How to Query Polymorphic Fields in SOQL?

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Question:

When working with polymorphic fields like Owner or What in Salesforce, which can reference multiple object types, how can I construct a SOQL query that avoids errors when querying fields that may not exist on all possible related objects? For example, if I query the Owner field and include fields specific to User or Group, the query may fail if the field is unavailable on certain object types. How can I dynamically query such fields without running into these errors?

Answer

To dynamically query polymorphic fields in Salesforce without encountering errors, you can use the TYPEOF clause in SOQL. This allows you to specify different fields to retrieve based on the object type being referenced by the polymorphic field.

For example, when querying the What field in the Event object, you can use the TYPEOF clause to specify fields for different object types:

SELECT 
  TYPEOF What
    WHEN Account THEN Phone, NumberOfEmployees
    WHEN Opportunity THEN Amount, CloseDate
    ELSE Name, Email
  END
FROM Event

In this query:

If What refers to an Account, it retrieves Phone and NumberOfEmployees.
If What refers to an Opportunity, it retrieves Amount and CloseDate.
For all other object types, it retrieves Name and Email.

This ensures that the query executes successfully regardless of the referenced object type.

Using the `TYPEOF` Clause in Apex:

If you need to dynamically construct and execute the query in Apex, you can do it like this:

String soqlQuery = 'SELECT TYPEOF What ' +
                   'WHEN Account THEN Phone, NumberOfEmployees ' +
                   'WHEN Opportunity THEN Amount, CloseDate ' +
                   'ELSE Name, Email ' +
                   'END ' +
                   'FROM Event';

List<SObject> results = Database.query(soqlQuery);

This approach ensures that you dynamically handle the object type of the polymorphic field in a single query.

Alternative Approach Without `TYPEOF`:

If the TYPEOF clause is unavailable or unsuitable for your use case, you can query each object type separately and handle the results programmatically. For example:

List<Event> accountEvents = [SELECT Id, WhatId, What.Phone, What.NumberOfEmployees FROM Event WHERE What.Type = 'Account'];
List<Event> opportunityEvents = [SELECT Id, WhatId, What.Amount, What.CloseDate FROM Event WHERE What.Type = 'Opportunity'];

// Combine the results as needed
List<Event> allEvents = new List<Event>();
allEvents.addAll(accountEvents);
allEvents.addAll(opportunityEvents);

While this works, it is less efficient because it requires multiple queries and additional processing in Apex.

Best Practices:

Use the TYPEOF clause whenever possible for a more efficient and cleaner solution.
Ensure the fields being queried are accessible based on your organization’s field-level security and user permissions.
Avoid querying unnecessary fields to minimize the risk of query failures and optimize performance.

By leveraging TYPEOF, you can handle polymorphic fields effectively in SOQL, avoiding errors and maintaining query efficiency.

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