1DBIx::Class::Manual::JoUisneirngC(o3n)tributed Perl DocuDmBeInxt:a:tCiloanss::Manual::Joining(3)
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6 DBIx::Class::Manual::Joining - Manual on joining tables with
7 DBIx::Class
8
10 This document should help you to use DBIx::Class if you are trying to
11 convert your normal SQL queries into DBIx::Class based queries, if you
12 use joins extensively (and also probably if you don't).
13
15 If you ended up here and you don't actually know what joins are yet,
16 then you should likely try the DBIx::Class::Manual::Intro instead. Skip
17 this part if you know what joins are..
18 But I'll explain anyway. Assuming you have created your database in a
20 more or less sensible way, you will end up with several tables that
21 contain "related" information. For example, you may have a table
22 containing information about "CD"s, containing the CD title and its
23 year of publication, and another table containing all the "Track"s for
24 the CDs, one track per row.
25 When you wish to extract information about a particular CD and all its
27 tracks, You can either fetch the CD row, then make another query to
28 fetch the tracks, or you can use a join. Compare:
29 SELECT ID, Title, Year FROM CD WHERE Title = 'Funky CD';
31 # .. Extract the ID, which is 10
32 SELECT Name, Artist FROM Tracks WHERE CDID = 10;
33 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';
35 So, joins are a way of extending simple select statements to include
37 fields from other, related, tables. There are various types of joins,
38 depending on which combination of the data you wish to retrieve, see
39 MySQL's doc on JOINs:
40 <http://dev.mysql.com/doc/refman/5.0/en/join.html>.
41
43 In DBIx::Class each relationship between two tables needs to first be
44 defined in the ResultSource for the table. If the relationship needs to
45 be accessed in both directions (i.e. Fetch all tracks of a CD, and
46 fetch the CD data for a Track), then it needs to be defined for both
47 tables.
48 For the CDs/Tracks example, that means writing, in "MySchema::CD":
50 MySchema::CD->has_many('tracks', 'MySchema::Tracks');
52 And in "MySchema::Tracks":
54 MySchema::Tracks->belongs_to('cd', 'MySchema::CD', 'CDID');
56 There are several other types of relationships, they are more
58 comprehensively described in DBIx::Class::Relationship.
59
61 Once you have defined all your relationships, using them in actual
62 joins is fairly simple. The type of relationship that you chose e.g.
63 "has_many", already indicates what sort of join will be performed.
64 "has_many" produces a "LEFT JOIN" for example, which will fetch all the
65 rows on the left side, whether there are matching rows on the right
66 (table being joined to), or not. You can force other types of joins in
67 your relationship, see the DBIx::Class::Relationship docs.
68 When performing either a search or a find operation, you can specify
70 which "relations" to also refine your results based on, using the join
71 attribute, like this:
72 $schema->resultset('CD')->search(
74 { 'Title' => 'Funky CD',
75 'tracks.Name' => { like => 'T%' }
76 },
77 { join => 'tracks',
78 order_by => ['tracks.id'],
79 }
80 );
81 If you don't recognise most of this syntax, you should probably go read
83 "search" in DBIx::Class::ResultSet and "ATTRIBUTES" in
84 DBIx::Class::ResultSet, but here's a quick break down:
85 The first argument to search is a hashref of the WHERE attributes, in
87 this case a restriction on the Title column in the CD table, and a
88 restriction on the name of the track in the Tracks table, but ONLY for
89 tracks actually related to the chosen CD(s). The second argument is a
90 hashref of attributes to the search, the results will be returned
91 sorted by the "id" of the related tracks.
92 The special 'join' attribute specifies which "relationships" to include
94 in the query. The distinction between "relationships" and "tables" is
95 important here, only the "relationship" names are valid.
96 This slightly nonsense example will produce SQL similar to:
98 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';
100
102 Another common use for joining to related tables, is to fetch the data
103 from both tables in one query, preventing extra round-trips to the
104 database. See the example above in "WHAT ARE JOINS".
105 Three techniques are described here. Of the three, only the "prefetch"
107 technique will deal sanely with fetching related objects over a
108 "has_many" relation. The others work fine for 1 to 1 type
109 relationships.
110 Whole related objects
112 To fetch entire related objects, e.g. CDs and all Track data, use the
113 'prefetch' attribute:
114 $schema->resultset('CD')->search(
116 { 'Title' => 'Funky CD',
117 },
118 { prefetch => 'tracks',
119 order_by => ['tracks.id'],
120 }
121 );
122 This will produce SQL similar to the following:
124 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';
126 The syntax of 'prefetch' is the same as 'join' and implies the joining,
128 so there is no need to use both together.
129 Subset of related fields
131 To fetch a subset or the related fields, the '+select' and '+as'
132 attributes can be used. For example, if the CD data is required and
133 just the track name from the Tracks table:
134 $schema->resultset('CD')->search(
136 { 'Title' => 'Funky CD',
137 },
138 { join => 'tracks',
139 '+select' => ['tracks.Name'],
140 '+as' => ['track_name'],
141 order_by => ['tracks.id'],
142 }
143 );
144 Which will produce the query:
146 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';
148 Note that the '+as' does not produce an SQL 'AS' keyword in the output,
150 see the DBIx::Class::Manual::FAQ for an explanation.
151 This type of column restriction has a downside, the returned $result
153 object will have no 'track_name' accessor:
154 while(my $result = $search_rs->next) {
156 print $result->track_name; ## ERROR
157 }
158 Instead "get_column" must be used:
160 while(my $result = $search_rs->next) {
162 print $result->get_column('track_name'); ## WORKS
163 }
164 Incomplete related objects
166 In rare circumstances, you may also wish to fetch related data as
167 incomplete objects. The usual reason to do is when the related table
168 has a very large field you don't need for the current data output. This
169 is better solved by storing that field in a separate table which you
170 only join to when needed.
171 To fetch an incomplete related object, supply the dotted notation to
173 the '+as' attribute:
174 $schema->resultset('CD')->search(
176 { 'Title' => 'Funky CD',
177 },
178 { join => 'tracks',
179 '+select' => ['tracks.Name'],
180 '+as' => ['tracks.Name'],
181 order_by => ['tracks.id'],
182 }
183 );
184 Which will produce same query as above;
186 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';
188 Now you can access the result using the relationship accessor:
190 while(my $result = $search_rs->next) {
192 print $result->tracks->name; ## WORKS
193 }
194 However, this will produce broken objects. If the tracks id column is
196 not fetched, the object will not be usable for any operation other than
197 reading its data. Use the "Whole related objects" method as much as
198 possible to avoid confusion in your code later.
199 Broken means: Update will not work. Fetching other related objects will
201 not work. Deleting the object will not work.
202
204 Across multiple relations
205 For simplicity in the example above, the "Artist" was shown as a simple
206 text field in the "Tracks" table, in reality, you'll want to have the
207 artists in their own table as well, thus to fetch the complete set of
208 data we'll need to join to the Artist table too.
209 In "MySchema::Tracks":
211 MySchema::Tracks->belongs_to('artist', 'MySchema::Artist', 'ArtistID');
213 The search:
215 $schema->resultset('CD')->search(
217 { 'Title' => 'Funky CD' },
218 { join => { 'tracks' => 'artist' },
219 }
220 );
221 Which is:
223 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';
225 To perform joins using relations of the tables you are joining to, use
227 a hashref to indicate the join depth. This can theoretically go as deep
228 as you like (warning: contrived examples!):
229 join => { room => { table => 'leg' } }
231 To join two relations at the same level, use an arrayref instead:
233 join => { room => [ 'chair', 'table' ] }
235 Or combine the two:
237 join => { room => [ 'chair', { table => 'leg' } ] }
239 Table aliases
241 As an aside to all the discussion on joins, note that DBIx::Class uses
242 the "relation names" as table aliases. This is important when you need
243 to add grouping or ordering to your queries:
244 $schema->resultset('CD')->search(
246 { 'Title' => 'Funky CD' },
247 { join => { 'tracks' => 'artist' },
248 order_by => [ 'tracks.Name', 'artist.Artist' ],
249 }
250 );
251 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;
253 This is essential if any of your tables have columns with the same
255 names.
256 Note that the table of the resultsource the search was performed on, is
258 always aliased to "me".
259 Joining to the same table twice
261 There is no magic to this, just do it. The table aliases will
262 automatically be numbered:
263 join => [ 'room', 'room' ]
265 The aliases are: "room" and "room_2".
267
269 Check the list of additional DBIC resources.
270
272 This module is free software copyright by the DBIx::Class (DBIC)
273 authors. You can redistribute it and/or modify it under the same terms
274 as the DBIx::Class library.
275perl v5.34.0 2021-07-22 DBIx::Class::Manual::Joining(3)