在 findOneDocument 与返回文档上的精益（）之后调用精益（） - 为什么在返回的文档上调用精益（）不起作用？

Question

这是我得到的错误 => "TypeError: apartment.lean is not a function"

在这段代码中，我收到一条错误消息，提示lean() 不是函数：

let apartment = await Apartment.findOne({
    'address.placeId':req.body.placeId,
    'address.apartmentNumber':req.body.apartmentNumber
});

if(apartment){
    //fetch references of pictures belonging to the apartment and send them back to the user
    const pictureReferences = fetchPictureReferences(apartment._id);
    let apartmentPoJo = apartment.lean();
    apartmentPoJo.pictures=[...pictureReferences];

    
    return res.status(200).json({msg:'Apartment Found',apartment:apartmentPoJo});
}

在该代码中，它运行良好，基本上是在 findOne() 完成后立即调用lea() 函数：

let apartment = await Apartment.findOne({
    'address.placeId':req.body.placeId,
    'address.apartmentNumber':req.body.apartmentNumber
}).lean();

if(apartment){
    //fetch references of pictures belonging to the apartment and send them back to the user
    const pictureReferences = fetchPictureReferences(apartment._id);
    apartment.pictures=[...pictureReferences];

    
    return res.status(200).json({msg:'Apartment Found',apartment});
}

你能告诉我为什么它们不同吗？我假设 findOne() 函数返回一个文档。是当场执行还是在我创建的新变量上执行？

感谢您的澄清！

Answer 1

简而言之，lean 只是Model（或Query）类的一个方法。返回的Document 类没有lean 方法。

Models 和 Documents 都是 Mongoose 类，但它们并不相同。在 Model 上调用 Query 方法（如 findOne）会返回一个文档实例。

Document Methods

Model Methods

Query Methods（大部分these methods是Model类的一部分）

const findApartment = async (req, res) => {
  try {
    const { apartmentNumber, placeId } = req.body;
    if (!apartmentNumber || !placeId) throw String("You must supply a valid apartment and id!"); 
  
    /* 
       Using the Apartment model, we invoke Query methods "findOne" and "lean".
       
       The result of these two queries is a document instance transformed into 
       a plain Javascript object (a plain object where Mongoose virtuals are removed) 
       
       If you ran the query without invoking "lean", then "findOne" 
       would return an instance of a Document, which contain the methods
       associated with Documents -- as shown above, the Document class does not 
       contain the "lean" method.
    */
    const apartment = await Apartment.findOne({
      'address.placeId': placeId,
      'address.apartmentNumber': apartmentNumber
    }).lean();
    if (!apartment) throw String("Unable to locate the apartment with that address!");

    return res.status(200).json({msg: 'Apartment Found', apartment });
  } catch (err) {
    return res.status(404).json({ msg: err.toString() });
  }
};

令人困惑的部分可能是模型和文档可能共享相同的方法名称（例如填充：D、M），但它们并不相同。

更多关于精益的信息可以在here找到。