Use Case whenever we will select a particular travel request(through lookup) from expense then data
from that travel request (Name + account, related to that travel request) concatenately should
populate Claim purpose field in expense.

Datapopulate:-

public with sharing class DataPopulate {
 public Travel_Request__c trq {
  get;
  set;
 }

 private ApexPages.StandardController stdCtrl;

 public DataPopulate(ApexPages.StandardController std) {
  stdCtrl = std;
 }

 public void doInsert() {
  Expense__c exp = (Expense__c) stdCtrl.getRecord();
  if (exp.Travel_Request__c == null) {
   return;
  } else {
   trq = [select Name , Proposed_Client_to_Visit__r.Name, Status__c from Travel_Request__c where Id =
: exp.Travel_Request__c LIMIT 1];
    System.debug('The record fetched '+trq);
 
   //exp.Branch__c = trq.Branch__c;
   //exp.Period_To__c = trq.Travel_Start_Date__c;
  // exp.Period_From__c = trq.Travel_End_Date__c;
   //exp.Department__c=trq.Department__c;

   exp.Claim_Purpose__c= trq.Name +''+ ' -- '+''+ trq.Proposed_Client_to_Visit__r.Name ;
   upsert exp;
  System.debug('The record insert '+exp);
  }
 }
}

test class:

//Test class for DataPopulate


@isTest
public class DataPopulateTest
{
 //setup test data for Class
 @testSetup  static void testSetDate()
 {
    Travel_Request__c trq = new Travel_Request__c();
    trq.Name='TestTravel' ;
    trq.Proposed_Client_to_Visit__c='T-System'; 
    insert trq;
     
    Expense__c expObj = new Expense__c();
    expObj.Travel_Request__c='TestTravel';
   // expObj.Period_To__c= system.today();
    //expObj.Period_From__c= system.today();
    insert expObj;
 }
 @isTest static void testPopulate()
 {
    Travel_Request__c trq = [SELECT id from Travel_Request__c where Name = 'TestTravel' ][0];
    
    Test.setCurrentPage(page.Expense);  
    
    Apexpages.currentPage().getParameters().put('Id',trq.Id);
    
   
     
     ApexPages.StandardController stdCon = new ApexPages.StandardController(trq);
     DataPopulate dClassObj = new  DataPopulate(stdCon);
    
     dClassObj.doInsert();
    // List<Expense__c> expObj=[SELECT Claim_Purpose__c from Expense__c WHERE
Travel_Request__c=:trq.id Limit 1];
    // Boolean result = expObj.equals('TestTravelT-System');
    // System.assertEquals(result,false);
 }
 
}