Subdocument Array of Objects Schema causes "Cast to array failed for value" validation error when you use ... spread operator versus toObject() for inheritance

[niftylettuce]

Hey @vkarpov15 - hope all is well.

I'm writing as this was a frustrating bug to track down. I believe this worked in version 5 of Mongoose, but I could be wrong and this totally has never worked since I switched to use ... spread operator here for inheriting the object.

Here's the diff showing the fix I had to do to avoid the issue:

  // swap the user group based off ctx.request.body.group
  ctx.state.domain.members = ctx.state.domain.members.map((member) => ({
-    ...member,
+    ...member.toObject(),
    group:
      member.user.toString() === ctx.params.member_id
        ? ctx.request.body.group
        : member.group
  }));

// ...

await ctx.state.domain.save(); // save the domain here causes the validation error

The schema is defined here https://github.com/forwardemail/forwardemail.net/blob/master/app/models/domain.js#L131 and here https://github.com/forwardemail/forwardemail.net/blob/master/app/models/domain.js#L45-L58 (pretty simple).

[niftylettuce]

Note this only happens when you don't use lean() and don't have a Mongoose object.

[Uzlopak]

toObject is basically transforming the hydrated Document to a lean Object. Probably when casting we have to check if it is a hydrated Document and lean it.

[vkarpov15]

I'll try to repro this. However, in general, I'd recommend using .toObject() with spread operator. Because spread operator iterates through the actual object's keys, doing ...member gives you a POJO with Mongoose document internals like $__ and _doc.

[vkarpov15]

Confirmed and fixed. Although I still would strongly recommend doing the below instead:

for (const member of ctx.state.domain.members) {
  member.group = member.user.toString() === ctx.params.member_id
        ? ctx.request.body.group
        : member.group;
}

You're not writing React here - by shallow cloning the array and all the documents you're incurring an unnecessary performance penalty and making your code more complex.

[titanism]

@vkarpov15 this rewrite to use the loop will throw the following error Cast to ObjectId failed for value "[Circular]" (type string) at path "_id" on latest version of Mongoose

  for (const member of ctx.state.domain.members) {
    if (member.user.toString() === ctx.params.member_id)
      member.group = ctx.request.body.group;
  }

[titanism]

@vkarpov15 we've confirmed that the fix is the following:

  // swap the user group based off ctx.request.body.group
  ctx.state.domain.members = ctx.state.domain.members.map((member) => {
    return {
      user: member.user,
      group:
        member.user.toString() === ctx.params.member_id
          ? ctx.request.body.group
          : member.group
    };
  });

[vkarpov15]

@titanism I'm trying to figure out why your original example doesn't work. Looking at forwardemail/forwardemail.net@9e3ec85#diff-93a5248c3ab689a90e0344028135658b7cf18f44872ed83ecc329f9e912342c9, why is the original code:

  for (const member of domain.members) {
    if (member.user.toString() === ctx.params.member_id)
      member.group = ctx.request.body.group;
  }

and not the below? Loop should be over ctx.state.domain.members

for (const member of ctx.state.domain.members) {
    if (member.user.toString() === ctx.params.member_id)
      member.group = ctx.request.body.group;
  }