How to run document workflow command with PW Web Services Gateway ?

Hello,

I am trying to use the WSG to execute document workflow commands (like Previous state, Next state or something customizable in the workflow definition)

I managed to retrieve possible commands for a specific document using the DocumentCommand relation.

Now the question is, how to run a command ?

I noticed another relation, DocumentCommandRun, which links a Document instance to a CommandRun instance.

The CommandRun class has the following properties:

 CommandId
 Comment
 Messages
 ObjectId
 Status
 StatusMessage

I thought ObjectId would be the Guid of a Document and CommandId the id of the associated Command.

But when I tried to create a CommandRun, I keep getting the error, no metter what I put in the properties :

"Parent instance id must be specified for specified creation."

I am clueness. Could anyone tell me how to run a document command ?

Thank you