Update lookup field by using trigger

Hi Guys,
Today iam going to explain update lookup field, I have lot of developer faced this issue.

Business case : Customer whats to update company(lookup) on expense__c based on another company__c(lookup) of object resource__c


Trigger


trigger updatelookup on Expense__c (before Insert){ 
    set regionsID = new set(); for(Expense__c expRep: Trigger.New){ 
            regionsID.add(expRep.region__c);
         } 
        map mapRegionToCompanyName = new map(); 
            for(region__c Region : [select id,resource__r.company__c from region__c where id in:regionsID]){ 
             mapRegionToCompanyName.put(region.id,region.resource__r.company__c);
      } 
          for(Expense__c expRep: Trigger.New){ 
           expRep.company__c = mapRegionToCompanyName.get(expRep.region__c); 
     } 
     }

Test Class


@isTest
public class updatecootest {
    static testmethod void myunittest(){ 
    
    Region__c r = new Region__c(Name ='China');
    insert r; 
    
    Company__c com = new Company__c(Name ='ForceLearn');
    insert com; 
    
    Expense__c er = new Expense__c(Name='Travel',Region__c= r.id,Company__c= com.id);
    insert er; 
    
    Expense__c erc = [SELECT Id, Company__c, name FROM Expense__c];
    test.starttest();
    update erc;
    test.stopTest();
    }
   }

Hola !! , we have update lookup field



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