limit dialect for SQL::Abstract::Limit
-In some cases, SQL::Abstract::Limit cannot determine the dialect of the remote
-SQL-server by looking at the database-handle. This is a common problem when
-using the DBD::JDBC, since the DBD-driver only know that in has a Java-driver
-available, not which JDBC-driver the Java component has loaded.
-This specifically sets the limit_dialect to Microsoft SQL-server (Se more names
-in SQL::Abstract::Limit -documentation.
+In some cases, SQL::Abstract::Limit cannot determine the dialect of
+the remote SQL server by looking at the database handle. This is a
+common problem when using the DBD::JDBC, since the DBD-driver only
+know that in has a Java-driver available, not which JDBC driver the
+Java component has loaded. This specifically sets the limit_dialect
+to Microsoft SQL-server (See more names in SQL::Abstract::Limit
+-documentation.
__PACKAGE__->storage->sql_maker->limit_dialect('mssql');
-The JDBC-bridge is one way of getting access to a MSSQL-server from a platform
+The JDBC bridge is one way of getting access to a MSSQL server from a platform
that Microsoft doesn't deliver native client libraries for. (e.g. Linux)
-=head2 Setting quotes for the generated SQL.
+=head2 Setting quoting for the generated SQL.
-If the database contains columnames with spaces and/or reserved words, the
-SQL-query needs to be quoted. This is done using:
+If the database contains column names with spaces and/or reserved words, they
+need to be quoted in the SQL queries. This is done using:
__PACKAGE__->storage->sql_maker->quote_char([ qw/[ ]/] );
__PACKAGE__->storage->sql_maker->name_sep('.');
-The first sets the quotesymbols. If the quote i "symmetric" as " or '
+The first sets the quote characters. Either a pair of matching
+brackets, or a C<"> or C<'>:
__PACKAGE__->storage->sql_maker->quote_char('"');
-is enough. If the left quote differs form the right quote, the first
-notation should be used. name_sep needs to be set to allow the
-SQL generator to put the quotes the correct place.
+Check the documentation of your database for the correct quote
+characters to use. C<name_sep> needs to be set to allow the SQL
+generator to put the quotes the correct place.
=head2 Overloading methods
}
Note that the hard work is done by the call to C<next::method>, which
-redispatches your call to store_column to the superclass(es).
+redispatches your call to store_column in the superclass(es).
=head3 Automatically creating related objects
-You might have a class C<Artist> which has many C<CD>s. Further, you
+You might have a class C<Artist> which has many C<CD>s. Further, if you
want to create a C<CD> object every time you insert an C<Artist> object.
You can accomplish this by overriding C<insert> on your objects:
=head2 Retrieving a row object's Schema
-It is possible to get a Schema object from a row object like so,
+It is possible to get a Schema object from a row object like so:
my $schema = $cd->result_source->schema;
- my $artist_rs = $schema->resultset('Artist');
- # for example
+ # use the schema as normal:
+ my $artist_rs = $schema->resultset('Artist');
This can be useful when you don't want to pass around a Schema object to every
method.
=head2 Dynamic Sub-classing DBIx::Class proxy classes
(AKA multi-class object inflation from one table)
-L<DBIx::Class> classes are proxy classes, therefore some different techniques
-need to be employed for more than basic subclassing. In this example we have
-a single user table that carries a boolean bit for admin. We would like
-like to give the admin users objects(L<DBIx::Class::Row>) the same methods as
-a regular user but also special admin only methods. It doesn't make sense to
-create two seperate proxy-class files for this. We would be copying all the
-user methods into the Admin class. There is a cleaner way to accomplish this.
-
-Overriding the C<inflate_results()> method within the User proxy-class gives
-us the effect we want. This method is called by L<DBIx::Class::ResultSet> when
-inflating a result from storage. So we grab the object being returned, inspect
-the values we are looking for, bless it if it's an admin object, and then
-return it. Running the test file below will confirm this works.
+L<DBIx::Class> classes are proxy classes, therefore some different
+techniques need to be employed for more than basic subclassing. In
+this example we have a single user table that carries a boolean bit
+for admin. We would like like to give the admin users
+objects(L<DBIx::Class::Row>) the same methods as a regular user but
+also special admin only methods. It doesn't make sense to create two
+seperate proxy-class files for this. We would be copying all the user
+methods into the Admin class. There is a cleaner way to accomplish
+this.
+
+Overriding the C<inflate_result> method within the User proxy-class
+gives us the effect we want. This method is called by
+L<DBIx::Class::ResultSet> when inflating a result from storage. So we
+grab the object being returned, inspect the values we are looking for,
+bless it if it's an admin object, and then return it. See the example
+below:
B<Schema Definition>
### Defined what our admin class is for ensure_class_loaded
my $admin_class = __PACKAGE__ . '::Admin';
- __PACKAGE__->load_components(qw/PK::Auto Core/);
+ __PACKAGE__->load_components(qw/Core/);
__PACKAGE__->table('users');
### The statement below will print
print "I can do admin stuff\n" if $admin->can('do_admin_stuff');
+=head2 Skip object creation for faster results
+
+DBIx::Class is not built for speed, it's built for convenience and
+ease of use, but sometimes you just need to get the data, and skip the
+fancy objects. Luckily this is also fairly easy using
+C<inflate_result>:
+
+ # Define a class which just returns the results as a hashref:
+ package My::HashRefInflator;
+
+ ## $me is the hashref of cols/data from the immediate resultsource
+ ## $prefetch is a deep hashref of all the data from the prefetched
+ ## related sources.
+
+ sub mk_hash {
+ my ($me, $rest) = @_;
+
+ return { %$me,
+ map { ($_ => mk_hash(@{$rest->{$_}})) } keys %$rest
+ };
+ }
+
+ sub inflate_result {
+ my ($self, $source, $me, $prefetch) = @_;
+ return mk_hash($me, $prefetch);
+ }
+
+ # Change the object inflation to a hashref for just this resultset:
+ $rs->result_class('My::HashRefInflator');
+
+ my $datahashref = $rs->next;
+ foreach my $col (keys %$datahashref) {
+ if(!ref($datahashref->{$col})) {
+ # It's a plain value
+ }
+ elsif(ref($datahashref->{$col} eq 'HASH')) {
+ # It's a related value in a hashref
+ }
+ }
+
+=head2 Want to know if find_or_create found or created a row?
+
+Just use C<find_or_new> instead, then check C<in_storage>:
+
+ my $obj = $rs->find_or_new({ blah => 'blarg' });
+ unless ($obj->in_storage) {
+ $obj->insert;
+ # do whatever else you wanted if it was a new row
+ }
+
+=head3 Wrapping/overloading a column accessor
+
+Problem: Say you have a table "Camera" and want to associate a description
+with each camera. For most cameras, you'll be able to generate the description from
+the other columns. However, in a few special cases you may want to associate a
+custom description with a camera.
+
+Solution:
+
+In your database schema, define a description field in the "Camera" table that
+can contain text and null values.
+
+In DBIC, we'll overload the column accessor to provide a sane default if no
+custom description is defined. The accessor will either return or generate the
+description, depending on whether the field is null or not.
+
+First, in your "Camera" schema class, define the description field as follows:
+
+ __PACKAGE__->add_columns(description => { accessor => '_description' });
+
+Next, we'll define the accessor-wrapper subroutine:
+
+ sub description {
+ my $self = shift;
+
+ # If there is an update to the column, we'll let the original accessor
+ # deal with it.
+ return $self->_description(@_) if @_;
+
+ # Fetch the column value.
+ my $description = $self->_description;
+
+ # If there's something in the description field, then just return that.
+ return $description if defined $description && length $descripton;
+
+ # Otherwise, generate a description.
+ return $self->generate_description;
+ }
+
=cut
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