While working on a MERN stack project, I came across a situation where I wanted to populate a field but also populate a field inside that populated field (I know it's confusing. Bear with me :p ). So, I solved it and decided to share it with you all. Nice idea, isn't it ? Let's get started then !
I assume you know the basics of mongoose, mongodb and nodejs. In this post, I will cover populate. What it is, How it works, and how to use it to populate documents in mongodb.
What is Population ??
Population is way of automatically replacing a path in document with actual documents from other collections. E.g. Replace the user id in a document with the data of that user. Mongoose has an awesome method populate to help us. We define refs in ours schema and mongoose uses those refs to look for documents in other collection.
Some points about populate:
- If no document is found to populate, then field will be
null. - In case of array of documents, if documents are not found, it will be an empty array.
- You can chain populate method for populating multiple fields.
- If two populate methods, populate same field, second populate overrides the first one.
First things first. We need an example to work with !!
We are going to create 3 collections with 3 schemas:
- User
- Blog
- Comment
const mongoose = require('mongoose');
const Schema = mongoose.Schema;
const UserSchema = new Schema({
name: String,
email: String,
blogs: [{
type: mongoose.Schema.Types.ObjectId,
ref: "Blog"
}]
});
const BlogSchema = new Schema({
title: String,
user: {
type: mongoose.Schema.Types.ObjectId,
ref: "User"
},
body: String,
comments: [{
type: mongoose.Schema.Types.ObjectId,
ref: "Comment"
}]
})
const CommentSchema = new Schema({
user: {
type: mongoose.Schema.Types.ObjectId,
ref: "User"
},
blog: {
type: mongoose.Schema.Types.ObjectId,
ref: "Blog"
},
body: String
})
const User = mongoose.model("Author", UserSchema);
const Blog = mongoose.model("Blog", BlogSchema);
const Comment = mongoose.model("Comment", CommentSchema);
module.exports = {User, Blog, Comment}
Minimal schemas with references to other schemas that will help us use populate method.
How Populate Works
Now let's see how populate works. I won't be writing the whole code. Only the important parts.
Suppose you want a user by id with its blogs. If you do it without populate, you will get the user document with his blog ids array. But we want blog documents instead of ids !!
So let's see how to do it.
// in your node js file
const User = require("path/to/userSchema");
User
.findOne({_id: userId })
.populate("blogs") // key to populate
.then(user => {
res.json(user);
});
/*
OUTPUT:
{
_id: userid, // obviously it will be id generated by mongo
name: "john doe",
email: "john@email.com",
blogs: [
{
_id: blogid,
title: "how to do nothing",
body: "Interesting matter in 11111the blog...",
comments: [commentId_1, commentId_2]
}
]
}
*/
Easy right? Populate is awesome for joining documents like that. You will get user with all blog documents in blogs array.
But if you see the output you will notice comments array is still full of comment ids instead of documents from comments. How do we populate them ??? Keep reading to know...
Nested Populate in a document !
Let's see how to do nested populate in a query and populate comments in user blogs.
User
.findOne({_id: userId })
.populate({
path: "blogs", // populate blogs
populate: {
path: "comments" // in blogs, populate comments
}
})
.then(user => {
res.json(user);
});
/*
OUTPUT:
{
_id: userid, // obviously it will be id generated by mongo
name: "john doe",
email: "john@email.com",
blogs: [
{
_id: blogid,
title: "how to do nothing",
body: "Interesting matter in the blog...",
comments: [
{
user: userId,
blog: blogId,
body: "your blog is awesome !"
}
]
}
]
}
*/
So that was it. If you want to select specific fields while populating, you can use select key to specify fields inside an object.
// simple populate
User
.findOne({_id: userId })
.populate("blogs", { name: 1 }) // get name only
// nested populate
User
.findOne({_id: userId})
.populate({
path: "blogs",
populate: {
path: "comments",
select: { body: 1 }
}
})
EXTRA : Populate After Save !?
Sometimes (rarely), you may want to populate a document after saving it to mongodb. Example, you create a new comment and save it, but when you send it with response you want to add user info in it instead of just user id.
const Comment = require("/path/to/commentSchema");
let newComment = new Comment({
user: userId,
blog: blogId,
body: "this is a new comment"
});
newComment.save().then(result => {
Comment
.populate(newComment, { path: "user" })
.then(comment => {
res.json({
message: "Comment added",
comment
});
})
})
/*
OUTPUT: Comment
{
user: {
_id: userid,
name: "john doe",
email: "johndoe@email.com",
blogs: [blogId_1, blogId_2
},
blog: blogId,
body: "your blog is awesome !"
}
*/
Any Suggestions are much appreciated.
Hope you find it useful and learned something new from it.
Happy Coding :)
If you want to learn MongoDB, do checkout my Learn MongoDB Series
Oldest comments (55)
Awesome . I knew this In django , Finally Got one for Node π€
Absolute delight to read.
thanks !!
From the outside, this seems like basically creating
JOINfunctionality from an RDBMS. That's not a bad thing! It turns out that sometimes an RDBMS needs document-style records, and sometimes document DBs need relations :) Just wanted to make the comparison in case it helps anyone else (and to make sure I understand it correctly).Correct !!...And Nice comparision. :)
Cool tip didnβt know this one.
Glad you found it useful :)
does populate in mongoose issue a separate DB call for fetching the referenced documents or does it translate to a $lookup and issue a single query?
No, it doesn't translate to a $lookup and Yes, it makes another query to db to get the required data.
Good question !
I see. yeah clientside joins are a difficult problem to solve. I've been trying to do that with my c# mongodb library but still haven't found a proper/ user friendly way. was expecting to steal it from mongoose if they've managed it π
hahaha...i think you have to use $lookup and $aggregation :p
I wonder why the populate method totally refused to work in my Express.js project. Tried everything as stated in the Mongoose Populate() documentation, yet nothing works.
really ?!.. what error are you getting ?
Glad you found t, It's very helpfull
Thank you !! :)
i don't know why populate is not working for me i get a empty array
don't forget to add ids in that array
yes that's why i was getting an empty array, thank you
:) good luck !
This is so much simple and concise. Thanks, Paras.
Thank you !!...Glad you found it useful. :)
Thank you for great article.
But I have a concern, what if I use my custom Id:string as a key, what will the type be rather than mongoose.Schema.Types.ObjectId?
There is a concept called "Virtuals". I came across it when I was thinking the same thing. I can only give you hint about it because I have not learned or applied them. In virtuals, you define the conditions for virtuals: 'localField' and 'foreignField'. Local field refers to the name of the field of your current Schema. Foreign field is the name of the field which you are using in other schema to refer to your current Schema. Then you use populate normally.
There maybe some mistake in above example but hope it helps you understand the basics atleast.
Maybe once I get time to study, I will write about it as well. :)
Thanks, that's helpful. But the "yahoo" could be anything, when we call
.populate("field name of Authors"). There are something still not clear. However, I get over it by modify data type of _id field.Good...things are not 100% clear to me as well...but I am sure after trying and testing we can understand it better :)
Hello paras wanted to know how you can find or filter items based on the populated items
Hi Neural. I don't think, we can find or filter items based on populated items. It may also slow down the query as well.
I would say, go for aggregation, because in aggregation we have better control over how we structure and filter data in different steps. Populate is good for simple to intermediate scenarios but aggregation is more helpful when you need to handle a little complex to advance scenarios
For anyone that wants more info, you can read more here. mongoosejs.com/docs/tutorials/virt...
Thanks @paras594 for the article, really helpful
Welcome and thanks for the reference !! You made the article more useful :)
Thank you for helpfull explaination. You have explained all in easy way and clearly. I wish you good luck!
I am glad you found it helpful. Thank you as well :)
You too Good Luck !!