MongoDB unique index with missing/null field

Question

How can I define a unique index that achieves the following goal:

If name field is missing (or null), allow only one document with the same id, but if the name exists, allow multiple instances of it as long as (id,name) is unique.

Good

{
    "id" : "a",
    "name" : "John"
},
{
    "id" : "a",
    "name" : "Hugo"
}

Good

{
    "id" : "a"
}

Bad

{
    "id" : "a",
    "name" : "John"
},
{
    "id" : "a",
    "name" : "John"
}

Bad

{
    "id" : "a",
}
{
    "id" : "a",
    "name" : "John"
}

Answer 1

Conditional indexes can be implemented with Partial indexes.

If I get the requirement it's just these 2 indexes:

  {
    "name": "id_idx",
    "key": {
      "id": 1
    },
    "partialFilterExpression": {
      "name": null
    },
    "unique": true
  },
  {
    "name": "id_name_idx",
    "key": {
      "id": 1,
      "name": 1
    },
    "partialFilterExpression": {
      "name": {
        "$gt": ""
      }
    },
    "unique": true
  }