I recently had to create a Logic App, which is responsible for copying data from one Cosmos DB database to another.
To do this, you can use the standard building blocks available in a Logic App workflow.

While doing so, I did stumble across one issue. When you supply a Partition Key for a specific collection, and some entries have null as a value for this Partition Key, you’ll get into a bit of trouble.

For my regular flow, for documents WITH an actual Partition Key, the step I used looks pretty much like the following piece of code

"The_name_of_my_step": {
    "type": "ApiConnection",
    "inputs": {
        "host": {
            "connection": {
                "name": "@parameters('$connections')['documentdb']['connectionId']"
            }
        },
        "method": "post",
        "body": "@items('For_each_document')",
        "headers": {
            "x-ms-documentdb-is-upsert": true,
            "x-ms-documentdb-raw-partitionkey": "@{items('For_each_document')['MyPrimaryKeyValue']}"
        },
        "path": "/dbs/@{encodeURIComponent('myDatabase')}/colls/@{encodeURIComponent(items('For_each_collection'))}/docs"
    },
    "runAfter": {}
}

Over here, I create a POST request to the endpoint of my Cosmos DB collection. I’m specifying the Partition Key and telling the engine it should be an Upsert in the header. Relatively straightforward and something you’ll get out of the box.

However, when the value of @{items('For_each_document')['MyPrimaryKeyValue']} is a null value, you’ll receive an error message stating the following:

The partition key supplied in x-ms-partitionkey header has fewer components than defined in the collection

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