Partial Filter Expressions on unique indexes are unsupported?

[ghost]

I have a collection that contains heterogeneous documents. Some documents are uniquely identified by the value of field a (and do not contain field b) and the others are uniquely identified by field b (and do not contain field a).

For example:

{ "a": "foo", "value": "qux" }
{ "b": 1, "value": "quux" }

I have created indexes with the following definitions:

[
  {
    "key": {
      "_id": 1
    },
    "name": "_id_",
    "ns": "my.collection",
    "v": 2
  },
  {
    "key": {
      "a": 1
    },
    "name": "aUniqueIndex",
    "ns": "my.collection",
    "unique": true,
    "partialFilterExpression": {
      "a": {
        "$exists": true
      }
    },
    "v": 2
  },
  {
    "key": {
      "b": 1
    },
    "name": "bUniqueIndex",
    "ns": "my.collection",
    "unique": true,
    "partialFilterExpression": {
      "b": {
        "$exists": true
      }
    },
    "v": 2
  }
]

Inserting the following documents:

{ "a": "foo", "value": "qux" }
{ "b": 1, "value": "quux" }
{ "a": "bar", "value": "quuux" }

... results in a unique constraint violation on bUniqueIndex where the unique value being violated is b: null

These documents should not be present in bUniqueIndex. I looked through the code and could find no mention of partial filter expressions on unique indexes, leading me to believe that they are unsupported at this time.

[msl12]

It seems that it does not support partial index.

[bwaldvogel]

It’s not yet implemented. I’m happy to receive a PR.