java - Is there a workaround for foreignKeyExtractor restriction for KTable-KTable leftJoin in Kafka Streams?

Question

How to implement simple operation such as this in Kafka Streams?

#### department
id | name | type | active

#### employee
id | full_name | department_id

-- show number of employee for every active internal department
SELECT d.id, d.name, count(e.id) AS employee_count
FROM department d 
-- left join, foreign key extracted from the right table
LEFT JOIN employee e ON d.id = e.department_id
WHERE d.type = 'I' AND d.active = true
GROUP BY d.id, d.name

For some reason Kafka Streams API doesn't allow extracting foreign key from the right table:

// key: id, value: department
KTable<Long, Department> kTableDepartment = ...;
// key: id, value: employee
KTable<Long, Employee> kTableEmployee = ...;
        
kTableDepartment.leftJoin(
        kTableEmployee,
        // foreign key extractor is available only for primary source (left table)
        (Department department) -> ...,
        new ValueJoiner<>() {
            @Override
            public Object apply(Department department, Employee employee) {
                return ...;
            }
        });

question from:https://stackoverflow.com/questions/65921488/is-there-a-workaround-for-foreignkeyextractor-restriction-for-ktable-ktable-left

Answer 1

Waitting for answers