Introduction
In Salesforce, activities (Tasks and Events) use two special polymorphic lookup fields — WhoId and WhatId — to relate records to people and business objects. Understanding these fields is essential for reporting, querying, and writing Apex that works with Tasks and Events.
What is WhoId?
WhoId is a polymorphic lookup on Task and Event that points to a record representing a person. The WhoId can reference either the Contact or the Lead object. Use WhoId when the activity is related to an individual person.
What is WhatId?
WhatId is a polymorphic lookup on Task and Event that points to a record representing a business object. The WhatId can reference objects like Account, Opportunity, Case, Campaign, custom objects, and more. Use WhatId when the activity is related to an account, opportunity, or other non-person entity.
Key Characteristics
– Both fields are polymorphic lookup fields (they accept multiple sObject types).
– They are available on both Task and Event.
– An activity can have one or both fields populated — e.g., a meeting with a contact about an opportunity (WhoId = Contact, WhatId = Opportunity).
SOQL and Polymorphic Fields
When querying polymorphic fields in SOQL, you can use TYPEOF (API version 30.0+) to handle different types, or use getSObject() in Apex to dynamically access the referenced record.
// Example SOQL using TYPEOF
SELECT Id, Subject, WhoId, WhatId,
TYPEOF WhoId WHEN Contact THEN Name, Email WHEN Lead THEN Name, Company END
FROM Task
WHERE WhoId != null
Apex examples
Accessing the polymorphic reference in Apex:
// Using getSObject() and instanceof
Task t = [SELECT Id, WhoId, WhatId FROM Task LIMIT 1];
SObject who = t.getSObject('WhoId');
if (who instanceof Contact) {
Contact c = (Contact) who;
System.debug('Contact name: ' + c.Name);
} else if (who instanceof Lead) {
Lead l = (Lead) who;
System.debug('Lead company: ' + l.Company);
}
Common Use Cases
– Relate a Task to a Contact (WhoId) and an Opportunity (WhatId).
– Use reports to show activities by Contact or by Account depending on which field is used.
– Trigger logic that needs to behave differently when WhoId points to a Lead vs Contact.
Limitations & Best Practices
– Polymorphic fields cannot be used in certain cross-object formulas.
– In validation rules and workflow rules, check the WhoId or WhatId prefix (e.g., LEFT(WhoId,3)) is not reliable. Prefer checking related record types in Apex or use custom fields when strict relationships are required.
– When building integrations, be explicit about the sObject type you expect for WhoId/WhatId to avoid runtime errors.
Short Summary
WhoId links an activity to a person (Contact or Lead). WhatId links an activity to a business object (Account, Opportunity, Case, custom objects, etc.). Both are polymorphic fields on Task and Event used to model relationships between activities and other records in Salesforce.