How to find a document of an ID from an array in JavaScript (Mongoose)

Question

I have a function on my server that is supposed to get a post by its ID. The function works up until the "foundPost" constant, where I can't seem to find one of the documents from the "posts" array. I've tried substituting findOne for find and the ObjectIds work for the const 'post'. I've double checked that post_id is 62067c1211eea1531d5872f4

Here is the function to find a post:

const postById = async (req, res) => {
    const userId = req.params.userId;
    const post_id = req.params.post_id;
    const posts = await Post.findOne({ user: userId });
    console.log(posts);  //see this below
    const foundPost = await posts.findOne({ "upload": post_id });  //error here
    console.log(foundPost);
    return res.json({ success: true, Post: foundPost });
};

Here is what 'console.log(posts)' returns:

[
   {
     upload: new ObjectId("623b681bdf85df9086417723"),
     edited: false,
     title: 'Test 1',
     description: 'testing post 1',
     name: 'John ',
     sharedPost: 0,
   },
   {
     upload: new ObjectId("62067c1211eea1531d5872f4"),
     edited: false,
     title: 'Test 2',
     description: 'testing post 2',
     name: 'John ',
     sharedPost: 0,
   }
]

I'm hoping that the function will return:

{
  success: true,
  {
     upload: new ObjectId("62067c1211eea1531d5872f4"),
     edited: false,
     title: 'Test 2',
     description: 'testing post 2',
     name: 'John ',
     sharedPost: 0,
   },
}

Can anyone see why the line const foundPost = await posts.findOne({ "upload": post_id }); isn't working? Thank you for your help.

****** Response to answer ******

Hello, thanks a lot for your answer, unfortunately it's still giving an error. Please see below the model for the code I'm using:

const PostSchema = new Schema({
  user: {
    type: Schema.Types.ObjectId,
    ref: "user",
  },
  post: [
    {
      user: {
        type: Schema.Types.ObjectId,
        ref: "user",
      },
      upload: {
        type: Schema.Types.ObjectId,
        ref: "upload",
      },
      title: {
        type: String,
      },
      description: {
        type: String,
      },
    },
  ],
  date: {
    type: Date,
    default: Date.now,
  },
});

I noticed that you used 'userProfile.posts' which I adapted to 'userProfile.post' to match this schema. I'm also not sure if you wanted to use 'subdoc' or 'subDoc' in line 11 of your code, but I tried both with the same error for each. I determined that the code stuck at the const subDocs = userPosts.filter(filter); line. I've looked into the.filter method you've used and can't find any potential errors. Not sure what the issue is.

Thanks

Answer 1

The issue is that you cannot run another mongo query on objects that were result of a previous query.

//will return a single document if found, or null if not found.
const posts = await Post.findOne({ user: userId });
//this will not work because at this point 'posts' will be either a Document or null value
//So the object will not have the method 'findOne' available.
const foundPost = await posts.findOne({ "upload": post_id });

The solution is to deal correctly with the types of objects you have.

Below is a functional and safe implementation that solves your issue:

const userPosts = await Post.findOne({ user: userId }).exec();
if (!userPosts) {
  // document not found with provided userId
  return res.json({ success: false });
}
//here we have a Document
//check if document has 'posts' property and is an array
if (userPosts.posts) {
  //filter the posts array

  const filter = function(subDoc) {
    return subdoc.upload === post_id
  }

  const subDocs = userPosts.filter(filter);
  //filter returns an array, so we must check if has itens
  //then we grab the first item
  if (subDocs.length > 0) {
    const foundPost = subDocs[0];
    return res.json({ success: true, Post: foundPost });
  }
  
  //subDoc not found, return correct response
  return res.json({ success: false });
}

If your Post model schema is what I'm supposing to be, this code will work perfectly.

const schema = mongoose.schema({
  user: Number,
  posts: [{ upload: Number }]
})

In case of error, please add the code of the model schema structure.