DBIx::Class::Manual::JUsernContributed Perl DocDBIx::Class::Manual::Joining(3)NAMEDBIx::Class::Manual::Joining - Manual on joining tables with
DBIx::Class
DESCRIPTION
This document should help you to use DBIx::Class if you are trying to
convert your normal SQL queries into DBIx::Class based queries, if you
use joins extensively (and also probably if you don't).
WHAT ARE JOINS
If you ended up here and you don't actually know what joins are yet,
then you should likely try the DBIx::Class::Manual::Intro instead. Skip
this part if you know what joins are..
But I'll explain anyway. Assuming you have created your database in a
more or less sensible way, you will end up with several tables that
contain "related" information. For example, you may have a table
containing information about "CD"s, containing the CD title and it's
year of publication, and another table containing all the "Track"s for
the CDs, one track per row.
When you wish to extract information about a particular CD and all it's
tracks, You can either fetch the CD row, then make another query to
fetch the tracks, or you can use a join. Compare:
SELECT ID, Title, Year FROM CD WHERE Title = 'Funky CD';
# .. Extract the ID, which is 10
SELECT Name, Artist FROM Tracks WHERE CDID = 10;
SELECT cd.ID, cd.Title, cd.Year, tracks.Name, tracks.Artist FROM CD JOIN Tracks ON CD.ID = tracks.CDID WHERE cd.Title = 'Funky CD';
So, joins are a way of extending simple select statements to include
fields from other, related, tables. There are various types of joins,
depending on which combination of the data you wish to retrieve, see
MySQL's doc on JOINs:
<http://dev.mysql.com/doc/refman/5.0/en/join.html>.
DEFINING JOINS AND RELATIONSHIPS
In DBIx::Class each relationship between two tables needs to first be
defined in the ResultSource for the table. If the relationship needs to
be accessed in both directions (i.e. Fetch all tracks of a CD, and
fetch the CD data for a Track), then it needs to be defined for both
tables.
For the CDs/Tracks example, that means writing, in "MySchema::CD":
MySchema::CD->has_many('tracks', 'MySchema::Tracks');
And in "MySchema::Tracks":
MySchema::Tracks->belongs_to('cd', 'MySchema::CD', 'CDID');
There are several other types of relationships, they are more
comprehensively described in DBIx::Class::Relationship.
USING JOINS
Once you have defined all your relationships, using them in actual
joins is fairly simple. The type of relationship that you chose e.g.
"has_many", already indicates what sort of join will be performed.
"has_many" produces a "LEFT JOIN" for example, which will fetch all the
rows on the left side, whether there are matching rows on the right
(table being joined to), or not. You can force other types of joins in
your relationship, see the DBIx::Class::Relationship docs.
When performing either a search or a find operation, you can specify
which "relations" to also refine your results based on, using the join
attribute, like this:
$schema->resultset('CD')->search(
{ 'Title' => 'Funky CD',
'tracks.Name' => { like => 'T%' }
},
{ join => 'tracks',
order_by => ['tracks.id'],
}
);
If you don't recognise most of this syntax, you should probably go read
"search" in DBIx::Class::ResultSet and "ATTRIBUTES" in
DBIx::Class::ResultSet, but here's a quick break down:
The first argument to search is a hashref of the WHERE attributes, in
this case a restriction on the Title column in the CD table, and a
restriction on the name of the track in the Tracks table, but ONLY for
tracks actually related to the chosen CD(s). The second argument is a
hashref of attributes to the search, the results will be returned
sorted by the "id" of the related tracks.
The special 'join' attribute specifies which "relationships" to include
in the query. The distinction between "relationships" and "tables" is
important here, only the "relationship" names are valid.
This slightly nonsense example will produce SQL similar to:
SELECT cd.ID, cd.Title, cd.Year FROM CD cd JOIN Tracks tracks ON cd.ID = tracks.CDID WHERE cd.Title = 'Funky CD' AND tracks.Name LIKE 'T%' ORDER BY 'tracks.id';
FETCHING RELATED DATA
Another common use for joining to related tables, is to fetch the data
from both tables in one query, preventing extra round-trips to the
database. See the example above in "WHAT ARE JOINS".
Three techniques are described here. Of the three, only the "prefetch"
technique will deal sanely with fetching related objects over a
"has_many" relation. The others work fine for 1 to 1 type
relationships.
Whole related objects
To fetch entire related objects, e.g. CDs and all Track data, use the
'prefetch' attribute:
$schema->resultset('CD')->search(
{ 'Title' => 'Funky CD',
},
{ prefetch => 'tracks',
order_by => ['tracks.id'],
}
);
This will produce SQL similar to the following:
SELECT cd.ID, cd.Title, cd.Year, tracks.id, tracks.Name, tracks.Artist FROM CD JOIN Tracks ON CD.ID = tracks.CDID WHERE cd.Title = 'Funky CD' ORDER BY 'tracks.id';
The syntax of 'prefetch' is the same as 'join' and implies the joining,
so there is no need to use both together.
Subset of related fields
To fetch a subset or the related fields, the '+select' and '+as'
attributes can be used. For example, if the CD data is required and
just the track name from the Tracks table:
$schema->resultset('CD')->search(
{ 'Title' => 'Funky CD',
},
{ join => 'tracks',
'+select' => ['tracks.Name'],
'+as' => ['track_name'],
order_by => ['tracks.id'],
}
);
Which will produce the query:
SELECT cd.ID, cd.Title, cd.Year, tracks.Name FROM CD JOIN Tracks ON CD.ID = tracks.CDID WHERE cd.Title = 'Funky CD' ORDER BY 'tracks.id';
Note that the '+as' does not produce an SQL 'AS' keyword in the output,
see the DBIx::Class::Manual::FAQ for an explanation.
This type of column restriction has a downside, the resulting $row
object will have no 'track_name' accessor:
while(my $row = $search_rs->next) {
print $row->track_name; ## ERROR
}
Instead "get_column" must be used:
while(my $row = $search_rs->next) {
print $row->get_colum('track_name'); ## WORKS
}
Incomplete related objects
In rare circumstances, you may also wish to fetch related data as
incomplete objects. The usual reason to do is when the related table
has a very large field you don't need for the current data output. This
is better solved by storing that field in a separate table which you
only join to when needed.
To fetch an incomplete related object, supply the dotted notation to
the '+as' attribute:
$schema->resultset('CD')->search(
{ 'Title' => 'Funky CD',
},
{ join => 'tracks',
'+select' => ['tracks.Name'],
'+as' => ['tracks.Name'],
order_by => ['tracks.id'],
}
);
Which will produce same query as above;
SELECT cd.ID, cd.Title, cd.Year, tracks.Name FROM CD JOIN Tracks ON CD.ID = tracks.CDID WHERE cd.Title = 'Funky CD' ORDER BY 'tracks.id';
Now you can access the result using the relationship accessor:
while(my $row = $search_rs->next) {
print $row->tracks->name; ## WORKS
}
However, this will produce broken objects. If the tracks id column is
not fetched, the object will not be usable for any operation other than
reading its data. Use the "Whole related objects" method as much as
possible to avoid confusion in your code later.
Broken means: Update will not work. Fetching other related objects will
not work. Deleting the object will not work.
COMPLEX JOINS AND STUFF
Across multiple relations
For simplicity in the example above, the "Artist" was shown as a simple
text field in the "Tracks" table, in reality, you'll want to have the
artists in their own table as well, thus to fetch the complete set of
data we'll need to join to the Artist table too.
In "MySchema::Tracks":
MySchema::Tracks->belongs_to('artist', 'MySchema::Artist', 'ArtistID');
The search:
$schema->resultset('CD')->search(
{ 'Title' => 'Funky CD' },
{ join => { 'tracks' => 'artist' },
}
);
Which is:
SELECT me.ID, me.Title, me.Year FROM CD me JOIN Tracks tracks ON CD.ID = tracks.CDID JOIN Artists artist ON tracks.ArtistID = artist.ID WHERE me.Title = 'Funky CD';
To perform joins using relations of the tables you are joining to, use
a hashref to indicate the join depth. This can theoretically go as deep
as you like (warning: contrived examples!):
join => { room => { table => 'leg' } }
To join two relations at the same level, use an arrayref instead:
join => { room => [ 'chair', 'table' ] }
Or combine the two:
join => { room => [ 'chair', { table => 'leg' } ]
Table aliases
As an aside to all the discussion on joins, note that DBIx::Class uses
the "relation names" as table aliases. This is important when you need
to add grouping or ordering to your queries:
$schema->resultset('CD')->search(
{ 'Title' => 'Funky CD' },
{ join => { 'tracks' => 'artist' },
order_by => [ 'tracks.Name', 'artist.Artist' ],
}
);
SELECT me.ID, me.Title, me.Year FROM CD me JOIN Tracks tracks ON CD.ID = tracks.CDID JOIN Artists artist ON tracks.ArtistID = artist.ID WHERE me.Title = 'Funky CD' ORDER BY tracks.Name, artist.Artist;
This is essential if any of your tables have columns with the same
names.
Note that the table of the resultsource the search was performed on, is
always aliased to "me".
Joining to the same table twice
There is no magic to this, just do it. The table aliases will
automatically be numbered:
join => [ 'room', 'room' ]
The aliases are: "room" and "room_2".
perl v5.14.2 2012-01-22 DBIx::Class::Manual::Joining(3)