Problem with Select Field

I want to use a plugin with Flexicontent and have created a select field to do this.

I can get it to work fine, but there is a small problem.

What it saves into the database, rather than the "value" of the option, it stores the text of the option.

I have used this:

SELECT id AS value, unit_name AS text FROM jos_rentalot_plus_units ORDER by unit_name ASC

To get it to work, I have to reverse it, but that means on the actual dropdown I display the ID number rather than the name of the unit.

Where in flexicontent can I change it so it saves the value part of the select field?