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lalitha kellalalitha kella 

This is the trigger i wrote but it is not working please help me(It is throwing read-only exception)

trigger Branch on Student_details__c(After update) {
if (Trigger.isAfter) {
for (Student_details__c  a: Trigger.new) {

if(a.Branch__c=='C.S.E')
a.Branch__c= 'Civil';
update a;
}}

}
suresh sanneboina 4suresh sanneboina 4
Hi,

You are trying to update the record on after update, on After operations the records will be on Read-only you cannot edit those values. you can only use for furthur processing. use the below code 
trigger Branch on Student_details__c(Before update) {
if (Trigger.update) {

	for (Student_details__c  a: Trigger.new) {
		if(a.Branch__c=='C.S.E')
		{
			a.Branch__c= 'Civil';
		}
		//update a;
	}
}

}


 
Vasani ParthVasani Parth
You cannot update fields on triggered records when you are in an after trigger
http://raydehler.com/cloud/clod/salesforce-trigger-when-to-use-before-versus-after.html

Imagine you have updated the opportunity ,again your record will invoke the trigger since an update happened and there will be infinite recurssion loop that will happen for your case.
NagendraNagendra (Salesforce Developers) 
Hi Lalitha,

When you are trying to update the field on the same object, then you can use before event. If you use after event, the records which get saved into the database will be in read-only mode. So the reason you are getting the above error.

Example: If you want to update some XYZ field on branch object we can use before event.

When you are trying to cross update the fields we can make use of after event.

Example: if you update some XYZ fields in A object then some XYZ fields in B objects have to be updated, in this scenario, we can use after event.
  
trigger Branch on Student_details__c(Before update) {
if (Trigger.update) {

	for (Student_details__c  a: Trigger.new) {
		if(a.Branch__c=='C.S.E')
		{
			a.Branch__c= 'Civil';
		}
		//update a;
	}
}

}

Please mark this solution as the best answer if it helps you.

Best Regards,
Nagendra.P
Amit Chaudhary 8Amit Chaudhary 8
Hi lalitha kella,

I found below issue in your trigger
1) You are trying to update the record in After Trigger Event
2) You are doing DML inside the trigger

Please update your code like below 
trigger Branch on Student_details__c(Before update) // use before event
{
	// I just removed the if condition as no need of that because we are using single event
	for (Student_details__c  a: Trigger.new) 
	{
		if(a.Branch__c=='C.S.E')
		{
			a.Branch__c= 'Civil';
		}
		//update a; // In before event no need of DML
	}

}
Please check below post for trigger best practices
1) http://amitsalesforce.blogspot.com/2015/06/trigger-best-practices-sample-trigger.html

1) One Trigger Per Object
A single Apex Trigger is all you need for one particular object. If you develop multiple Triggers for a single object, you have no way of controlling the order of execution if those Triggers can run in the same contexts

2) Logic-less Triggers
If you write methods in your Triggers, those can’t be exposed for test purposes. You also can’t expose logic to be re-used anywhere else in your org. 

3) Context-Specific Handler Methods
Create context-specific handler methods in Trigger handlers

4) Bulkify your Code
Bulkifying Apex code refers to the concept of making sure the code properly handles more than one record at a time.

5) Avoid SOQL Queries or DML statements inside FOR Loops
An individual Apex request gets a maximum of 100 SOQL queries before exceeding that governor limit. So if this trigger is invoked by a batch of more than 100 Account records, the governor limit will throw a runtime exception

6) Using Collections, Streamlining Queries, and Efficient For Loops
It is important to use Apex Collections to efficiently query data and store the data in memory. A combination of using collections and streamlining SOQL queries can substantially help writing efficient Apex code and avoid governor limits

7) Querying Large Data Sets
The total number of records that can be returned by SOQL queries in a request is 50,000. If returning a large set of queries causes you to exceed your heap limit, then a SOQL query for loop must be used instead. It can process multiple batches of records through the use of internal calls to query and queryMore

8) Use @future Appropriately
It is critical to write your Apex code to efficiently handle bulk or many records at a time. This is also true for asynchronous Apex methods (those annotated with the @future keyword). The differences between synchronous and asynchronous Apex can be found

9) Avoid Hardcoding IDs
When deploying Apex code between sandbox and production environments, or installing Force.com AppExchange packages, it is essential to avoid hardcoding IDs in the Apex code. By doing so, if the record IDs change between environments, the logic can dynamically identify the proper data to operate against and not fail



Let us know if this will help you

Thanks
Amit Chaudhary