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DBIx-Class-0.08099_01
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/ DBIx::Class::Manual::FAQ

NAME

DBIx::Class::Manual::FAQ - Frequently Asked Questions (in theory)

DESCRIPTION

This document is intended as an anti-map of the documentation. If you know what you want to do, but not how to do it in DBIx::Class, then look here. It does not contain much code or examples, it just gives explanations and pointers to the correct pieces of documentation to read.

FAQs

How Do I:

Getting started

.. create a database to use?

First, choose a database. For testing/experimenting, we reccommend DBD::SQLite, which is a self-contained small database (i.e. all you need to do is to install DBD::SQLite from CPAN, and it's usable).

Next, spend some time defining which data you need to store, and how it relates to the other data you have. For some help on normalisation, go to http://b62.tripod.com/doc/dbbase.htm or http://209.197.234.36/db/simple.html.

Now, decide whether you want to have the database itself be the definitive source of information about the data layout, or your DBIx::Class schema. If it's the former, look up the documentation for your database, eg. http://sqlite.org/lang_createtable.html, on how to create tables, and start creating them. For a nice universal interface to your database, you can try DBI::Shell. If you decided on the latter choice, read the FAQ on setting up your classes manually, and the one on creating tables from your schema.

.. use DBIx::Class with Catalyst?

Install Catalyst::Model::DBIC::Schema from CPAN. See its documentation, or below, for further details.

.. set up my DBIx::Class classes automatically from my database?

Install DBIx::Class::Schema::Loader from CPAN, and read its documentation.

.. set up my DBIx::Class classes manually?

Look at the DBIx::Class::Manual::Example and come back here if you get lost.

.. create my database tables from my DBIx::Class schema?

Create your classes manually, as above. Write a script that calls "deploy" in DBIx::Class::Schema. See there for details, or the DBIx::Class::Manual::Cookbook.

.. connect to my database?

Once you have created all the appropriate table/source classes, and an overall Schema class, you can start using them in an application. To do this, you need to create a central Schema object, which is used to access all the data in the various tables. See "connect" in DBIx::Class::Schema for details. The actual connection does not happen until you actually request data, so don't be alarmed if the error from incorrect connection details happens a lot later.

Relationships

.. tell DBIx::Class about relationships between my tables?

There are a vareity of relationship types that come pre-defined for you to use. These are all listed in DBIx::Class::Relationship. If you need a non-standard type, or more information, look in DBIx::Class::Relationship::Base.

.. define a one-to-many relationship?

This is called a has_many relationship on the one side, and a belongs_to relationship on the many side. Currently these need to be set up individually on each side. See DBIx::Class::Relationship for details.

.. define a relationship where this table contains another table's primary key? (foreign key)

Create a belongs_to relationship for the field containing the foreign key. See "belongs_to" in DBIx::Class::Relationship.

.. define a foreign key relationship where the key field may contain NULL?

Just create a belongs_to relationship, as above. If the column is NULL then the inflation to the foreign object will not happen. This has a side effect of not always fetching all the relevant data, if you use a nullable foreign-key relationship in a JOIN, then you probably want to set the join_type to left.

.. define a relationship where the key consists of more than one column?

Instead of supplying a single column name, all relationship types also allow you to supply a hashref containing the condition across which the tables are to be joined. The condition may contain as many fields as you like. See DBIx::Class::Relationship::Base.

.. define a relatiopnship across an intermediate table? (many-to-many)

Read the documentation on "many_to_many" in DBIx::Class::Relationship.

.. stop DBIx::Class from attempting to cascade deletes on my has_many relationships?

By default, DBIx::Class cascades deletes and updates across has_many relationships. If your database already does this (and that is probably better), turn it off by supplying cascade_delete => 0 in the relationship attributes. See DBIx::Class::Relationship::Base.

.. use a relationship?

Use its name. An accessor is created using the name. See examples in "Using relationships" in DBIx::Class::Manual::Cookbook.

Searching

.. search for data?

Create a $schema object, as mentioned above in ".. connect to my database". Find the ResultSet that you want to search in, and call search on it. See "search" in DBIx::Class::ResultSet.

.. search using database functions?

Supplying something like:

->search({'mydatefield' => 'now()'})

to search, will probably not do what you expect. It will quote the text "now()", instead of trying to call the function. To provide literal, unquoted text you need to pass in a scalar reference, like so:

->search({'mydatefield' => \'now()'})
.. sort the results of my search?

Supply a list of columns you want to sort by to the order_by attribute. See "order_by" in DBIx::Class::ResultSet.

.. sort my results based on fields I've aliased using as?

You don't. You'll need to supply the same functions/expressions to order_by, as you did to select.

To get "fieldname AS alias" in your SQL, you'll need to supply a literal chunk of SQL in your select attribute, such as:

->search({}, { select => [ \'now() AS currenttime'] })

Then you can use the alias in your order_by attribute.

.. group the results of my search?

Supply a list of columns you want to group on, to the group_by attribute, see "group_by" in DBIx::Class::ResultSet.

.. group my results based on fields I've aliased using as?

You don't. You'll need to supply the same functions/expressions to group_by, as you did to select.

To get "fieldname AS alias" in your SQL, you'll need to supply a literal chunk of SQL in your select attribute, such as:

->search({}, { select => [ \'now() AS currenttime'] })

Then you can use the alias in your group_by attribute.

.. filter the results of my search?

The first argument to search is a hashref of accessor names and values to filter them by, for example:

->search({'created_time' => { '>=', '2006-06-01 00:00:00' } })

Note that to use a function here you need to make the whole value into a scalar reference:

->search({'created_time' => \'>= yesterday()' })
.. search in several tables simultaneously?

To search in two related tables, you first need to set up appropriate relationships between their respective classes. When searching you then supply the name of the relationship to the join attribute in your search, for example when searching in the Books table for all the books by the author "Fred Bloggs":

->search({'authors.name' => 'Fred Bloggs'}, { join => 'authors' })

The type of join created in your SQL depends on the type of relationship between the two tables, see DBIx::Class::Relationship for the join used by each relationship.

.. create joins with conditions other than column equality?

Currently, DBIx::Class can only create join conditions using equality, so you're probably better off creating a view in your database, and using that as your source. A view is a stored SQL query, which can be accessed similarly to a table, see your database documentation for details.

.. search using greater-than or less-than and database functions?

To use functions or literal SQL with conditions other than equality you need to supply the entire condition, for example:

my $interval = "< now() - interval '12 hours'";
->search({last_attempt => \$interval})

and not:

my $interval = "now() - interval '12 hours'";
->search({last_attempt => { '<' => \$interval } })
.. search with an SQL function on the left hand side?

To use an SQL function on the left hand side of a comparison:

->search({}, { where => \'YEAR(date_of_birth)=1979' });

Or, if you have quoting off:

->search({ 'YEAR(date_of_birth' => 1979 });
.. find more help on constructing searches?

Behind the scenes, DBIx::Class uses SQL::Abstract to help construct its SQL searches. So if you fail to find help in the DBIx::Class::Manual::Cookbook, try looking in the SQL::Abstract documentation.

Fetching data

.. fetch as much data as possible in as few select calls as possible?

See the prefetch examples in the Cookbook.

.. fetch a whole column of data instead of a row?

Call get_column on a DBIx::Class::ResultSet, this returns a DBIx::Class::ResultSetColumn, see it's documentation and the Cookbook for details.

.. fetch a formatted column?

In your table schema class, create a "private" column accessor with:

__PACKAGE__->add_columns(my_column => { accessor => '_hidden_my_column' });

Then, in the same class, implement a subroutine called "my_column" that fetches the real value and does the formatting you want.

See the Cookbook for more details.

.. fetch a single (or topmost) row?

Sometimes you many only want a single record back from a search. A quick way to get that single row is to first run your search as usual:

->search->(undef, { order_by => "id DESC" })

Then call "slice" in DBIx::Class::ResultSet and ask it only to return 1 row:

->slice(0,1)

These two calls can be combined into a single statement:

->search->(undef, { order_by => "id DESC" })->slice(0,1)

Why slice instead of "first" in DBIx::Class::ResultSet or "single" in DBIx::Class::ResultSet? If supported by the database, slice will use LIMIT/OFFSET to hint to the database that we really only need one row. This can result in a significant speed improvement.

.. refresh a row from storage?

Use "discard_changes" in DBIx::Class::PK.

$row->discard_changes

Discarding changes and refreshing from storage are two sides fo the same coin. When you want to discard your local changes, just re-fetch the row from storage. When you want to get a new, fresh copy of the row, just re-fetch the row from storage. "discard_changes" in DBIx::Class::PK does just that by re-fetching the row from storage using the row's primary key.

Inserting and updating data

.. insert a row with an auto incrementing primary key?

In versions of DBIx::Class less than 0.07, you need to ensure your table class loads the PK::Auto component. This will attempt to fetch the value of your primary key from the database after the insert has happened, and store it in the created object. In versions 0.07 and above, this component is automatically loaded.

.. insert a row with a primary key that uses a sequence?

You need to create a trigger in your database that updates your primary key field from the sequence. To help PK::Auto find your inserted key, you can tell it the name of the sequence in the column_info supplied with add_columns.

->add_columns({ id => { sequence => 'mysequence', auto_nextval => 1 } });
.. insert many rows of data efficiently?
.. update a collection of rows at the same time?

Create a resultset using a search, to filter the rows of data you would like to update, then call update on the resultset to change all the rows at once.

.. use database functions when updating rows?
.. update a column using data from another column?

To stop the column name from being quoted, you'll need to supply a scalar reference:

->update({ somecolumn => \'othercolumn' })
.. store JSON/YAML in a column and have it deflate/inflate automatically?

You can use DBIx::Class::InflateColumn to accomplish YAML/JSON storage transparently.

If you want to use JSON, then in your table schema class, do the following:

use JSON;

__PACKAGE__->add_columns(qw/ ... my_column ../)
__PACKAGE__->inflate_column('my_column', {
    inflate => sub { jsonToObj(shift) },
    deflate => sub { objToJson(shift) },
});

For YAML, in your table schema class, do the following:

use YAML;

__PACKAGE__->add_columns(qw/ ... my_column ../)
__PACKAGE__->inflate_column('my_column', {
    inflate => sub { YAML::Load(shift) },
    deflate => sub { YAML::Dump(shift) },
});

This technique is an easy way to store supplemental unstructured data in a table. Be careful not to overuse this capability, however. If you find yourself depending more and more on some data within the inflated column, then it may be time to factor that data out.

Misc

How do I store my own (non-db) data in my DBIx::Class objects?

You can add your own data accessors to your classes.

How do I use DBIx::Class objects in my TT templates?

Like normal objects, mostly. However you need to watch out for TT calling methods in list context. When calling relationship accessors you will not get resultsets, but a list of all the related objects.

Starting with version 0.07, you can use "search_rs" in DBIx::Class::ResultSet to work around this issue.

See the SQL statements my code is producing?

Turn on debugging! See DBIx::Class::Storage for details of how to turn on debugging in the environment, pass your own filehandle to save debug to, or create your own callback.

Why didn't my search run any SQL?

DBIx::Class runs the actual SQL statement as late as possible, thus if you create a resultset using search in scalar context, no query is executed. You can create further resultset refinements by calling search again or relationship accessors. The SQL query is only run when you ask the resultset for an actual row object.

How do I deal with tables that lack a primary key?

If your table lacks a primary key, DBIx::Class can't work out which row it should operate on, for example to delete or update. However, a UNIQUE constraint on one or more columns allows DBIx::Class to uniquely identify the row, so you can tell DBIx::Class::ResultSource these columns act as a primary key, even if they don't from the database's point of view:

$resultset->set_primary_key(@column);

Notes for CDBI users

Is there a way to make an object auto-stringify itself as a particular column or group of columns (a-la cdbi Stringfy column group, or stringify_self method) ?

See "Stringification" in DBIx::Class::Manual::Cookbook