Firebase: Realtime Database Update and Delete

In our previous section, we learned how we could read and write data into the database. Now, we will learn how we can modify and delete the data from the database.

Update

For updating a single node in our JSON database, we simply use setValue() on the correct child reference.

correct child reference. 
myRef.setValue("Hello, World")
myRef.child("someChild").child("name").setValue(name)

If we want to write to a specific child of a node without overwriting other child nodes, we use the updateChildren() method. When we call updateChildren(), we can update lower-level child values by specifying a path for the key.

For example

//writeNewPost function for chat application
private fun Newpost(userId:String, username:String, title:String, body:String){
	//creating new post at /user-posts/$userid/$postid simultaneously
	//Using push key
	val key=database.child("posts").push().key
	//Checking it is null or not
if(key==null){
		Log.w(TAG,"Could not get push key for posts")
		return
	}
	//Creating Post object
	val post=Post(userId, username, title, body)
	//Push it to a map
val postValues=post.toMap()
	val childUpdates=HashMap<String,Any>()
	childUpdates["/posts/$key"]=postValues
	childUpdates["/user-posts/$userId/$key"]=postValues
	database.updateChildren(childUpdates)
}

The push() method is used to create a post in the node containing post for all user at /posts/$postid and simultaneously retrieves the key with getKey() method. The key is then used to create a second entry in the user's posts at /user-posts/$userid/$postid. Using /user-posts/$userid/$postid path, we can simultaneously update to multiple locations in the JSON tree with a single call to updateChildren(). Updates are made in a way that they are atomic means either all updates succeed or all updates fail.

Adding a Completion Callback

If we want to know when our data has been committed, we can add a completion listener. Both the setValue() and updateChildren() take an optional completion listener. It will be called when the write has been successfully committed to the database. The listener is passed an error object, if the call was unsuccessful. This error object indicates why the failure occurred.

database.child("users").child(userId).setValue(user)
	.addOnSuccessListener{
		//Write was successful!
		//?.
	}
.addOnFailureListener{
		//Write failed
		//?
}

Delete

The simplest way for deleting data is to call removeValue() on a reference to the location of that data. We can also delete data by specifying null as the value for another write operation such as setValue() or updateChildren(). We can use this technique with updateChildren() to delete multiple children in a single API call.

activity_upd_del.xml

upd_del.kt

package com.example.firebaserealtimedatabase

import androidx.appcompat.app.AppCompatActivity
import android.os.Bundle
import android.text.TextUtils
import android.util.Log
import android.view.View
import android.widget.Toast
import com.google.firebase.auth.FirebaseAuth
import com.google.firebase.database.*
import kotlinx.android.synthetic.main.activity_upd_del.*
import kotlinx.android.synthetic.main.activity_welcome.*
import kotlinx.android.synthetic.main.activity_welcome.email

class upd_del : AppCompatActivity() {

    private var mFirebaseDatabase: DatabaseReference? = null
        private var mFirebaseInstance: FirebaseDatabase? = null

        private var userId: String? = null

        override fun onCreate(savedInstanceState: Bundle?) {
            super.onCreate(savedInstanceState)
            setContentView(R.layout.activity_upd_del)

            mFirebaseInstance = FirebaseDatabase.getInstance()

            // get reference to 'users' node
            mFirebaseDatabase = mFirebaseInstance!!.getReference("users")

            val user = FirebaseAuth.getInstance().getCurrentUser()

            // add it only if it is not saved to database
            userId = user?.getUid()

        }


        private fun updateUser(name: String, email: String) {
            // updating the user via child nodes
            if (!TextUtils.isEmpty(name) && !TextUtils.isEmpty(email)) {
                mFirebaseDatabase!!.child(userId!!).child("name").setValue(name)
                mFirebaseDatabase!!.child(userId!!).child("email").setValue(email)
                Toast.makeText(applicationContext, "Successfully updated user", Toast.LENGTH_SHORT).show()
            }
            else
                Toast.makeText(applicationContext, "Unable to update user", Toast.LENGTH_SHORT).show()

        }

        fun onUpdateClicked(view: View) {
            val name = name.getText().toString()
            val email = email.getText().toString()

            //Calling updateUser function 
            updateUser(name, email)
        }

        fun onDeleteClicked(view: View) {
            //Remove value from child 
            mFirebaseDatabase!!.child(userId!!).removeValue()
            Toast.makeText(applicationContext, "Successfully deleted user", Toast.LENGTH_SHORT).show()

            // clear information
            txt_user.setText("")
            email.setText("")
            name.setText("")
        }

        companion object {
            private val TAG = upd_del::class.java.getSimpleName()
        }
    }

Output