Firebase-database - Cloud function query

Question

I'm saving tokens for firebase cloud messages in fcmTokens. Each user has up to three tokens.

{
  "uid1" : {
    "-Kt8Skx5sa-IeXhKBs4G" : {
      "uid1" : "token1"
    }
  },
  "uid2" : {
    "-Kt8Xt1lT7OeyKJBs1ps" : {
      "uid2" : "token1"
    },
    "-Kt8Xt1lT7OeyKJz892k" : {
      "uid2" : "token2"
    }
  }
}

ids is an array which contains user ids that need to receive messages. I can't seem to get the tokens for each user.

Error:

Error: Registration token(s) provided to sendToDevice() must be a non-empty string or a non-empty array.

Below is what I have now:

function sendFcm(ids) {

  const payload = {
    notification: {
      title: 'You have been invited to an event!',
      body: 'Event body',
      icon: "https://placeimg.com/250/250/people"
    }
  };

  for (const key in ids) {
    if (ids.hasOwnProperty(key)) {

      admin.database()
        .ref(`/fcmTokens/${ids[key]}/{pushId}/${ids[key]}`)
        .once('value')
        .then(token => token.val())
        .then(userFcmToken => {
          return admin.messaging().sendToDevice(userFcmToken, payload)
        })
        .then(res => {
          console.log("Sent Successfully"), res
        })
        .catch(err => {
          console.log(err);
        })
    }
  }
}

Answer 1

Your data structure seems inefficient for what you're doing with it. If you have a set of tokens per user, keep precisely that: a set of tokens per user.

{
  "uid1" : {
    "token1": true
  },
  "uid2" : {
    "token1": true,
    "token2": true,
    "token3": true
  }
}

And then:

for (const key in ids) {
  if (ids.hasOwnProperty(key)) {
    admin.database()
      .ref(`/fcmTokens/${ids[key]}`)
      .once('value')
      .then(snapshot => {
        snapshot.forEach((tokenSnapshot) => {
          let userFcmToken = tokenSnapshot.key;
          admin.messaging().sendToDevice(userFcmToken, payload);
        })
      })
    }