Do I need to have a property set as partitionKey to be on resource?

[adamsosinski]

Hello,

I'm integrating CosmosDb into my solution, similar to the code below.

private readonly PartitionKey = new("1234");
...
string idStr = id.Value.ToString(); // passed as method argument
var response = await _container.ReadItemAsync<Document>(idStr, _partitionKey, cancellationToken: cancellationToken).ConfigureAwait(false);
var dto = response.Resource.ToDto(); // ToDto() converts Document into matching dto
...
// logic modyfing dto
...
var document = Document.From(dto) with { PartitionKey = "1234" };
await _container.ReplaceItemAsync(document, idStr, _partitionKey, cancellationToken: cancellationToken).ConfigureAwait(false); 

And the Document looks like this

public record Document
{
	[JsonProperty("id")]
	public Guid DocumentId {get; init;}
	[JsonProperty("partitionKey")]
	public string PartitionKey {get;init;}
	...
	// other properties
}

Trying different approaches I came to the conclusion that PartitionKey property is required.
This makes me question what is the reason for providing partitionKey into ReplaceItemAsync()?
I have figured out that there is a validation done against it. But, I thought that it will use that provided value as partitionKey and set it on the document, so there is no need to have it on the class representing the document.