Connected to MLab, but won't connect to localhost - mongodb

Question

I had this working fine on both localhost and MLab, but then had to switch databases. After much trying I got the database up on MLab, but now it's not connecting to my localhost. Here is my server.js file:
const path = require("path");
const PORT = process.env.PORT || 3001;
const app = express();
const mongoose = require("mongoose");
const routes = require("./routes");
// Connect to the Mongo DB
mongoose.connect(process.env.MONGODB_URI || 'mongodb://XXUSERXX:XXPASSWORDXX#ds217388-a0.mlab.com:17388,ds217388-a1.mlab.com:17388/<dbname>?replicaSet=rs-ds217388', { useNewUrlParser: true });
mongoose.connection.on("open", function (ref) {
console.log("Connected to mongo server.");
});
mongoose.connection.on('error', function (err) { console.log(err) });
// Define middleware here
app.use(express.urlencoded({ extended: true }));
app.use(express.json());
// Serve up static assets (usually on heroku)
if (process.env.NODE_ENV === "production") {
app.use(express.static("client/build"));
}
// Add routes, both API and view
app.use(routes);
// Define API routes here
// Send every other request to the React app
// Define any API routes before this runs
app.get("*", (req, res) => {
res.sendFile(path.join(__dirname, "./client/build/index.html"));
});
app.listen(PORT, () => {
console.log(`🌎 ==> API server now on port ${PORT}!`);
});
The only line of code I changed was this one below, this is what it was previously:
mongoose.connect(process.env.MONGODB_URI || 'mongodb://localhost:27017/wineDB', { useNewUrlParser: true });
I have this app connected in Heroku and had MONGODB_URI defined in the Config Vars, but it wasn't working with the second database until I manually put the connection string in my server.js file. It worked fine with the first one, I don't understand why!
How do I get it to connect to find localhost when it's not running off of MLAB so I can test? Thanks for the help.

Answer 1

It looks like the confusion is from a few different combinations of whether the environment variable being defined or not, as well as whether or not your app is using the variable, instead of falling back to what is defined.
The MONGODB_URI environment variable should contain the connection string for your mLab database and be defined in your Heroku environment both locally and when deployed. I'm assuming that the variable process.env.LOCAL will only be present on your local environment, in situations where your app should be connecting to the local database.
In these cases, something like the following should work:
if(process.env.LOCAL || process.env.MONGODB_URI) {
mongoose.connect(process.env.LOCAL || process.env.MONGODB_URI, { useNewUrlParser: true });
...
} else {
console.log("MongoDB connection string not defined!");
}
We place process.env.LOCAL first, followed the by ||, to say that it gets preference when connecting. Mongoose should then connect to whatever is defined in process.env.LOCAL if present (i.e. your local MongoDB database), falling back to process.env.MONGODB_URI (i.e. mLab) otherwise.
Lastly, it's wrapped in a simple if-else to print out an error message if both values are not defined.