3 DBIx::Class::Manual::FAQ - Frequently Asked Questions (in theory)
7 This document is intended as an anti-map of the documentation. If you
8 know what you want to do, but not how to do it in L<DBIx::Class>, then
9 look here. It does B<not> contain much code or examples, it just gives
10 explanations and pointers to the correct pieces of documentation to
17 =head2 Getting started
21 =item .. create a database to use?
23 First, choose a database. For testing/experimenting, we reccommend
24 L<DBD::SQLite>, which is a self-contained small database (i.e. all you
25 need to do is to install L<DBD::SQLite> from CPAN, and it's usable).
27 Next, spend some time defining which data you need to store, and how
28 it relates to the other data you have. For some help on normalisation,
29 go to L<http://b62.tripod.com/doc/dbbase.htm> or
30 L<http://209.197.234.36/db/simple.html>.
32 Now, decide whether you want to have the database itself be the
33 definitive source of information about the data layout, or your
34 DBIx::Class schema. If it's the former, look up the documentation for
35 your database, eg. L<http://sqlite.org/lang_createtable.html>, on how
36 to create tables, and start creating them. For a nice universal
37 interface to your database, you can try L<DBI::Shell>. If you decided
38 on the latter choice, read the FAQ on setting up your classes
39 manually, and the one on creating tables from your schema.
41 =item .. use DBIx::Class with L<Catalyst>?
43 Install L<Catalyst::Model::DBIC::Schema> from CPAN. See its
44 documentation, or below, for further details.
46 =item .. set up my DBIx::Class classes automatically from my database?
48 Install L<DBIx::Class::Schema::Loader> from CPAN, and read its documentation.
50 =item .. set up my DBIx::Class classes manually?
52 Look at the L<DBIx::Class::Manual::Example> and come back here if you get lost.
54 =item .. create my database tables from my DBIx::Class schema?
56 Create your classes manually, as above. Write a script that calls
57 L<DBIx::Class::Schema/deploy>. See there for details, or the
58 L<DBIx::Class::Manual::Cookbook>.
60 =item .. connect to my database?
62 Once you have created all the appropriate table/source classes, and an
63 overall L<Schema|DBIx::Class::Schema> class, you can start using
64 them in an application. To do this, you need to create a central
65 Schema object, which is used to access all the data in the various
66 tables. See L<DBIx::Class::Schema/connect> for details. The actual
67 connection does not happen until you actually request data, so don't
68 be alarmed if the error from incorrect connection details happens a
78 =item .. tell DBIx::Class about relationships between my tables?
80 There are a variety of relationship types that come pre-defined for
81 you to use. These are all listed in L<DBIx::Class::Relationship>. If
82 you need a non-standard type, or more information, look in
83 L<DBIx::Class::Relationship::Base>.
85 =item .. define a one-to-many relationship?
87 This is called a C<has_many> relationship on the one side, and a
88 C<belongs_to> relationship on the many side. Currently these need to
89 be set up individually on each side. See L<DBIx::Class::Relationship>
92 =item .. define a relationship where this table contains another table's primary key? (foreign key)
94 Create a C<belongs_to> relationship for the field containing the
95 foreign key. See L<DBIx::Class::Relationship/belongs_to>.
97 =item .. define a foreign key relationship where the key field may contain NULL?
99 Just create a C<belongs_to> relationship, as above. If the column is
100 NULL then the inflation to the foreign object will not happen. This
101 has a side effect of not always fetching all the relevant data, if you
102 use a nullable foreign-key relationship in a JOIN, then you probably
103 want to set the C<join_type> to C<left>.
105 =item .. define a relationship where the key consists of more than one column?
107 Instead of supplying a single column name, all relationship types also
108 allow you to supply a hashref containing the condition across which
109 the tables are to be joined. The condition may contain as many fields
110 as you like. See L<DBIx::Class::Relationship::Base>.
112 =item .. define a relatiopnship across an intermediate table? (many-to-many)
114 Read the documentation on L<DBIx::Class::Relationship/many_to_many>.
116 =item .. stop DBIx::Class from attempting to cascade deletes on my has_many and might_have relationships?
118 By default, DBIx::Class cascades deletes and updates across
119 C<has_many> and C<might_have> relationships. You can disable this
120 behaviour on a per-relationship basis by supplying
121 C<< cascade_delete => 0 >> in the relationship attributes.
123 The cascaded operations are performed after the requested delete or
124 update, so if your database has a constraint on the relationship, it
125 will have deleted/updated the related records or raised an exception
126 before DBIx::Class gets to perform the cascaded operation.
128 See L<DBIx::Class::Relationship>.
130 =item .. use a relationship?
132 Use its name. An accessor is created using the name. See examples in
133 L<DBIx::Class::Manual::Cookbook/Using relationships>.
141 =item .. search for data?
143 Create a C<$schema> object, as mentioned above in ".. connect to my
144 database". Find the L<ResultSet|DBIx::Class::Manual::Glossary/ResultSet>
145 that you want to search in, and call C<search> on it. See
146 L<DBIx::Class::ResultSet/search>.
148 =item .. search using database functions?
150 Supplying something like:
152 ->search({'mydatefield' => 'now()'})
154 to search, will probably not do what you expect. It will quote the
155 text "now()", instead of trying to call the function. To provide
156 literal, unquoted text you need to pass in a scalar reference, like
159 ->search({'mydatefield' => \'now()'})
161 =item .. sort the results of my search?
163 Supply a list of columns you want to sort by to the C<order_by>
164 attribute. See L<DBIx::Class::ResultSet/order_by>.
166 =item .. sort my results based on fields I've aliased using C<as>?
168 You don't. You'll need to supply the same functions/expressions to
169 C<order_by>, as you did to C<select>.
171 To get "fieldname AS alias" in your SQL, you'll need to supply a
172 literal chunk of SQL in your C<select> attribute, such as:
174 ->search({}, { select => [ \'now() AS currenttime'] })
176 Then you can use the alias in your C<order_by> attribute.
178 =item .. group the results of my search?
180 Supply a list of columns you want to group on, to the C<group_by>
181 attribute, see L<DBIx::Class::ResultSet/group_by>.
183 =item .. group my results based on fields I've aliased using C<as>?
185 You don't. You'll need to supply the same functions/expressions to
186 C<group_by>, as you did to C<select>.
188 To get "fieldname AS alias" in your SQL, you'll need to supply a
189 literal chunk of SQL in your C<select> attribute, such as:
191 ->search({}, { select => [ \'now() AS currenttime'] })
193 Then you can use the alias in your C<group_by> attribute.
195 =item .. filter the results of my search?
197 The first argument to C<search> is a hashref of accessor names and
198 values to filter them by, for example:
200 ->search({'created_time' => { '>=', '2006-06-01 00:00:00' } })
202 Note that to use a function here you need to make the whole value into
205 ->search({'created_time' => \'>= yesterday()' })
207 =item .. search in several tables simultaneously?
209 To search in two related tables, you first need to set up appropriate
210 relationships between their respective classes. When searching you
211 then supply the name of the relationship to the C<join> attribute in
212 your search, for example when searching in the Books table for all the
213 books by the author "Fred Bloggs":
215 ->search({'authors.name' => 'Fred Bloggs'}, { join => 'authors' })
217 The type of join created in your SQL depends on the type of
218 relationship between the two tables, see L<DBIx::Class::Relationship>
219 for the join used by each relationship.
221 =item .. create joins with conditions other than column equality?
223 Currently, L<DBIx::Class> can only create join conditions using
224 equality, so you're probably better off creating a C<view> in your
225 database, and using that as your source. A C<view> is a stored SQL
226 query, which can be accessed similarly to a table, see your database
227 documentation for details.
229 =item .. search using greater-than or less-than and database functions?
231 To use functions or literal SQL with conditions other than equality
232 you need to supply the entire condition, for example:
234 my $interval = "< now() - interval '12 hours'";
235 ->search({last_attempt => \$interval})
239 my $interval = "now() - interval '12 hours'";
240 ->search({last_attempt => { '<' => \$interval } })
242 =item .. search with an SQL function on the left hand side?
244 To use an SQL function on the left hand side of a comparison:
246 ->search({}, { where => \'YEAR(date_of_birth)=1979' });
250 (When the bind arg ordering bug is fixed, the previous example can be
251 replaced with the following.)
253 ->search({}, { where => \'YEAR(date_of_birth)=?', bind => [ 1979 ] });
257 Or, if you have quoting off:
259 ->search({ 'YEAR(date_of_birth' => 1979 });
261 =item .. find more help on constructing searches?
263 Behind the scenes, DBIx::Class uses L<SQL::Abstract> to help construct
264 its SQL searches. So if you fail to find help in the
265 L<DBIx::Class::Manual::Cookbook>, try looking in the SQL::Abstract
274 =item .. fetch as much data as possible in as few select calls as possible?
276 See the prefetch examples in the L<Cookbook|DBIx::Class::Manual::Cookbook>.
278 =item .. fetch a whole column of data instead of a row?
280 Call C<get_column> on a L<DBIx::Class::ResultSet>, this returns a
281 L<DBIx::Class::ResultSetColumn>, see it's documentation and the
282 L<Cookbook|DBIx::Class::Manual::Cookbook> for details.
284 =item .. fetch a formatted column?
286 In your table schema class, create a "private" column accessor with:
288 __PACKAGE__->add_columns(my_column => { accessor => '_hidden_my_column' });
290 Then, in the same class, implement a subroutine called "my_column" that
291 fetches the real value and does the formatting you want.
293 See the Cookbook for more details.
295 =item .. fetch a single (or topmost) row?
297 Sometimes you many only want a single record back from a search. A quick
298 way to get that single row is to first run your search as usual:
300 ->search->(undef, { order_by => "id DESC" })
302 Then call L<DBIx::Class::ResultSet/slice> and ask it only to return 1 row:
306 These two calls can be combined into a single statement:
308 ->search->(undef, { order_by => "id DESC" })->slice(0)
310 Why slice instead of L<DBIx::Class::ResultSet/first> or L<DBIx::Class::ResultSet/single>?
311 If supported by the database, slice will use LIMIT/OFFSET to hint to the database that we
312 really only need one row. This can result in a significant speed improvement.
314 =item .. refresh a row from storage?
316 Use L<DBIx::Class::PK/discard_changes>.
318 $row->discard_changes
320 Discarding changes and refreshing from storage are two sides fo the same coin. When you
321 want to discard your local changes, just re-fetch the row from storage. When you want
322 to get a new, fresh copy of the row, just re-fetch the row from storage.
323 L<DBIx::Class::PK/discard_changes> does just that by re-fetching the row from storage
324 using the row's primary key.
328 =head2 Inserting and updating data
332 =item .. insert a row with an auto incrementing primary key?
334 In versions of L<DBIx::Class> less than 0.07, you need to ensure your
335 table class loads the L<PK::Auto|DBIx::Class::PK::Auto>
336 component. This will attempt to fetch the value of your primary key
337 from the database after the insert has happened, and store it in the
338 created object. In versions 0.07 and above, this component is
339 automatically loaded.
341 =item .. insert a row with a primary key that uses a sequence?
343 You need to create a trigger in your database that updates your
344 primary key field from the sequence. To help PK::Auto find your
345 inserted key, you can tell it the name of the sequence in the
346 C<column_info> supplied with C<add_columns>.
348 ->add_columns({ id => { sequence => 'mysequence', auto_nextval => 1 } });
350 =item .. insert many rows of data efficiently?
352 =item .. update a collection of rows at the same time?
354 Create a resultset using a search, to filter the rows of data you
355 would like to update, then call update on the resultset to change all
358 =item .. use database functions when updating rows?
360 =item .. update a column using data from another column?
362 To stop the column name from being quoted, you'll need to supply a
365 ->update({ somecolumn => \'othercolumn' })
367 But note that when using a scalar reference the column in the database
368 will be updated but when you read the value from the object with e.g.
372 you still get back the scalar reference to the string, B<not> the new
373 value in the database. To get that you must refresh the row from storage
374 using C<discard_changes()>. Or chain your function calls like this:
376 ->update->discard_changes
378 to update the database and refresh the object in one step.
380 =item .. store JSON/YAML in a column and have it deflate/inflate automatically?
382 You can use L<DBIx::Class::InflateColumn> to accomplish YAML/JSON storage transparently.
384 If you want to use JSON, then in your table schema class, do the following:
388 __PACKAGE__->add_columns(qw/ ... my_column ../)
389 __PACKAGE__->inflate_column('my_column', {
390 inflate => sub { jsonToObj(shift) },
391 deflate => sub { objToJson(shift) },
394 For YAML, in your table schema class, do the following:
398 __PACKAGE__->add_columns(qw/ ... my_column ../)
399 __PACKAGE__->inflate_column('my_column', {
400 inflate => sub { YAML::Load(shift) },
401 deflate => sub { YAML::Dump(shift) },
404 This technique is an easy way to store supplemental unstructured data in a table. Be
405 careful not to overuse this capability, however. If you find yourself depending more
406 and more on some data within the inflated column, then it may be time to factor that
415 =item How do I store my own (non-db) data in my DBIx::Class objects?
417 You can add your own data accessors to your classes.
419 =item How do I use DBIx::Class objects in my TT templates?
421 Like normal objects, mostly. However you need to watch out for TT
422 calling methods in list context. When calling relationship accessors
423 you will not get resultsets, but a list of all the related objects.
425 Starting with version 0.07, you can use L<DBIx::Class::ResultSet/search_rs>
426 to work around this issue.
428 =item See the SQL statements my code is producing?
430 Turn on debugging! See L<DBIx::Class::Storage> for details of how
431 to turn on debugging in the environment, pass your own filehandle to
432 save debug to, or create your own callback.
434 =item Why didn't my search run any SQL?
436 L<DBIx::Class> runs the actual SQL statement as late as possible, thus
437 if you create a resultset using C<search> in scalar context, no query
438 is executed. You can create further resultset refinements by calling
439 search again or relationship accessors. The SQL query is only run when
440 you ask the resultset for an actual row object.
442 =item How do I deal with tables that lack a primary key?
444 If your table lacks a primary key, DBIx::Class can't work out which row
445 it should operate on, for example to delete or update. However, a
446 UNIQUE constraint on one or more columns allows DBIx::Class to uniquely
447 identify the row, so you can tell L<DBIx::Class::ResultSource> these
448 columns act as a primary key, even if they don't from the database's
451 $resultset->set_primary_key(@column);
455 =head2 Notes for CDBI users
459 =item Is there a way to make an object auto-stringify itself as a
460 particular column or group of columns (a-la cdbi Stringfy column
461 group, or stringify_self method) ?
463 See L<DBIx::Class::Manual::Cookbook/Stringification>