=head1 External custom fields

=head2 Description

C<External custom fields> is extension to custom fields that allow
you to define CF with a dynamic list of values. Loading values into
this custom fields requires writing a little Perl code to fetch the
data from the external source; the code that we added to RT 3.7
allows it to load data from arbitrary external sources.

=head2 Introduction into writing source of values

For each type of data source that you want, you'll need to put a file
in F</opt/rt3/local/lib/RT/CustomFieldValues/> (or equivilent if you
installed RT someplace other than F</opt/rt3>). To get a sense of the
code that you'll need to write, take a look at the code in 
L</opt/rt3/lib/RT/CustomFieldValues/Groups.pm> for a simple example
which just uses RT's API to pull in a list of RT's groups.

Running C<perldoc /opt/rt3/lib/RT/CustomFieldValues/External.pm> will
show you the documentation for the API that needs to be fulfilled, by
copying and editing the C<Groups> example is probably a fine place to
start.

Later in this doc we'll describe the example a little bit more.

=head2 Configuration

After the custom code is written, you need to tell RT about its
existence by adding something like following to your RT_SiteConfig.pm:
Set(@CustomFieldValuesSources, "RT::CustomFieldValues::MySource");

The value in quotes should be the name of the class that you created.

Stop and start your web server to enable any config changes. Open the web interface with
as an administrative user (such as root) create new custom field. In order to use
a custom source of values, you should select a custom field type
that lets users pick values from a list, for example it could be C<Select *>
or a type with autocompletion. It shouldn't be C<Enter one value> type
as this type doesn't allow user to choose values. Other types that
don't have a defined list of values are also unacceptable.

Save the changes, now you have ability to select a "source" for values.
Choose the class you wrote from the list and save changes.

=head2 How to write custom source

You have to implement a subclass of L<RT::CustomFieldValues::External>.
There are two main methods you want to override:

=over 4

=item SourceDescription

This method should return a string describing the data source; this is
the identifier which the adimistrator will see in the dropdown in the web
interface. See L</Configuration>.

=item ExternalValues

This method should return an array reference of hash references.  The
hash references should contain keys for C<name>, C<description>, and
C<sortorder>. C<name> is most important one. The others are optional

=back

Here's a simple static example:

  package RT::CustomFieldValues::MySource;
  
  # define class inheritance
  use base qw(RT::CustomFieldValues::External);

  # admin friendly description, the default valuse is the name of the class
  sub SourceDescription {
      return 'My Source';
  }
  
  # actual values provider method
  sub ExternalValues {
      # return reference to array ([])
      return [
          # each element of the array is a reference to hash that describe a value
          # possible keys are name, description and sortorder
          { name => 'value1', description => 'external value', sortorder => 1 },
          { name => 'value2', description => 'another external value', sortorder => 2 },
          # values without description are also valid, the default description is empty string
          { name => 'value3', sortorder => 3 },
          # you can skip sortorder too, but note that the default sortorder is 0 (zero)
          { name => 'value3' },
      ];
  }
  
  1; # don't forget to return some true value

That's all. Install and configure your class as described in the L</Configuration> section.