I have a DB called participants with structure:

{
  _id: '123',
  infos: [
    {
      myId: 'my123',
      otherId: 'other123'
    }
  ]
}

However, studyInfos can be empty and the objects inside of studyInfos can contain a myId without a otherId. I want to create a unique index on

{ myId: 1, otherId: 1}

I am using ensureIndex like so:

  Participants._ensureIndex(
    { 'infos.myId': 1, 'infos.otherId': 1 },
    {
      unique: true,
      sparse: true,
      partialFilterExpression: {
        'infos.otherId': { $exists: true, $type: String, $ne: null },
        'infos.myId': { $exists: true, $type: String, $ne: null },
      },
    },
  );

However, when i try to add a second user with infos empty or two users with otherId missing in both it fails due to duplicate index. Basically the only duplicate I don't want is

{
  _id: '123',
  infos: [
    {
      myId: 'my123',
      otherId: 'other123'
    }
  ]
},
{
  _id: '456',
  infos: [
    {
      myId: 'my123',
      otherId: 'other123'
    }
  ]
},

But its not allowing:

{
  _id: '123',
  infos: []
},
{
  _id: '456',
  infos: []
},

or

{
  _id: '123',
  infos: [
    {
      myId: 'my123',
    }
  ]
},
{
  _id: '456',
  infos: [
    {
      myId: 'my123',
    }
  ]
},

either despite my usage of partialFilterExpression and sparse.

Help.