SObject row was retrieved via SOQL without querying the requested field error is not coming

Question

This trigger is first inserting related contact when account is inserted and then updating a custom field on Account name 'Client_Contact__c' with inserted contact lastname.

trigger practicetrigger on Account ( after insert ) {
        if ( Trigger.isinsert && Trigger.isafter ) {
           Set<Id> accId = new Set<Id>();
           list<Contact> conList = new list<Contact>();
           list<Account> accToUpdate = new list<Account>();
           map<Id, Account> accMap = new map<Id, Account>();
        for ( Account a : Trigger.New ) {
           Contact c = new Contact( LastName = a.Name, AccountId=a.Id );
           conList.add ( c );
           accId.add( a.Id );
        }
        if ( !conList.isempty() ) {
           insert conList;
        }
        for ( Account ac : **[select Id from Account where Id =: accId]** ) {
           ac.Client_Contact__c = ' new contact ';
           system.debug( 'Account ac '+ac.Client_Contact__c);
           accMap.put( ac.Id , ac );
        }
        if ( conList.size() > 0 ) {
           for ( Contact con : conList ) {
               if ( accMap.containskey( con.AccountId ) ) {
                    accMap.get( con.AccountId ).Client_Contact__c = con.lastname;
                    System.debug( ' accMap '+accMap.get(con.AccountId));
                    accToUpdate.add( accMap.get(con.AccountId) );
               }
           }
        }
        if ( !accToUpdate.isempty() ) {
           update accToUpdate;
        }
       }
    }

In the bold part of code, I have not queried the custom field Client_Contact__c but still error is not coming and code is executing successfully. Why error is not coming? Can anybody tell what i am missing here and In what situation this error do not come?

Answer 1

You can always set a field to new value even if you didn't query it. Not sure what you think the problem is?

In a normal database and SQL you can do UPDATE TABLE Users SET Name = 'Joe' WHERE Id = 123, why something similar shouldn't be possible in Apex? With different syntax, yes - but still, same idea?