const PostSchema = new Schema({
    ...
    comments: [
        {
            user: {
                type: Schema.Types.ObjectId,
                ref: 'user',
            },
            body: {
                type: String,
                required: [true, 'Content required'],
            },
        }
    ],
   ...
});

PostRouter.put('/posts/comments', (req, res) => {
    const { id } = req.query;
    const userID = req.body.user;
    const body = req.body.body;
    const comment = {
        user: mongoose.Types.ObjectId(userID),
        body: body,
    };

    Posts
        .update({ _id: mongoose.Types.ObjectId(id) }, { $push: { comments: comment }})
        .then(result => {
            res.status(200).json(result.ok);
        })
        .catch(err => console.log(err));
});