CakePHP 3.x ORM:
hasOne relations with custom select() must include fields from related hasOne tables.

Sample:
$query = $mainTable-> find()
  -> select([
    'field1' => 'MainTables.last_name',
    'field2' => 'MainTables.first_name',
    'MainTables.id',
    'SecondTable.id',
    'SecondTable.location',
  ])
  -> contain(['SecondTable'])
  -> first();