[[!template id=plugin name=field author="[[rubykat]]"]]
[[!tag type/meta]]
[[!toc]]
## NAME

IkiWiki::Plugin::field - front-end for per-page record fields.

## SYNOPSIS

    # activate the plugin
    add_plugins => [qw{goodstuff field ....}],

    # simple registration
    field_register => [qw{meta}],

    # simple registration with priority
    field_register => {
	meta => 'last'
	foo => 'DD'
    },

    # allow the config to be queried as a field
    field_allow_config => 1,

    # flag certain fields as "tags"
    field_tags => {
	BookAuthor => '/books/authors',
	BookGenre => '/books/genres',
	MovieGenre => '/movies/genres',
    }

## DESCRIPTION

This plugin is meant to be used in conjunction with other plugins
in order to provide a uniform interface to access per-page structured
data, where each page is treated like a record, and the structured data
are fields in that record.  This can include the meta-data for that page,
such as the page title.

Plugins can register a function which will return the value of a "field" for
a given page.  This can be used in a few ways:

* In page templates; all registered fields will be passed to the page template in the "pagetemplate" processing.
* In PageSpecs; the "field" function can be used to match the value of a field in a page.
* In SortSpecs; the "field" function can be used for sorting pages by the value of a field in a page.
* By other plugins, using the field_get_value function, to get the value of a field for a page, and do with it what they will.

## CONFIGURATION OPTIONS

The following options can be set in the ikiwiki setup file.

**field_allow_config**

    field_allow_config => 1,

Allow the $config hash to be queried like any other field; the 
keys of the config hash are the field names with a prefix of "CONFIG-".

**field_register**

    field_register => [qw{meta}],

    field_register => {
	meta => 'last'
	foo => 'DD'
    },

A hash of plugin-IDs to register.  The keys of the hash are the names of the
plugins, and the values of the hash give the order of lookup of the field
values.  The order can be 'first', 'last', 'middle', or an explicit order
sequence between 'AA' and 'ZZ'.  If the simpler type of registration is used,
then the order will be 'middle'.

This assumes that the plugins in question store data in the %pagestatus hash
using the ID of that plugin, and thus the field values are looked for there.

This is the simplest form of registration, but the advantage is that it
doesn't require the plugin to be modified in order for it to be
registered with the "field" plugin.

**field_tags**

    field_tags => {
	BookAuthor => '/books/authors',
	BookGenre => '/books/genres',
	MovieGenre => '/movies/genres',
    }

A hash of fields and their associated pages.  This provides a faceted
tagging system.

The way this works is that a given field-name will be associated with a given
page, and the values of that field will be linked to sub-pages of that page,
the same way that the \[[!tag ]] directive does.

This also provides a field with the suffix of `-tagpage` which gives
the name of the page to which that field-value is linked.

For example:

	BookGenre: SF

will link to "/books/genres/SF", with a link-type of "bookgenre".

If one was using a template, then the following template:

	Genre: <TMPL_VAR BOOKGENRE>
	GenrePage: <TMPL_VAR BOOKGENRE-TAGPAGE>
	GenreLink: \[[<TMPL_VAR BOOKGENRE-TAGPAGE>]]

would give:

	Genre: SF
	GenrePage: /books/genres/SF
	GenreLink: <a href="/books/genres/SF/">SF</a>

## PageSpec

The `field` plugin provides a few PageSpec functions to match values
of fields for pages.

* field
  * **field(*name* *glob*)**
  * field(bar Foo\*) will match if the "bar" field starts with "Foo".
* destfield
  * **destfield(*name* *glob*)**
  * as for "field" but matches against the destination page (i.e when the source page is being included in another page).
* field_item
  * **field_item(*name* *glob*)**
  * field_item(bar Foo) will match if one of the values of the "bar" field is "Foo".
* destfield_item
  * **destfield_item(*name* *glob*)**
  * as for "field_item" but matches against the destination page.
* field_null
  * **field_null(*name*)**
  * matches if the field is null, that is, if there is no value for that field, or the value is empty.
* field_tagged
  * **field_tagged(*name* *glob*)**
  * like `tagged`, but this uses the tag-bases and link-types defined in the `field_tags` configuration option.
* destfield_tagged
  * **destfield_tagged(*name* *glob*)**
  * as for "field_tagged" but matches against the destination page.

## SortSpec

The "field" SortSpec function can be used to sort a page depending on the value of a field for that page.  This is used for directives that take sort parameters, such as **inline** or **report**.

field(*name*)

For example:

sort="field(bar)" will sort by the value og the "bar" field.

Additionally, the "field_natural" SortSpec function will use the
Sort::Naturally module to do its comparison (though it will fail if that
module is not installed).

## FUNCTIONS

### field_register

field_register(id=>$id);

Register a plugin as having field data.  The above form is the simplest, where
the field value is looked up in the %pagestatus hash under the plugin-id.

Additional Options:

**call=>&myfunc**

A reference to a function to call rather than just looking up the value in the
%pagestatus hash.  It takes two arguments: the name of the field, and the name
of the page.  It is expected to return (a) an array of the values of that field
if "wantarray" is true, or (b) a concatenation of the values of that field
if "wantarray" is not true, or (c) undef if there is no field by that name.

    sub myfunc ($$) {
	my $field = shift;
	my $page = shift;

	...

	return (wantarray ? @values : $value);
    }

**first=>1**

Set this to be called first in the sequence of calls looking for values.  Since
the first found value is the one which is returned, ordering is significant.
This is equivalent to "order=>'first'".

**last=>1**

Set this to be called last in the sequence of calls looking for values.  Since
the first found value is the one which is returned, ordering is significant.
This is equivalent to "order=>'last'".

**order=>$order**

Set the explicit ordering in the sequence of calls looking for values.  Since
the first found value is the one which is returned, ordering is significant.

The values allowed for this are "first", "last", "middle", or a two-character
ordering-sequence between 'AA' and 'ZZ'.

### field_get_value($field, $page)

    my $value = field_get_value($field, $page);

    my $value = field_get_value($field, $page, foo=>'bar');

Returns the value of the field for that page, or undef if none is found.
It is also possible to override the value returned by passing in
a value of your own.

## DOWNLOAD

* browse at GitHub: <http://github.com/rubykat/ikiplugins/blob/master/IkiWiki/Plugin/field.pm>
* git repo at git://github.com/rubykat/ikiplugins.git
