CODEWITHSUNDEEP
mongodbmongoosemongoose-populate
David
David

Reputation: 373

How does mongoose populate work under the hood

Could somebody tell me how

I have a collection

a {
 b: String
 c: Date
 d: ObjectId --> j
}

j {
 k: String
 l: String
 m: String
}

when I carry out a:

a.find({ b: 'thing' }).populate('d').exec(etc..)

in the background is this actually carrying out two queries against the MongoDB in order to return all the items 'j'?

I have no issues getting populate to work, what concerns me is the performance implications of the task.

Thanks

Upvotes: 21

Views: 6923

Answers (3)

kgangadhar
kgangadhar

Reputation: 5088

Adding to @JohnnyHK answer on the performance implications of the task you worried about, I believe no matter what, these queries have to execute sequentially whether we use the mongoose provided populate() method or the one you will implement the server-side, both will have the same time complexity.

This is because in order to populate we need to have the results from the first query, after getting the result uuid will be used to query the document in the other collection.

So I believe it's a waste to make these changes the server-side than to use the mongoose provided method. The performance will remain the same.

Upvotes: 2

Ibad Shaikh
Ibad Shaikh

Reputation: 3536

Basically the model 'a' is containing an attribute 'd' which is referencing(pointing) towards the model 'j'.

So whenever we use

a.find({ b: 'thing' }).populate('d').exec(etc..)

Then through populate we can individually call properties of 'j' like :

  • d.k
  • d.l
  • d.m

Populate() helps us to call properties of other models.

Upvotes: 1

JohnnyHK
JohnnyHK

Reputation: 312005

Mongoose uses two queries to fulfill the request.

The a collection is queried to get the docs that match the main query, and then the j collection is queried to populate the d field in the docs.

You can see the queries Mongoose is using by enabling debug output:

mongoose.set('debug', true);

Upvotes: 34

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