MongoDB Partial Unique Index on Object Field in Array Field - mongodb

Question

Lets say I have a collection whose objects have a structure like this:
{
"a": [
{"name": "foo", "meh": "whatever"},
{"name": "bar", "meh": "hem"}
],
"other_stuff": { }
}
It's possible for an object in this collection to not have the "a" field. I'd like to enforce a constraint on the database such that, if an object has the "a" field, that none of the "name" fields in objects contained in that "a" field are duplicates across the entire collection.
So for example, the following object would be flagged as a constraint violation if the above object were already in the collection:
{
"a": [
{"name": "bar", "meh": "meh meh"}
],
"other_stuff": { }
}
Furthermore, the following object would be flagged as a constraint violation regardless of other documents already in the collection:
{
"a": [
{"name": "boo", "meh": "blub"},
{"name": "boo", "meh": "glub"}
],
"other_stuff": { }
}
Is it possible to specify a partial unique index for MongoDB? If it's different between Mongo 3.2, 3.4, 3.6 and 4.0 it would be nice to know that too - I don't care about earlier than 3.2.
I was thinking it might be something like this to prevent duplication across documents (because in Javscript [] > '' === false and undefined > '' === false and ['anything'] > '' === true for the passive reader):
db.MyCollection.createIndex(
{ "a.name": 1 },
{
"unique": true,
"partialFilterExpression": {
"a": { "$gt": '' }
}
}
)
... and I think the only way to prevent duplicate values within a document is to enforce in application logic the use of the $addToSet operator when operating on the "a" field.
I'd be happy to be corrected or corroborated on either count.