'Populate the dropdown widget for responding to an Incident Report
based on those vehicles which aren\'t already on-call'

@staticmethod
def irs_vehicle_requires():

    s3db = current.s3db
    table = s3db.asset_asset
    ltable = s3db.irs_ireport_vehicle
    asset_represent = s3db.asset_asset_id.represent
    query = (((table.type == s3db.asset_types['VEHICLE']) & (table.deleted == False)) & (((ltable.id == None) | (ltable.closed == True)) | (ltable.deleted == True)))
    left = ltable.on((table.id == ltable.asset_id))
    requires = IS_EMPTY_OR(IS_ONE_OF(current.db(query), 'asset_asset.id', asset_represent, left=left, sort=True))
    return requires
