How to group array data returned based from entity column value - TypeORM / Postgres in NestJS

Question

I have following 2 tables with following values:

course_tbl

id	title
1	course_1
2	course_2

modules_table

id	title	is_finished	course_id
1	module_1	true	1
2	module_2	true	1
3	module_3	false	1
4	module_4	true	2
5	module_5	false	2

Basically, the format or the return I am looking for is something like this:

[
    {
        id: 1
        title: course_1
        finishedModules: [ 
            {
                id: 1
                title: module_1
            }, 
            {
                id: 2
                title: module_2
            }
        ],
        unfinishedModules: [ 
            {
                id: 3
                title: module_3
            }
        ]
    },
    {
        id: 2
        title: course_2
        finishedModules: [ 
            {
                id: 4
                title: module_4
            }
        ],
        unfinishedModules: [ 
            {
                id: 5
                title: module_5
            }
        ]
    }
]

I tried doing, this.courseRepository.find({ relations: ['modules'] }) , but it only does the basic left join query.

Answer 1

There's a transformer option @Column decorator which could group data in desired format: https://orkhan.gitbook.io/typeorm/docs/entities#column-options

In your CourseEntity you could do this

@Column({ 
  transformer: {
    to: (value: DatabaseType) => <if is_finished true return object, otherwise do nothing>,
    from: (value: DatabaseType) => value
  }
})
finishedModules: Module

@Column({ 
  transformer: {
    to: (value: DatabaseType) => <if is_finished false return object, otherwise do nothing>,
    from: (value: DatabaseType) => value
  }
})
unfinishedModules: Module

In theory, transformer function should be triggered on find, and DatabaseType should contain query results, but I'm not sure how DatabaseType looks like in reality, since it's undocumented.

But, this is quite inefficient method, maybe it would be easier to add some reducer on find query result with finishedModules and unfinishedModules as a reducer response.