Doctrine\MongoDB\Query\Expr::field PHP Method

    public function field($field)
    {
        $this->currentField = (string) $field;
        return $this;
    }

 public function testOperatorWithCurrentField()
 {
     $expr = new Expr();
     $expr->field('field');
     $this->assertSame($expr, $expr->operator('$op', 'value'));
     $this->assertEquals(array('field' => array('$op' => 'value')), $expr->getQuery());
 }