In our little town we have a (1945fair.com) fair each year so that local homebased businesses have a place to show and sell their wares. I am the web developer (78 years old and forgetful) and I was asked to make a way the 85 spaces could be assigned without different people being assigned the same space. I thought I can do that! I used Unique in a number field and set it to 0 to 85 with steps of 1. The issue is the folks that need to assign the space need to change assignments from time to time. For example, if someone makes green widgets and they have space 3 and later another person wants a space and only 2 and 4 are available, and they also makes green widgets, we don not want them next to each other.  I guess this takes javascript or php or something else over my head to make it were a space can be changed and when it is changed the "old" number goes back into a pot for reassignment to someone else. I attached the form I was working with.  Anyone that can help is greatly appreciated.    

I got the part to work that prevents the same space being assigned to different people.   I am using two spaces as a maximum for spaces for one person.  I installed the Woody plugin so I could use the php code below. I created a short cut via Woody for the php code and pasted in the page. I have gtwo fields that are Unique from 1 - 99.

What I would like to understand is where is the information stored and how could I delete a space that has been assigned (spaces get moved around to accomodate not having two person with the same thing next to each other)

add_filter('frm_validate_field_entry', 'two_fields_unique', 10, 2);

function two_fields_unique( $errors, $posted_field ) {

  $first_field_id = 125; // change 125 to the id of the first field

  $second_field_id = 126; // change 126 to the id of the second field

  if ( $posted_field->id == $first_field_id ) {

    $entry_id = isset( $_POST['id'] ) ? absint( $_POST['id'] ) : 0;

    $values_used = FrmDb::get_col( 'frm_item_metas',

           array( 'item_id !' => $entry_id,

                array( 'or' => 1,

                     array( 'field_id' => $first_field_id, 'meta_value' => $_POST['item_meta'][ $first_field_id ] ),

                     array( 'field_id' => $second_field_id, 'meta_value' => $_POST['item_meta'][ $second_field_id ] ),

                )

           ), 'item_id', array( 'group_by' => 'item_id', 'having' => 'COUNT(*) > 1' )

     );

     if ( ! empty( $values_used ) ) {

           $errors[ 'field'. $first_field_id ] = 'You have already selected that option';

           $errors[ 'field'. $second_field_id ] = 'You have already selected that option';

     }

  }

  return $errors;