I have two Mongo schemas:
User:
{
_id: ObjectId,
name: String,
country: ObjectId // Reference to schema Country
}
Country:
{
_id: ObjectId,
name: String
}
I want to get all users who have the country name "VietNam".
What kind of query (only to the User schema) can I use for this case?
I want it to look like this SQL query:
SELECT *
FROM User
JOIN Country
ON User.country = Country._id
WHERE Country.name = 'VietNam'
You can use below aggregation with mongodb 3.6 and above
db.country.aggregate([
{ "$match": { "name": "VietNam" } },
{ "$lookup": {
"from": Users.collection.name,
"let": { "countryId": "$_id" },
"pipeline": [
{ "$match": { "$expr": { "$eq": [ "$country", "$$countryId" ] } } },
],
"as": "users",
}},
{ "$unwind": "$users" },
{ "$replaceRoot": { "newRoot": "$users" }}
])
Unlike relational databases, this isn't something that Mongo is good at, and you should generally structure your schemas in a different way when you're using NoSQL. In this case, you could add a countryName field to the user collection (and perhaps an index on country) so that you can query {countryName: "Vietnam"}. (Instead of country-name, it'd likely make more sense to use the iso-2 country code)
If you do need to do a "join" you can use the $lookup operator in the aggregation pipeline -- just keep in mind that aggregations don't scale well and tend to be a little hard to write.
if you want to associate 2 or more collections you must define the reference object of the collection
my solution please check this https://mongoosejs.com/docs/populate.html
Heres the code I generally use:
Models/users.js
const mongoose = require("mongoose");
const Countries = require("./countries");
const UsersSchema = new mongoose.Schema({
name: Sting,
country: Countries.schema
});
module.exports = mongoose.model("Users", UsersSchema);
controllers/users.js
const express = require("express");
const router = express.Router();
const Users = require("../models/users");
router.post("/register", async(req, res) => {
try{
const Me = await Users.create({name: req.body.name, country: req.body.country})
/* If you make the request correctly on the frontend req.body.name is a
string and req.body.country is an object with one property, it's name */
res.status(200).json({
data: Me
});
}
catch(err){
res.status(400).json({message:err.message})
Since the subschema is an object you reference it as Me.country.name. Hope this helps!
Users.findMany({ country: req.body.country}). I guess I read your question wrong but this is a great reference to all things MongoDB.