lean() function bug in document's field with Buffer type!

[DelavariMohsen]

Hi,
I have this model:

_id: mongoose.Types.ObjectId,
name: { type: String, required: true },
mimetype: String,
content: Buffer,

I return content field with this code in Nest js framework:

res.contentType(f.mimetype);
res.send(f.content);

When I find document with this code:

this.files.findById(imageId);

It's OK.
but when I use .lean() function in findById Or findOne and return image to client, Image is not valid image file!

I use:
MongoDb cloud v 4.0.
mongoose: ^5.5.7
@nestjs/mongoose: ^6.1.2,

thanks.

[DelavariMohsen]

repro script is ready in my Github

result with lean():

[vkarpov15]

Unfortunately this is expected behavior and there isn't much Mongoose can do about it because you're using lean(). The MongoDB driver stores buffers as MongoDB's Binary type. Mongoose converts MongoDB Binary objects into Buffers based on your schema. But we don't do that if you use lean(), because the whole point of lean() is to avoid schema conversion to minimize performance overhead..

TLDR:

    server.get('/getImageWithLean', async (req, res) => {
        const file = await fileModel.findOne({}).lean();
        if (file == null) {
            res.sendStatus(400).send();
            return;
        }
        res.contentType(file.mimetype);
        // Pull out the underlying buffer from the MongoDB Binary instance
        res.send(file.content.buffer);
    });