Question

In my custom post type, if one of the fields are left blank, how can I insert a default value before it is saved to the db? This happens after the user clicks on Publish before I insert the default value so it all happens behind the scenes.

Answer 1

You can use pre_post_update or save_post hook :
http://codex.wordpress.org/Plugin_API/Action_Reference/save_post

Or instead saving this default value in db, you can simply display it when the field is blank.