Authenticating while connecting to multiple database using mongoose - mongodb

Question

I have set up auth in my MongoDB as mentioned here. Initially, in my project I was accessing a single database say, firstdb from mongoose using
let url = "mongodb://localhost:27017/firstdb";
let options = {
server:{
socketOptions:{
keepAlive:120
}
},
user:"username1",
pass:"mypassword1"
};
mongoose.connect(url,options,callback);
The user with username and mypassword was created in the firstdb itself giving it readWrite perms. I did this while logged in with my admin user.
Things were working smoothly. Then I had a requirement of connecting to a second database. So I changed my code as such
let url1 = "mongodb://localhost:27017/firstdb";
let options1 = {
server:{
socketOptions:{
keepAlive:120
}
},
user:"username1",
pass:"mypassword1",
auth:{
authdb:"firstdb"
}
};
let connection1 = mongoose.createConnection(url1,options1);
let url2 = "mongodb://localhost:27017/seconddb";
let options2 = {
server:{
socketOptions:{
keepAlive:120
}
},
user:"username2",
pass:"mypassword2",
auth:{
authdb:"seconddb"
}
};
let connection2 = mongoose.createConnection(url2,options2);
This time I created user username2 the same way in the seconddb database. But now mongoose is unable to perform any operation and is failing with Not authorized to execute command. I can access the db through mongo shell though. I also spun up the code in my local system which doesn't have mongodb auth enabled and it works fine there. Please help

Answer 1

So, after shooting around in the blind, found something that works.
Use the username and password in the url itself like
let connection1 = mongoose.createConnection("mongodb://username1:password1#localhost:27017/firstdb");
Still, I would like to know why passing the parameters as options doesn't work.