Join two arrays of objects on property

Question

In MySQL if I have two tables like this:

appointments

appointment_id    patient_id    appointment_time
-------------------------------------------------------
1                 42            2017-10-17 08:00:00
2                 43            2017-10-17 08:30:00

and

patients

patient_id    name
---------------------------
42            Joe Smith
43            Sally Sue
44            Jim Bob

then I could left join the two tables like this

SELECT *
FROM appointments
LEFT JOIN patients ON appointments.patient_id = patients.patient_id

and I would get a result like this:

appointment_id    patient_id    appointment_time       patient_id    name
----------------------------------------------------------------------------------
1                 42            2017-10-17 08:00:00    42            Joe Smith
2                 43            2017-10-17 08:30:00    43            Sally Sue

So my question is, in Javascript (ES6+), if I have two arrays of objects like this:

const appointments = [
  {
    appointmentId: 1,
    patientId: 42,
    appointmentTime: '2017-10-17 08:00:00'
  },
  {
    appointmentId: 2,
    patientId: 43,
    appointmentTime: '2017-10-17 08:30:00'
  }
];

and

const patients = [
  {
    patientId: 42,
    name: 'Joe Smith'
  },
  {
    patientId: 43,
    name: 'Sally Sue'
  },
  {
    patientId: 44,
    name: 'Jim Bob'
  }
];

How can I left join the patients array onto the appointments array based on the patientId property?

[
  {
    appointmentId: 1,
    patientId: 42,
    appointmentTime: '2017-10-17 08:00:00',
    name: 'Joe Smith'
  },
  {
    appointmentId: 2,
    patientId: 43,
    appointmentTime: '2017-10-17 08:30:00',
    name: 'Sally Sue'
  }
]

I know that there's the unionBy method in lodash, but that will remove the properties in the appointments object.

dhilt · Accepted Answer · 2017-10-17T20:25:10.050

5

Do following:

let result = appointments.map(a => ({...patients.find(p => a.patientId === p.patientId), ...a}));

edited Oct 17 '17 at 20:25

answered Oct 17 '17 at 19:36

dhilt

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instead of `name: p.name` at the end, you can just use `...p` to merge in all the props of the patient. – Brian Glaz Oct 17 '17 at 19:40
@BrianGlaz Of course! I've changed the answer, thanks – dhilt Oct 17 '17 at 19:41
I'd be inclined to swap `patients` and `appointments` so the result is 1:1 mapped to appointments as asked for in the question :p – Jaromanda X Oct 17 '17 at 19:44
Great answer, except `patients` and `appointments` needs to be swapped as @JaromandaX stated. – Kyle Morgan Oct 17 '17 at 19:45
@KyleMorgan not obvious from the sample input data :p – Jaromanda X Oct 17 '17 at 19:45
@JaromandaX True. I should have added an extra patient that didn't have any appointments. – Kyle Morgan Oct 17 '17 at 19:48
You are right again, so I swapped them... – dhilt Oct 17 '17 at 19:51
`patients.find(p => p.patientId === p.patientId)` should be `patients.find(p => p.patientId === a.patientId)` – Jaromanda X Oct 17 '17 at 20:17

Join two arrays of objects on property

1 Answers1