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 vareity 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 relationships?
118 By default, DBIx::Class cascades deletes and updates across
119 C<has_many> relationships. If your database already does this (and
120 that is probably better), turn it off by supplying C<< cascade_delete => 0 >>
121 in the relationship attributes. See L<DBIx::Class::Relationship::Base>.
123 =item .. use a relationship?
125 Use its name. An accessor is created using the name. See examples in
126 L<DBIx::Class::Manual::Cookbook/Using relationships>.
134 =item .. search for data?
136 Create a C<$schema> object, as mentioned above in ".. connect to my
137 database". Find the L<ResultSet|DBIx::Class::Manual::Glossary/ResultSet>
138 that you want to search in, and call C<search> on it. See
139 L<DBIx::Class::ResultSet/search>.
141 =item .. search using database functions?
143 Supplying something like:
145 ->search({'mydatefield' => 'now()'})
147 to search, will probably not do what you expect. It will quote the
148 text "now()", instead of trying to call the function. To provide
149 literal, unquoted text you need to pass in a scalar reference, like
152 ->search({'mydatefield' => \'now()'})
154 =item .. sort the results of my search?
156 Supply a list of columns you want to sort by to the C<order_by>
157 attribute. See L<DBIx::Class::ResultSet/order_by>.
159 =item .. sort my results based on fields I've aliased using C<as>?
161 You don't. You'll need to supply the same functions/expressions to
162 C<order_by>, as you did to C<select>.
164 To get "fieldname AS alias" in your SQL, you'll need to supply a
165 literal chunk of SQL in your C<select> attribute, such as:
167 ->search({}, { select => [ \'now() AS currenttime'] })
169 Then you can use the alias in your C<order_by> attribute.
171 =item .. group the results of my search?
173 Supply a list of columns you want to group on, to the C<group_by>
174 attribute, see L<DBIx::Class::ResultSet/group_by>.
176 =item .. group my results based on fields I've aliased using C<as>?
178 You don't. You'll need to supply the same functions/expressions to
179 C<group_by>, as you did to C<select>.
181 To get "fieldname AS alias" in your SQL, you'll need to supply a
182 literal chunk of SQL in your C<select> attribute, such as:
184 ->search({}, { select => [ \'now() AS currenttime'] })
186 Then you can use the alias in your C<group_by> attribute.
188 =item .. filter the results of my search?
190 The first argument to C<search> is a hashref of accessor names and
191 values to filter them by, for example:
193 ->search({'created_time' => { '>=', '2006-06-01 00:00:00' } })
195 Note that to use a function here you need to make the whole value into
198 ->search({'created_time' => \'>= yesterday()' })
200 =item .. search in several tables simultaneously?
202 To search in two related tables, you first need to set up appropriate
203 relationships between their respective classes. When searching you
204 then supply the name of the relationship to the C<join> attribute in
205 your search, for example when searching in the Books table for all the
206 books by the author "Fred Bloggs":
208 ->search({'authors.name' => 'Fred Bloggs'}, { join => 'authors' })
210 The type of join created in your SQL depends on the type of
211 relationship between the two tables, see L<DBIx::Class::Relationship>
212 for the join used by each relationship.
214 =item .. create joins with conditions other than column equality?
216 Currently, L<DBIx::Class> can only create join conditions using
217 equality, so you're probably better off creating a C<view> in your
218 database, and using that as your source. A C<view> is a stored SQL
219 query, which can be accessed similarly to a table, see your database
220 documentation for details.
222 =item .. search using greater-than or less-than and database functions?
224 To use functions or literal SQL with conditions other than equality
225 you need to supply the entire condition, for example:
227 my $interval = "< now() - interval '12 hours'";
228 ->search({last_attempt => \$interval})
232 my $interval = "now() - interval '12 hours'";
233 ->search({last_attempt => { '<' => \$interval } })
235 =item .. search with an SQL function on the left hand side?
237 To use an SQL function on the left hand side of a comparison:
239 ->search({}, { where => \'YEAR(date_of_birth)=1979' });
243 (When the bind arg ordering bug is fixed, the previous example can be
244 replaced with the following.)
246 ->search({}, { where => \'YEAR(date_of_birth)=?', bind => [ 1979 ] });
250 Or, if you have quoting off:
252 ->search({ 'YEAR(date_of_birth' => 1979 });
254 =item .. find more help on constructing searches?
256 Behind the scenes, DBIx::Class uses L<SQL::Abstract> to help construct
257 its SQL searches. So if you fail to find help in the
258 L<DBIx::Class::Manual::Cookbook>, try looking in the SQL::Abstract
267 =item .. fetch as much data as possible in as few select calls as possible?
269 See the prefetch examples in the L<Cookbook|DBIx::Class::Manual::Cookbook>.
271 =item .. fetch a whole column of data instead of a row?
273 Call C<get_column> on a L<DBIx::Class::ResultSet>, this returns a
274 L<DBIx::Class::ResultSetColumn>, see it's documentation and the
275 L<Cookbook|DBIx::Class::Manual::Cookbook> for details.
277 =item .. fetch a formatted column?
279 In your table schema class, create a "private" column accessor with:
281 __PACKAGE__->add_columns(my_column => { accessor => '_hidden_my_column' });
283 Then, in the same class, implement a subroutine called "my_column" that
284 fetches the real value and does the formatting you want.
286 See the Cookbook for more details.
288 =item .. fetch a single (or topmost) row?
290 Sometimes you many only want a single record back from a search. A quick
291 way to get that single row is to first run your search as usual:
293 ->search->(undef, { order_by => "id DESC" })
295 Then call L<DBIx::Class::ResultSet/slice> and ask it only to return 1 row:
299 These two calls can be combined into a single statement:
301 ->search->(undef, { order_by => "id DESC" })->slice(0,1)
303 Why slice instead of L<DBIx::Class::ResultSet/first> or L<DBIx::Class::ResultSet/single>?
304 If supported by the database, slice will use LIMIT/OFFSET to hint to the database that we
305 really only need one row. This can result in a significant speed improvement.
307 =item .. refresh a row from storage?
309 Use L<DBIx::Class::PK/discard_changes>.
311 $row->discard_changes
313 Discarding changes and refreshing from storage are two sides fo the same coin. When you
314 want to discard your local changes, just re-fetch the row from storage. When you want
315 to get a new, fresh copy of the row, just re-fetch the row from storage.
316 L<DBIx::Class::PK/discard_changes> does just that by re-fetching the row from storage
317 using the row's primary key.
321 =head2 Inserting and updating data
325 =item .. insert a row with an auto incrementing primary key?
327 In versions of L<DBIx::Class> less than 0.07, you need to ensure your
328 table class loads the L<PK::Auto|DBIx::Class::PK::Auto>
329 component. This will attempt to fetch the value of your primary key
330 from the database after the insert has happened, and store it in the
331 created object. In versions 0.07 and above, this component is
332 automatically loaded.
334 =item .. insert a row with a primary key that uses a sequence?
336 You need to create a trigger in your database that updates your
337 primary key field from the sequence. To help PK::Auto find your
338 inserted key, you can tell it the name of the sequence in the
339 C<column_info> supplied with C<add_columns>.
341 ->add_columns({ id => { sequence => 'mysequence', auto_nextval => 1 } });
343 =item .. insert many rows of data efficiently?
345 =item .. update a collection of rows at the same time?
347 Create a resultset using a search, to filter the rows of data you
348 would like to update, then call update on the resultset to change all
351 =item .. use database functions when updating rows?
353 =item .. update a column using data from another column?
355 To stop the column name from being quoted, you'll need to supply a
358 ->update({ somecolumn => \'othercolumn' })
360 But note that when using a scalar reference the column in the database
361 will be updated but when you read the value from the object with e.g.
365 you still get back the scalar reference to the string, B<not> the new
366 value in the database. To get that you must refresh the row from storage
367 using C<discard_changes()>. Or chain your function calls like this:
369 ->update->discard_changes
371 to update the database and refresh the object in one step.
373 =item .. store JSON/YAML in a column and have it deflate/inflate automatically?
375 You can use L<DBIx::Class::InflateColumn> to accomplish YAML/JSON storage transparently.
377 If you want to use JSON, then in your table schema class, do the following:
381 __PACKAGE__->add_columns(qw/ ... my_column ../)
382 __PACKAGE__->inflate_column('my_column', {
383 inflate => sub { jsonToObj(shift) },
384 deflate => sub { objToJson(shift) },
387 For YAML, in your table schema class, do the following:
391 __PACKAGE__->add_columns(qw/ ... my_column ../)
392 __PACKAGE__->inflate_column('my_column', {
393 inflate => sub { YAML::Load(shift) },
394 deflate => sub { YAML::Dump(shift) },
397 This technique is an easy way to store supplemental unstructured data in a table. Be
398 careful not to overuse this capability, however. If you find yourself depending more
399 and more on some data within the inflated column, then it may be time to factor that
408 =item How do I store my own (non-db) data in my DBIx::Class objects?
410 You can add your own data accessors to your classes.
412 =item How do I use DBIx::Class objects in my TT templates?
414 Like normal objects, mostly. However you need to watch out for TT
415 calling methods in list context. When calling relationship accessors
416 you will not get resultsets, but a list of all the related objects.
418 Starting with version 0.07, you can use L<DBIx::Class::ResultSet/search_rs>
419 to work around this issue.
421 =item See the SQL statements my code is producing?
423 Turn on debugging! See L<DBIx::Class::Storage> for details of how
424 to turn on debugging in the environment, pass your own filehandle to
425 save debug to, or create your own callback.
427 =item Why didn't my search run any SQL?
429 L<DBIx::Class> runs the actual SQL statement as late as possible, thus
430 if you create a resultset using C<search> in scalar context, no query
431 is executed. You can create further resultset refinements by calling
432 search again or relationship accessors. The SQL query is only run when
433 you ask the resultset for an actual row object.
435 =item How do I deal with tables that lack a primary key?
437 If your table lacks a primary key, DBIx::Class can't work out which row
438 it should operate on, for example to delete or update. However, a
439 UNIQUE constraint on one or more columns allows DBIx::Class to uniquely
440 identify the row, so you can tell L<DBIx::Class::ResultSource> these
441 columns act as a primary key, even if they don't from the database's
444 $resultset->set_primary_key(@column);
448 =head2 Notes for CDBI users
452 =item Is there a way to make an object auto-stringify itself as a
453 particular column or group of columns (a-la cdbi Stringfy column
454 group, or stringify_self method) ?
456 See L<DBIx::Class::Manual::Cookbook/Stringification>