Collection schema validation uniqueItems doesn't work?

I have these valdation rules for my collection:

  "$jsonSchema": {
    "bsonType": "object",
    "required": ["CCF_CNAME", "CCF_NPARS"],
    "properties": {
      "CCF_CNAME": {
        "bsonType": "string",
        "uniqueItems": true,
        "description": "CCF_CNAME must be a unique string."
      "CCF_NPARS": {
        "bsonType": "int",
        "description": "CCF_NPARS must be an integer."

but for some reason when I try to insert a document with a CCF_CNAME string that already exists, I don’t get an error…

For example I insert these two rows:

the string is the same, but no error. I ran the command
db.getCollectionInfos( { name: "ccf" } )[0].options.validator

and I got my validation correctly, so that’s not the problem. Also trying to use a CCF_NPARS value that is a float for example does give an error.

What am I missing?

uniqueItems is a property of an array field; it checks that elements of the array are unique, that it’s a set. Schema validation only checks a single document that you are inserting/updating, it doesn’t compare different documents in a collection. To ensure uniqueness of the field value across documents, you can use unique indexes

Thank you for your reply!

Is there a way to somehow define unique columns in the json schema?