Trigger Context Variables Considerations

Be aware of the following considerations for trigger context variables:

1)       trigger.new and trigger.old cannot be used in Apex DML operations.

2)       You can use an object to change its own field values using trigger.new, but only in before triggers. In all after triggers, trigger.new is not saved, so a runtime exception is thrown.

3)       trigger.old is always read-only.

4)       You cannot delete trigger.new.

The following table lists considerations about certain actions in different trigger events:

Trigger Event	Can change fields using trigger.new	Can update original object using an update DML operation	Can delete original object using a delete DML operation
Before insert	Allowed.	Not applicable. The original object has not been created; nothing can reference it, so nothing can update it.	Not applicable. The original object has not been created; nothing can reference it, so nothing can update it.
After insert	Not allowed. A runtime error is thrown, as trigger.new is already saved.	Allowed.	Allowed, but unnecessary. The object is deleted immediately after being inserted.
Before update	Allowed.	Not allowed. A runtime error is thrown.	Not allowed. A runtime error is thrown.
After update	Not allowed. A runtime error is thrown, as trigger.new is already saved.	Allowed. Even though bad code could cause an infinite recursion doing this incorrectly, the error would be found by the governor limits.	Allowed. The updates are saved before the object is deleted, so if the object is undeleted, the updates become visible.
Before delete	Not allowed. A runtime error is thrown. trigger.new is not available in before delete triggers.	Allowed. The updates are saved before the object is deleted, so if the object is undeleted, the updates become visible.	Not allowed. A runtime error is thrown. The deletion is already in progress.
After delete	Not allowed. A runtime error is thrown. trigger.new is not available in after delete triggers.	Not applicable. The object has already been deleted.	Not applicable. The object has already been deleted.
After undelete	Not allowed. A runtime error is thrown. trigger.old is not available in after undelete triggers.	Allowed.	Allowed, but unnecessary. The object is deleted immediately after being inserted.

All triggers define implicit variables that allow developers to access runtime context. These variables are contained in the System.Trigger class:

Variable	Usage
isExecuting	Returns true if the current context for the Apex code is a trigger, not a Visualforce page, a Web service, or an executeanonymous() API call.
isInsert	Returns true if this trigger was fired due to an insert operation, from the Salesforce user interface, Apex, or the API.
isUpdate	Returns true if this trigger was fired due to an update operation, from the Salesforce user interface, Apex, or the API.
isDelete	Returns true if this trigger was fired due to a delete operation, from the Salesforce user interface, Apex, or the API.
isBefore	Returns true if this trigger was fired before any record was saved.
isAfter	Returns true if this trigger was fired after all records were saved.
isUndelete	Returns true if this trigger was fired after a record is recovered from the Recycle Bin (that is, after an undelete operation from the Salesforce user interface, Apex, or theAPI.)
new	Returns a list of the new versions of the sObject records. Note that this sObject list is only available in insert and update triggers, and the records can only be modified in before triggers.
newMap	A map of IDs to the new versions of the sObject records. Note that this map is only available in before update, after insert, and after update triggers.
old	Returns a list of the old versions of the sObject records. Note that this sObject list is only available in update and delete triggers.
oldMap	A map of IDs to the old versions of the sObject records. Note that this map is only available in update and delete triggers.
size	The total number of records in a trigger invocation, both old and new.