How to access fields returned by Polymorphic query in apex/soql?

Question

This might seem to be a fairly simple question, but I am unable to figure out how to use the fields returned by a Polymorphic SOQL query.

For example: I ran the following query in dev console's query editor:

SELECT TYPEOF WHAT
WHEN OPPORTUNITY THEN Name,StageName
WHEN ACCOUNT THEN Name
ELSE Email
END
FROM EVENT

It shows that I have one record which matches this criteria, but it doesn't show the record details. Here is the output:

When I run this segment of code in Anonymous window, the list only prints WhatId and the Id of the event itself.

list<Event> newlist=[SELECT TYPEOF WHAT
WHEN OPPORTUNITY THEN StageName
ELSE Email
END
FROM EVENT];

system.debug(newlist);

if(newlist.get(0).What instanceof Opportunity){
    //what should I do here to access fields of opportunity?
}

I understand that I can fetch the WhatId from the list and use that to query the fields on Opportunity object. But if I need to run the DML again to get the required fields, what is the use of the polymorphic DML statement?

Any help is appreciated. Many thanks.

Answer 1

Here is an example of how to retrieve field values returned from Polymorphic query.

list<Event> newlist=[SELECT TYPEOF WHAT
                    WHEN OPPORTUNITY THEN StageName
                    ELSE Email
                    END
                    FROM EVENT];

To access the StageName value of the Opportunity, first create an instance of Opportunity object based on evt.What as below and then you will get the StageName.

for(Event evt:newlist){
    if(evt.What instanceof Opportunity){
        //what should I do here to access fields of opportunity?
        System.debug('evt.WhatId=' + evt.WhatId);
        Opportunity opty = evt.What;
        System.debug('opty=' + opty);
        System.debug('opty.StageName=' + opty.StageName);
    }
}

But if you try to access StageName like this way evt.What.StageName without typecasting to Opportunity then compiler will complain as StageName is not an attribute of that Polymorphic object.