How to update a lookup field with the value from a formula or text field?

Question

I am not a developer and I do not yet write Apex code very well. I need an Apex Trigger to do the following:

1. If field "A" is null, update this field with the value from another field "B"
2. Field "A" is a lookup field tied to object "oA"
3. Field "B" is a formula field that gets its value from another object "ObjC"
4. Object "ObjA" and Object "ObjC" are not related
5. Fields "A" and "B" both reside in the Lead object
6. Objects "ObjA" and "ObjC" are related to the Lead object via lookup relationships

Following is my attempt to write a Lead Trigger:

trigger ForceFieldAName on Lead (after insert, after update ) {

  Set<String> sSources = new Set<String>();
  for(Lead l : Trigger.new) {        
    if(l.Field_B__c != null) sSources.add(l.Field_B__c);
  }

  Map<String, ObjC__c> mObjCs = new Map<String, ObjC__c>();  
  for(ObjC_c crInstance : [SELECT Id, Name FROM ObjC__c WHERE Name IN :sSources]) {
    mObjCs.put(crInstance.Name, crInstance);
  }

    for(Lead l : Trigger.new) {        
        if(l.Field_B__c != null) {
            if(mObjC_s.containsKey(l.Field_B__c)) {        
        l.Field_A__c = mObjCs.get(l.Field_B__c).Id;        
      }
        }
    }  
}

I get an error from the last row "l.Field_A__c = mObjCs.get(l.Field_B__c).Id;" that says the "Record is Read Only". Does this mean I cannot use the value from the formula field to update the value in a lookup field in the Apex Trigger?

Answer 1

The error is due to assigning a value to a field in the Trigger.new list, take a look here for trigger context.