Edit both fields of linked mjoin table?

I have a table structure similar to the example here: https://editor.datatables.net/manual/php/mjoin#Mjoin-class

My first table is "mechanics" with user_id and trade. The second table ("building_areas") uses building_area_id to identify the areas inside our buildings - it includes the building name and associated area name. The link table is "mechanic coverage" which is the mechanic id's and building_area id's. The mechanic id shows up several times in this table if the mechanic covers several areas.

I have successfully set up the mjoin and the table data is displaying as I had hoped. (The mechanic name, trade, and a concatenated list of the buildings and areas he/she covers). Picture attached.

My issue is that the field I would like to be able to edit (in the link table) is the mechanic id. When a mechanic calls in sick or has time off, a different mechanic covers those areas.

Editor::inst( $db, 'mechanic', 'user_id' )
->field(
        Field::inst( 'mechanic.days' ),
        Field::inst( 'mechanic.shift_start' ),
        Field::inst( 'mechanic.shift_end' ), ...
)
->join(
        Mjoin::inst('building_areas')
            ->link('mechanic.user_id', 'mech_coverage.today_mechanic')          
            ->link('building_areas.area_id', 'mech_coverage.building_area_id')
            ->fields(           
                Field::inst( 'area_id' )
                    ->validator( 'Validate::required' )
                    ->options( Options::inst() 
                        ->table( 'building_areas' )
                        ->value( 'area_id' )
                        ->label( array('building_name', 'area_name') )                          
                    ),
                Field::inst('building_name'),
                Field::inst('area_name')
                    ->setFormatter( 'Format::ifEmpty', null )                                               
            )
        )

I would like to edit mech_coverage.today_mechanic - not the building areas but I haven't been able to figure out how to do it. Is there something simple I'm missing or should I have approached this a completely different way?

A couple of key features I'm hoping not to lose: mechanic shows up only once in display, building_areas are a concatenated list in one column.

Thank you for any help.