Firebase database rules allow update but prevent create - firebase

Question

How to set the database rules with "allow to update but not allow to add new record"?
I tried to simulate below but doesn't work..
My Data:
{
users:{
-randomID001:{
email:user#email.com,
status:active
},
-randomID002:{
email:user2#email.com,
status:inactive
}
}
}
Simulate in the console:
{
"rules": {
".read": "auth != null",
"users":{
".indexOn": "email",
".write":"!newData.exists() && data.exists()"
}
}
}
This line of code show mean for allow to write if new record doesn't exist and existing record exist ??
".write":"!newData.exists() && data.exists()"
Below is the test data i submit:
{
"randomID001":{"status":"inactive"}
}
I got the below error:
Simulated update denied
Please advise.

Answer 1

You can use validate to check if email already exists in the database this will allow only update on the existing users.
"users":{
"$userId":{
".write":"auth!=null",
".validate":"newData.child('email').val()===data.child('email').val()"
}

Answer 2

Firebase Realtime Database Simulator allows you to test [READ, SET, UPDATE]
It looks like dealing with firebase object.
FIREBASE DB MODEL - here is yours
{
users:{
-randomID001:{
email:user#email.com,
status:active
},
-randomID002:{
email:user2#email.com,
status:inactive
}
}
}
Try with different Locations
/users/randomID001 // exists in your DB
/users/randomID002 // exists in your DB
/users/randomID003 // not exists in your DB
CREATE
// newData(json) exists and no randomID001,002 data => FALSE
// newData(json) exists and no randomID003 data => TRUE
".write": "newData.exists() && !data.exists()"
UPDATE
// newData(json) exists and target(randomID001,002) data exists => TRUE
// newData(json) exists and target(randomID003) data exists => FALSE
".write": "newData.exists() && data.exists()"
DELETE
//You are sending only HTTP DELETE SIGNAL, NOT JSON
//Target(randomID001,002) data exists => TRUE
//Target(randomID003) data exists => FALSE
".write":"!newData.exists() && data.exists()"
next example is allowing [create,update]
You can think like "CREATE || UPDATE || DELETE".
{
"rules": {
"users": {
"$uid":{
".write": "(newData.exists() && !data.exists()) || (newData.exists() && data.exists())"
}
}
}
}
Also validation rule is good idea to be secured.
VALIDATION
...
items: {
"$itemId": {
".validate": "newData.hasChildren(['name', 'message', 'timestamp'])",
"name": {
".validate": "newData.val().length > 0 && newData.val().length < 20"
},
"message": {
".validate": "newData.isString() && newData.val().length > 0 && newData.val().length < 50"
},
"timestamp": {
".validate": "newData.isNumber() && newData.val() == now"
}
}
...
For your code maintenance, Firebase bolt is good choice.
https://github.com/firebase/bolt/blob/master/docs/language.md