Editing Accessing Metadata Fields (section)

== The Structure of Values ==

On an eprint, the title is generally a simple metadata field.  When $dataobj->value is called, it returns a scalar value.

EPrints has two types of metadata field:

* Simple
* Compound

Both types can either be a single or multiple values.  An example of a compound multiple field is the creators field:

<pre>
my $creators = $dataobj->value('creators');
use Data::Dumper;
print Dumper $creators;
</pre>

Data::Dumper is a very useful library that will output the Perl data structure.  In the above case, the output may look something like this:

<pre>
$VAR1 = [
          {
            'name' => {
                        'lineage' => '',
                        'given' => 'Noura',
                        'honourific' => '',
                        'family' => 'Abbas'
                      },
            'id' => '10363'
          },
          {
            'name' => {
                        'lineage' => '',
                        'given' => 'Andrew',
                        'honourific' => '',
                        'family' => 'Gravell'
                      },
            'id' => '22'
          },
          {
            'name' => {
                        'lineage' => '',
                        'given' => 'Gary',
                        'honourific' => '',
                        'family' => 'Wills'
                      },
            'id' => '395'
          }
        ];
</pre>

The above output shows an array of hashes.  The structure can be compared to the configuration of the creators field (see cfg/cfg.d/eprint_fields.pl):

<pre>
          {
            'name' => 'creators',
            'type' => 'compound',
            'multiple' => 1,
            'fields' => [
                          {
                            'sub_name' => 'name',
                            'type' => 'name',
                            'hide_honourific' => 1,
                            'hide_lineage' => 1,
                            'family_first' => 1,
                          },
                          {
                            'sub_name' => 'id',
                            'type' => 'text',
                            'input_cols' => 20,
                            'allow_null' => 1,
                          }
                        ],
            'input_boxes' => 4,
          },
</pre>

Each element in the array consists of a hash that has a key for each sub field of the creator field.  Note the structure of the name part of the data dump.  The name datatype is also a hash.

To summarise:

<dl>
<dt>Simple
<dd>Usually a scalar value (though some datatypes return a hashref
<dt>Compound
<dd>A hashref containing a simple value for each subfield
<dt>Multiple
<dd>An arrayref containing simple or compound metadata fields
</dl>

The perl 'ref' function can help you to identify the shape of your data.