Views joins can use 'left_formula' instead of 'left_field' in order to use a SQL expression for the left part of the join

The Standard views join plugin accepts a new left_formula configuration entry, which can be used instead of left_field in order to use a SQL expression for the left part of the join. The setting of left_formula takes precedence over the setting left_field.

Code example:

$configuration = [
  'table' => 'two',
  'field' => 'field_b',
  'left_table' => 'one',
  'left_formula' => 'MAX(one.field_a)',
  'operator' => '=',
  'extra' => [
    0 => [
      'left_field' => 'field_c',
      'value' => 'some_val',
    ],
  ],
];
$join = Views::pluginManager('join')->createInstance('standard', $configuration);