Add bw_theme_field___post_parent() to theme the post_parent as a link

[bobbingwide]

On the officialcaravan.co.uk website I want to be able to display the post_parent as a link to the post's parent rather than just the value of the parent post ID.

Requirement

• Display the post_parent field as link to the post's parent.
• Except when the post_parent is 0, in which case display an empty string.

Proposed solution

• Implement bw_theme_field__post_parent() in libs\bw_fields.php
• Optionally add virtual field parent_ID for when the parent ID is needed instead of the link.