Invalid foreign key relationship error when referring to Task object related lookup record

Question

I am totally new at this and this actually one of my first triggers so I'm really proud that it is working (when used on a Case__c object) but it is not working when triggered on any other object. I'm probably making a rookie mistake here. We are a law firm so I wrote a trigger to make a billable record when specific strings of text are contained in a chatter post. This trigger works well for the Case__c object but I'm trying to include an "if" statement that will also allow it to work on a Task object.

trigger ChatterBillable on FeedItem (after insert, after update) {

    for (FeedItem f: trigger.new){
        if (f.body.contains('$b') || f.body.contains('$B')){
            Billable__c b               = new Billable__c();
            b.Billable_Date__c          = Date.today();
            String billtime             = f.body.substringAfter('=').substringBefore('hr');
            Decimal strToDec            = decimal.valueOf(billtime);
            system.debug('===strToDec=='+strToDec);
            b.Time__c                   = strToDec;
            b.Person_Billing__c         = f.CreatedById;
            b.Internal_Description__c   = f.Body;
            String xd                   = f.body.substringAfter('XD').substringBefore('::').replaceAll('<[^>]+>',' ');
            String idType               = f.ParentId.getSobjectType().getDescribe().getName();
            b.Service_Note__c           = xd + idType;
            b.RecordTypeId              = '012d0000001taJa';
            //if the chatter post is on a Case__c record this code is working by creating a Billable_c record as intended
            if (idType == 'Case__c') {
                b.Case__c       = f.ParentId;
            // However, if the chatter post triggering this code is on a Task record I want to relate the Billable__c record 
            // to the same Case__c record as the Task is related to.
            } else if (idType   == 'Task'){
                b.Case__c       = f.ParentId.What;
            }
            insert b;
        }
    }
}

I'm getting an error on the line 23 "Invalid foreign key relationship: FeedItem.ParentId"

b.Case__c = f.ParentId.What;

I wasn't sure if I was accessing the relationship record correctly so I also tried

b.Case__c = f.Parent.What;

And with this change I get an error "Invalid field What for SObject Name." This is confusing me, I went to look at the API names for Task and "What" appears to be the API name for a lookup relationship field which includes Case__c. Anyone know what I'm doing wrong? Help would be very much appreciated.

Answer 1

The field is WhatId. However, you have to query for the parent records, as you don't get cross-object data in a trigger context.

String taskPrefix = SObjectType.Task.getKeyPrefix();
Set<Id> taskIds = new Set<Id>();
for (FeedItem record : trigger.new)
    if (record.ParentId.getSObjectType() == Task.sObjectType)
        taskIds.add(record.ParentId);

Map<Id, Task> tasks = new Map<Id, Task>([SELECT WhatId FROM Task WHERE Id IN :taskIds]);
for (FeedItem record : trigger.new)
{
    Task parentTask = tasks.get(record.ParentId);
    if (parentTask != null)
    {
        Id parentWhatId = parentTask.WhatId;
    }
}

That said, you should also remove your insert statement from your loop:

List<Billable__c> billableRecords = new List<Billable__c>();
for (Task record : trigger.new)
{
    billableRecords.add(new Billable__c(
        // populate fields
    ));
}
insert billableRecords;

Answer 2

It should be b.Case__c = f.Parent.WhatId;

as you need to assign the WhatId to CaseId

Refer Understanding Polymorphic Keys and Relationships