Fire trigger only if we update custom field

Question

I have an object in Salesforce called File_uploaded__c with some fields like Filename__c, fileSize__c, IsMain, and I want to know if there's a way to implement a trigger that is only fired when we update the field IsMain and not every time we update the whole object

Answer 1

You can do this by comparing the current value of the IsMain field with the value in the trigger.oldMap collection. This will tell you if the field has changed.

Something like this will work

trigger FileUploadedTrigger on File_uploaded__c (after update) {

     list<File_uploaded__c> changedIsMainList = new list<File_uploaded__c>();
     for(File_uploaded__c fu : trigger.new){
          if(fu.IsMain__c != trigger.oldMap.get(fu.Id).IsMain__c){
               changedIsMainList.add(fu);
          }
     }
     //You now have a list of only those File_uploaded__c records where IsMain
     //has changed.  You can now perform your logic on only those records

     //YOUR LOGIC HERE
}

This should get you started. I would also recommend reading through Apex Code Best Practices as well.