When use Update message for dataverse plugin, I have the error "Exception Message: The given key was not present in the dictionary."

I have a dataverse plugin C# code like below, in the trace "stage 1" between "stage 2" (line 4),

OptionSetValue stagingType = (OptionSetValue)entity["stagingtype"];

there is a error:

> Exception Message: The given key was not present in the dictionary.

The code is executed to check the duplicate in a table, it works in Create message, but I create a new step, but it did not work in Update message, the error is above, I have copied the code here

          if (entity.LogicalName == "atmtransaction")
            {
                tracingService.Trace("Stage {0}", "1");
                OptionSetValue stagingType = (OptionSetValue)entity["stagingtype"];
                tracingService.Trace("Stage {0}", "2);

                string store = (string)entity["store"];
                tracingService.Trace("Stage {0}", "3");

                DateTime date = (DateTime)entity["date"];
                tracingService.Trace("Stage {0}", "4");

                if (stagingType.Value != 1)
                {

                    tracingService.Trace("Stage {0}", "5");
                    // Check if a record already exists with the same store and date
                    QueryExpression query = new QueryExpression("ada_atmtransaction");
                    query.Criteria.AddCondition("ada_store", ConditionOperator.Equal, store);
                    query.Criteria.AddCondition("ada_date", ConditionOperator.Equal, date);

                    EntityCollection results = service.RetrieveMultiple(query);
                    if (results.Entities.Count > 0)
                    {
                        tracingService.Trace("Stage {0}", "6");
                        throw new InvalidPluginExecutionException("A record already exists with the same store and date.");
                    }
                 }
            }

Anyone idea about this?

You get this error on below line

            OptionSetValue stagingType = (OptionSetValue)entity["stagingtype"];

Reason: On update most probably field stagingtype is not changed/updated and hence you will not find this field in entity object, which I suspect is the Target.
You will have to explicitly fetch the record with stagingtype field and then check.

In addition I would sugest look into preImage and postImage this will be helpful for you as well.

https://learn.microsoft.com/en-us/power-apps/developer/data-platform/understand-the-data-context#entity-images