How to get the sum of a table with a condition in Yii2 using AR?

Question

Yii

I'm trying to get the sum of records from a table with a date condition.
The code is the following:
$reportDate = '2015-10-01';

$saldo = Transaction::find()
                ->sum('')
                ->andWhere('<', '', $reportDate)
                ;

column type transaction_date = date.
Yii gives an error on this: Call to a member function andWhere() on string
If you remove the andWhere condition , then the request is processed normally.
I'm doing something wrong... :) But reading the sections of the documentation Active Record and sum() did not answer the question.
Tell me, please, how is it right?

Answer 1

The ->andWhere method is used when the ->where method has already been used.