Firebase realtime database - orderByChild not returning any result

Question

I am using the Firebase Realtime Database functions .orderByChild() and .equalTo() to fetch data nodes to be updated. However, I cannot seem to get any results back from my queries.

My database structure

{
  "4QEg0TWDbESiMX8Cu8cvUCm17so2" : {
    "-LmGtXsgJbAvVS8gv5-E" : {
      "createdAt" : 1565815876803,
      "message" : "Hello",
      "isSender" : false,
      "sender" : {
        "alias" : "",
        "name" : "Person A",
        "sid" : "mUO3DtYY2yRw3zkv4EmTlfldB3S2"
      },
      "sysStatus" : 0
    },
    "-LmGtt4nyuygG9B4s6__" : {
      "createdAt" : 1565815967746,
      "message" : "Hej!",
      "isSender" : true,
      "sender" : {
        "alias" : "",
        "name" : "Person B",
        "sid" : "4QEg0TWDbESiMX8Cu8cvUCm17so2"
      },
      "sysStatus" : 0
    },
    "-LmJxcvL_Y7JmojxPiiK" : {
      "createdAt" : 1565867281849,
      "isSender" : true,
      "message" : "111",
      "sender" : {
        "alias" : "",
        "name" : "Person B",
        "sid" : "4QEg0TWDbESiMX8Cu8cvUCm17so2"
      },
      "sysStatus" : 0
    }
  },
  "mUO3DtYY2yRw3zkv4EmTlfldB3S2" : {
    "222" : {
      "createdAt" : 1565867281849,
      "isSender" : true,
      "message" : "Test",
      "sender" : {
        "alias" : "",
        "name" : "Person A",
        "sid" : "mUO3DtYY2yRw3zkv4EmTlfldB3S2"
      },
      "sysStatus" : 0
    },
    "333" : { <-- This is one node I wish to fetch
      "createdAt" : 1565815967746,
      "isSender" : false,
      "message" : "123",
      "sender" : {
        "alias" : "",
        "name" : "Person B",
        "sid" : "4QEg0TWDbESiMX8Cu8cvUCm17so2" <-- This is the value I am trying to match
      },
      "sysStatus" : 0
    }
  },
  "rKNUGgdKqdP68T0ne6wmgJzcCE82" : {
    "-Lm_GTmNHyxqCqQl4D4Z" : { <-- This is one node I wish to fetch
      "createdAt" : 1566140917160,
      "isSender" : false,
      "message" : "Sesame",
      "sender" : {
        "alias" : "",
        "name" : "Person B",
        "sid" : "4QEg0TWDbESiMX8Cu8cvUCm17so2" <-- This is the value I am trying to match
      },
      "sysStatus" : 5
    }
  }
}
  

My code

dbRoot.child('messages')
.orderByChild('sid')
.equalTo(userId)
.once('value', (senderSnapshot) => {
  console.log('senderSnapshot', senderSnapshot.val())
  console.log('senderSnapshot amount', senderSnapshot.numChildren())
  senderSnapshot.forEach((sender)=>{
    //Do the work!
  })
})

The code logs

senderSnapshot null

senderSnapshot amount 0

I have manually checked that there are several nodes where "sid" is set to the "userId" I am looking for.

Why am I not getting any results back from my query?

It seems like I have to search dbRoot.child('messages/rKNUGgdKqdP68T0ne6wmgJzcCE82') to get my value. :/ (And then repeat the search for each user) How much extra data overhead would it be to download/collect all contacts and then loop thru each users contact?

Answer 1

Firebase Database queries allow you to search through the direct child nodes at a certain location for a value at a fixed path under each child. So: if you know the user whose message you want to search through, you can do:

dbRoot
  .child('messages')
  .child('mUO3DtYY2yRw3zkv4EmTlfldB3S2') // the user ID
  .orderByChild('sender/sid')
  .equalTo(userId)

Note the two changes I made here from your code:

1. We now start the search from messages/mUO3DtYY2yRw3zkv4EmTlfldB3S2, so that it only searches the messages for that one user.
2. We now order/filter on the sender/sid property for each child under messages/mUO3DtYY2yRw3zkv4EmTlfldB3S2.

By combining these we are essentially searching a flat list of child nodes, for a specific matching value at a path under each child node.

---

There is no way in your current data structure to find all messages for a specific sender/sid value across all users. To allow that you'll have to add an additional data structure, where you essentially invert the current data.

"sender_messages": {
  "mUO3DtYY2yRw3zkv4EmTlfldB3S2": {
    "4QEg0TWDbESiMX8Cu8cvUCm17so2/-LmGtXsgJbAvVS8gv5-E": true,
  },
  "4QEg0TWDbESiMX8Cu8cvUCm17so2": {
    "4QEg0TWDbESiMX8Cu8cvUCm17so2/-LmGtXsgJbAvVS8gv5-E": true,
    "4QEg0TWDbESiMX8Cu8cvUCm17so2/-LmGtt4nyuygG9B4s6__": true,
    "4QEg0TWDbESiMX8Cu8cvUCm17so2/-LmJxcvL_Y7JmojxPiiK": true,
    "mUO3DtYY2yRw3zkv4EmTlfldB3S2/333": true,
    "rKNUGgdKqdP68T0ne6wmgJzcCE82/Lm_GTmNHyxqCqQl4D4Z": true
  },
  "mUO3DtYY2yRw3zkv4EmTlfldB3S2": {
    "mUO3DtYY2yRw3zkv4EmTlfldB3S2/222": true,
  }
}

Now you can find the messages for a specific sender/sid by reading:

dbRoot
  .child('sender_messages')
  .child('4QEg0TWDbESiMX8Cu8cvUCm17so2') // the sender ID

And then looping over the results, and loading the individual messages from each path as needed.

This is quite common in NoSQL databases: you'll often have to modify your data structure to allow the use-cases you want to add to your app.

See also:

• Firebase query if child of child contains a value
• Firebase Query Double Nested
• Speed up fetching posts for my social network app by using query instead of observing a single event repeatedly (to learn why loading the additional messages is not nearly as slow as you may initially think)