NAME¶
DBIx::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 its 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 its 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';
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.
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.
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
}
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".