CastError: Cast to ObjectId failed for value "" at path "_id"

Question

Well, I see that here are a few posts like this, but they didn't help me ...

let me describe my problem:

I have two Schema's

var A = new Schema({
   someAttribut: {type: String},
   b: {type: ObjectId, ref:'B'}
});

var B = new Schema({
   someAttribut2: {type: Boolean, 'default': false'}
});

Now I'm in the situation that I already have a B-Object and I want to create an A-Object.

So i do it this way:

var a = new A(req.aFromClient); // {_id:null, someAttribute:'123', b:null}
//load b from Database ... 
a.b = existingBFromDatabase; // {_id: 'Sb~Õ5ÙÐDâb', someAttribute2: false}

The b object is loaded from my monogDB. The debugger shows me a valid ObjectId (53627ed535d9d04416e26218 or Sb~Õ5ÙÐDâb) for my b .

But when I save my new A-Object, i get the error: 'CastError: Cast to ObjectId failed for value "" at path "_id"'

I don't understand why I get this error. Firstly, I don't define the id in the schema, so mongoose should add it, which seems to work. Secondly, I think that mongoose should generate a new ID when I create an a object.

do you have any proposal?

Answer 1

Based on the comment in the code, _id does have a value (of null). So you need to remove _id from req.aFromClient before creating your new A doc from it:

delete req.aFromClient._id;
var a = new A(req.aFromClient);

Answer 2

You should do:

a.b = existingBFromDatabase._id;

Because mongoose only works with the id of the already existing object.