How can I update a related record when a specific field is modified in a parent record using a trigger?

To update a related record when a specific field is modified in a parent record using a trigger, you can follow these steps:

1. Identify the relationship between the parent and child objects. This could be a lookup or master-detail relationship.
2. Determine the specific field (let’s call it SummerVacation__c) You want to monitor for changes in the parent object.
3. Write a trigger on the parent object (let’s call it School__c) that executes after an update.
4. Inside the trigger, iterate over the trigger’s Trigger.new list to access the updated parent records.
5. Use the Trigger.oldMap to retrieve the previous values of the parent records.
6. Compare the specific field values of the old and new parent records to identify changes. You can use a if statement to check if the specific field has been modified.
7. If the specific field has been modified, retrieve the related child object (let’s call it Student__c)records based on the relationship between the parent and child objects. You can use SOQL queries or related fields to fetch the child records.
8. Modify the desired field(s) on the child records with the new values. You can assign new values directly or use a loop to update multiple child records.
9. Finally, perform an update operation on the child records to persist the changes.

Here’s an example trigger code that demonstrates these steps:

trigger UpdateRelatedRecord on School__c (after update) {
    Set<Id> parentIdsToUpdate = new Set<Id>();
    List<Student__c> childRecordsToUpdate = new List<Student__c>();
    
    for (School__c parentRecord : Trigger.new) {
        School__c oldParentRecord = Trigger.oldMap.get(parentRecord.Id);
        
        // Check if the specific field has been modified
        if (oldParentRecord.SummerVacation__c != parentRecord.SummerVacation__c) {
            parentIdsToUpdate.add(parentRecord.Id);
        }
    }
    
    if (!parentIdsToUpdate.isEmpty()) {
        // Retrieve related child records
        List<Student__c> childRecords = [SELECT Id, SummerVacation__c FROM Student__c WHERE School__c IN :parentIdsToUpdate];
        
        // Update desired field(s) on child records
        for (Student__c childRecord : childRecords) {
            childRecord.SummerVacation__c = true; // Update the SummerVacation__c field on the related record 
            childRecordsToUpdate.add(childRecord);
        }
        
        // Perform an update on child records
        if (!childRecordsToUpdate.isEmpty()) {
            update childRecordsToUpdate;
        }
    }
}

Write a trigger to count the number of related child records on the parent.

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