How do I check a user name is already there in the firebasedb?

MIT App Inventor Help
,
1

I have an app made with firedb and it's a sign up app
I want when I write a user name it get called and check if that user name is already in the db

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2

You should reorganise your data structure:

users
--John
  --email:john@gmail.com
  --password:abc123
--Ali
  --email:ali@gmail.com
  --password:123abc
--Juan
  --email:juan@gmail.com
  --password:234abc

Set users as your project bucket and call the taglist
This will return a list of your users
Do this on screen initialise, then you have that list to work with from the start