NodeJs and Postman - node.js

Question

I have 2 databases; First one is Users database where a registered user is an object with the 4 following keys (name, password, email and an array called "cats"). My next database is called Cats (again an object with 4 keys).
Now what I want to do is to update the User.cats each time the specific user uploads a cat in the Cats database. I have written all the requests for both databases (getUsers/getCats, postUser/postCat, putUser/putCat, patchUser/patchCat, deleteUser/deleteCat, getUserByID/GetCatById, getUserCount/getCatsCount);
I don't know how to access the users database so that User.cats gets updated whenever the user registers a new cat on his account.
These are the users:
{
"result": [
{
"name": "Razvan",
"email": "razvan#gmail.com",
"password": "Tricul",
"cats": []
},
{
"name": "mimisor",
"email": "mimi#hotmail.com",
"password": "MiMi",
"cats": []
},
{
"name": "Omar",
"email": "omar#gmail.com",
"password": "tataItz",
"cats": []
},
{
"name": "Dana",
"email": "danaraluca#gmail.com,",
"password": "MamaItz",
"cats": []
}
]
}
and these are the cats:
"result": [
{
"name": "Cocorico",
"age": "15 ani",
"breed": "Black Cat",
"owner": "UserId"
},
{
"name": "Mimi",
"age": "3 ani",
"breed": "Egyptian Mau",
"owner": "UserId"
},
{
"name": "Blondsky",
"age": "10 ani",
"breed": "Ginger Boy",
"owner": "UserId"
},
{
"name": "Billy",
"age": "1 ani",
"breed": "Aarabian Mau",
"owner": "UserId"
}
]
}

Answer 1

Basically you have two collections relationship.
I assume that you are using MongoDB. You can read about Mongoose to generate the component user and cat and use populate (I don't remember is MongoDB has something native to do that)
https://vegibit.com/mongoose-relationships-tutorial/
Other option for any DB, in your function you can read the users and do an iteration where for each user, you read from DB the cats that you have for that user and complete the field cats.