[SOLVED] Creating unique index for CosmosDB with MongoDB API fails

Creating unique index for CosmosDB with MongoDB API fails

I'm using Azure CosmosDB with the MongoDB API. I'm trying to execute the following:

db.createCollection('test')
db.test.createIndex({key: 1}, {name: 'key_1', unique: true})

However, doing so fails with the following error:

The unique index cannot be modified. To change the unique index, remove the collection and re-create a new one.

When reading about it in the documentation and on Stack Overflow, it's mentioned that you can only create a unique index for empty collections.

However, when I try the following command, it seems my collection is empty, so this apparantly isn't the reason why it isn't working:

db.test.find()

I tried to recreate the collection several times, but to no avail.

Solution

In my case, the problem was caused by a limitation of the continuous backup of CosmosDb.

As stated in the documentation:

Azure Cosmos DB API for SQL or MongoDB accounts that create unique index after the container is created aren't supported for continuous backup. Only containers that create unique index as a part of the initial container creation are supported. For MongoDB accounts, you create unique index using extension commands.

I had to create my unique index using the custom commands.

db.runCommand({ 
  customAction: "CreateCollection", 
  collection: "CollectionName", 
  indexes: [
    {
      key: { "FieldName": 1 }, 
      name: "FieldName_1", 
      unique: true
    }
  ]
})