Calling lean on document throws TypeError: lean is not a function

Question

I'm new to mongoose, and I'm trying to create an app that uses the OpenWeatherMap API. After requesting the data form the API, I save them to my MongoDB and then I want to return the results as a json, so I call the following function:

async function saveForecast(data) {

// Code here for creating the "forecastList" from the data and fetching the "savedLocation" from the DB

const newForecast = new Forecast({
    _id: new mongoose.Types.ObjectId(),
    location: savedLocation,
    city: {
        id: data.city.id,
        name: data.city.name,
        coordinates: data.city.coord,
        country: data.city.country
    },
    forecasts: forecastList
});

try {
    const savedForecast = await newForecast.save();
    return savedForecast.populate('location').lean().exec(); //FIXME: The lean() here throws  TypeError: savedForecast.populate(...).lean is not a function
} catch (err) {
    console.log('Error while saving forecast. ' + err);
}
}

The "newForecast" is being saved successfully in the DB, however when I try adding the .lean() after my populate I get the following error: TypeError: savedForecast.populate(...).lean is not a function

I've used lean() on find queries and it works fine, but I can't get it to work with my "newForecast" object, even though the "savedForecast" is a mongoose document, as the debugger shows me.

Any ideas why lean() is not working? Thank you!

I'm using Mongoose 5.2.15, and after updating to 5.3.6 the same error occurs. — Lucas P.
– Lucas P., Commented Oct 23, 2018 at 15:07

Volodymyr · Accepted Answer · 2018-10-23 23:59:47Z

5

The issue comes from the fact that Document does not have lean() method.

await newForecast.save(); does not return a Query but a Document. Then running populate on a Document also returns Document. To convert Document to a plain JS object you have to use Document.prototype.toObject() method:

try {
    const savedForecast = await newForecast.save();
    return savedForecast.populate('location').toObject(); // Wrong! `location` is not populated!
} catch (err) {
    console.log('Error while saving forecast. ' + err);
}

However this code will execute wrongly - population will not be called, because populate must receive a callback argument or execPopulate (which returns a Promise) has to be called on it. As far as you are using async/await I would suggest to use execPopulate instead of a callback. And last but not least - populated location needs to be leaned:

try {
    const savedForecast = await newForecast.save();
    return await savedForecast
      .populate({ path: 'location', options: { lean: true }})
      .execPopulate()
      .then(populatedForecast => populatedForecast.toObject());
} catch (err) {
    console.log('Error while saving forecast. ' + err);
}

answered Oct 23, 2018 at 23:59

Volodymyr

1,4081 gold badge10 silver badges12 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Lucas P. Over a year ago

Thank you so much! It finally works. I have misunderstood the use of lean() (and .toObject() for that matter). The weird thing is that my populate was working fine (when not calling .toObject() afterwards), so I got everything mixed up with promises returned by Mongoose. Again, thank you :)

yalcinozer Over a year ago

I know this is an old post. But I couldn't answer to my situation. When I log the populated field, I can see it as with all its keys on terminal. But if I try to access fields of it like appointment.populatedField.firstName it says Property 'firstName' does not exist on type 'string'. Populated field seen as string. After wasting many hours, I still have no idea what's the problem.

Collectives™ on Stack Overflow

Calling lean on document throws TypeError: lean is not a function

1 Answer 1

2 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

2 Comments

Your Answer

Sign up or log in

Post as a guest

Related