When creating documents in Azure Cosmos databases for MongoDB API you might get the error message MongoError: document does not contain shard key.

The issue occurs for partitioned collections that have been created via the Azure CLI, because of the way the partitionKey path is being stored in the collection settings.

Reproduction steps

1. From the Azure Portal, create a Cosmos DB account with the Azure Cosmos DB for MongoDB API.

Account Name: xyzreprodbaccount

API: Azure Cosmos DB for MongoDB API

2. Create a database.

Database name: xyzreprodatabase

3. Create a collection with a shard key.

Collection id: collection1

Shard key: tid

4. Create a new document and save it.

a. Browse to Data Explorer > Database > Collection > Documents.

b. Click on New Document.

c. In the editor type in the following text:

{
     "id" : "1",
     "tid": "aaa"
}

d. Click Save.

e. Observe that the document has been created.

5. Create another collection using the Azure CLI.

You can create the collection also from the Azure Shell (https://shell.azure.com/). Use the following command:

$partitionKeyPath = '/tid'

az cosmosdb collection create --resource-group-name XYZ-repro --name xyzreprodbaccount --db-name xyzreprodatabase --collection-name collection2 --throughput 400 --partition-key-path $partitionKeyPath 

6. From the Azure Portal, browse the collection2 created at step 5 and insert a new document.

a. Browse to Data Explorer > Database > Collection > Documents.

b. Click on New Document.

c. In the editor type in the following text:

{
     "id" : "1",
     "tid": "aaa"
}

d. Click Save.

Expected result: the document to be saved

Actual result: error message “Command insert failed: document does not contain shard key”

When saving a document from an application, the error message is “MongoError: document does not contain shard key”.

Problem analysis

Looking at the collection properties using the AZ CLI command

az cosmosdb collection show --resource-group-name XYZ-repro --name xyzreprodbaccount --db-name xyzreprodatabase --collection-name collection1

we can see that for the first collection created from the Azure Portal, the path for the partitionKey is stored as:

"partitionKey": {
      "kind": "Hash",
      "paths": [
        "/'$v'/tid/'$v'"
      ]
    } 

While for the second collection created via Azure CLI, the path for the partitionKey is stored as:

"partitionKey": {
      "kind": "Hash",
      "paths": [
        "/tid"
      ]
    } 

Workaround

When creating collections via Azure CLI, specify the partition key as /’$v’/tid/’$v’

Here is the AZ CLI command:

$partitionKeyPath = '/tid'
$Bugfix_partitionKeyPath = '/''$v''' + $partitionKeyPath + '/''$v'''

az cosmosdb collection create --resource-group-name XYZ-repro --name xyzreprodbaccount --db-name xyzreprodatabase --collection-name collection2 --throughput 400 --partition-key-path $Bugfix_partitionKeyPath 

[Edit October, 2020]

The Aure CLI command az cosmosdb collection create which was triggering this issue is now deprecated. Please use the new command to create a MongoDB collection:

az cosmosdb mongodb collection create

ex:

$partitionKeyPath = 'tid'

az cosmosdb mongodb collection create --resource-group XYZ-repro --account-name xyzreprodbaccount --database-name xyzreprodatabase --name collection2 --throughput 400 --shard $partitionKeyPath

The command az cosmosdb mongodb collection create does not have the issue discussed in this article.