what does populate in mongoose mean?

As you read this answer (and I appreciate it), But it is not complete. Therefore I decide to write my own answer (because at this time the edit queue is full).

But if you need to Populating across multiple levels. We can get a list of a user's friends, but if we wanna retrieve a user's friends of friends we can dot it like this:

const userSchema = new Schema({
  name: String,
  friends: [{ type: ObjectId, ref: 'User' }]
});

User.
  findOne({ name: 'Val' }).
  populate({
    path: 'friends',
    // Get friends of friends - populate the 'friends' array for every friend
    populate: { path: 'friends' }
  });

I came across that question randomly, but I feel I need to help here, even if it's old because I'm not convinced by the way it is explained :

Populate() function populate...

That might be very clear for natives English speaker, but maybe not for others.

In short

Populate will automatically replace the specified path in the document, with document(s) from other collection(s).

Long version

Let's take your example:

Story.findOne({ title: Nintendo })

Will return a Story of that kind :

{
  _creator : A0jfdSMmEJj9, //id of the creator (totally random, just for a clear example)
    title    : Nintendo,
    fans     : [r432i900fds09809n, fdsjifdsjfueu88] // again, totally random that I've typed here
  }
}

In some case, those kind of request would be enough, because we don't care about the author or the fans so, having some ID won't bother us much.

But in the case where i need that _creator's name, I'll need to make another request to find it in database. Except, that here in mongoose we have a clever function called populate() that we can chained to our previous request in order to directly get that information in our answer without explictly doing an additional request.

Story.findOne({ title: Nintendo }).populate('_creator')

will return

{
  _creator : {
       _id : A0jfdSMmEJj*9,
       name: Sai,
       age: 100,
       stories : [fdsfdsfdsew38u, 89hr3232, ...]
    },
    title    : Nintendo,
    fans     : [r432i900fds09809n, fdsjifdsjfueu88]
  }
}

But maybe, that's too much information, and we don't want the stories that he wrote and his age and name are enough. Populate can then take an other argument containing the field that we need

Story.findOne({ title: Nintendo }).populate('_creator', 'name age')

result ==>

{
  _creator : {
       name: Sai,
       age: 100,
    },
    title    : Nintendo,
    fans     : [r432i900fds09809n, fdsjifdsjfueu88]
  }
}

populate() function in mongoose is used for populating the data inside the reference. In your example StorySchema is having _creator field which will reference to the _id field which is basically the ObjectId of the mongodb document.

populate() function can accept a string or an object as an input.

Where string is the field name which is required to be populated. In your case that is _creator. After mongoose found one doc from mongodb and the result of that is like below

_creator: {
  name: "SomeName",
  age: SomeNumber,
  stories: [Set Of ObjectIDs of documents in stories collection in mongodb]
},
title: "SomeTitle",
fans: [Set of ObjectIDs of documents in persons collection in mongodb]

populate can also accept the object as an input.

You can find the documents of mongoose's populate() function here : http://mongoosejs.com/docs/2.7.x/docs/populate.html or https://mongoosejs.com/docs/populate.html

.populate() is being used in order to bring only needed information.

EXAMPLE without .populate()

User.findOne({ name: Bob })

Will return

{
  Bob : {
    _id : dasd348ew,
    email: [email protected],
    age: 25,
    job: teacher,
    nationality: American,
  }
}

EXAMPLE with .populate()

User.findOne({ name: Bob }).populate("Bob", "job email")

Will return

{
  Bob : {
    job: teacher,
    email: [email protected],
  }
}