Update order header through AIF

The following code sample shows how to update an existing order through AIF from outside Dynamics AX 2012 . Scenario is the we need to update some of the fields at header of Order.

 public static int UpdatePurchaseOrder(string _purchId, CallContext CallContextObj = null)  
     {  
       try  
       {  
         KeyField keyField = new KeyField() { Field = "PurchId", Value = _purchId };  
         EntityKey entityKey = new EntityKey();  
         entityKey.KeyData = new KeyField[1] { keyField };  
         EntityKey[] entityKeys = new EntityKey[1] { entityKey };  
         if (CallContextObj == null)  
         {  
           CallContextObj = CallContextSettings.getCallContextObject();  
         }  
         using (ANPurchOrderServiceClient client = new ANPurchOrderServiceClient())  
         {  
           client.ClientCredentials.Windows.ClientCredential.Domain = CredentialsSettings.Domain;  
           client.ClientCredentials.Windows.ClientCredential.UserName = CredentialsSettings.UserName;  
           client.ClientCredentials.Windows.ClientCredential.Password = CredentialsSettings.Password;  
           AxdANPurchOrder PurchaseOrderDoc = client.read(null, entityKeys);  
           AxdEntity_PurchTable purchTable = PurchaseOrderDoc.PurchTable.First();  
           purchTable.ANPublishedState = AxdEnum_ANPublishedState.Published;  
           purchTable.ANPublishedStateSpecified = true;  
           purchTable.action = AxdEnum_AxdEntityAction.update;  
           purchTable.actionSpecified = true;  
           purchTable.PurchLine = null;  
           // Invoke the update operation  
           AxdANPurchOrder newOrder = new AxdANPurchOrder()  
           {  
             PurchTable = new[] { purchTable }  
           };  
           client.update(CallContextObj, entityKeys, newOrder);  
           return 1;  
         }  
       }  
       catch (Exception ex)  
       {  
         Console.WriteLine(ex.ToString());  
       }   
       return 0;  
     }  

With above code we did as per need, read the purchase order document through read operation, set some fields and last set actions also. when I try to test the application I got some funny strange error

“Invalid entity action.”

This error confuse me also, as I have set the action also. After some research I came to know that read operation of service is returning both header and lines also, which I don’t need that for my scenario. So the fix is that I need to set the line object to null also and then send for update operation.

purchTable.PurchLine = null;

Question after this fix came In my mind is that can we only read header or specific line through read operation ? I will research about this more. If anyone knows this please share your thoughts with me. THANKS