How can I add a lookup relationship using an external id on a before insert/update trigger in Salesforce?

Hello,

trigger surveyTrigger on Survey__c (before insert) {
if(Trigger.isBefore){
if(Trigger.isInsert){
Map<String, String> extMap = new Map<String, String>();
Set<String> extIdSet = new Set<String>();
for(Survey__c survey : Trigger.new){
extIdSet.add(survey.Account_Key__c);
}
for(Account a : [select Id, Foreign_Key__c from Account where Foreign_Key__c IN :extIdSet]){
extMap.put(a.Foreign_Key__c, a.Id);
}
for(Survey__c survey : Trigger.new){
survey.Account__c = extMap.get(survey.Account_Key__c);
}
}
}
}

Test class @isTest
private class TestSurveyTrigger {
static testMethod void myUnitTest() {
Account testAccount = new Account(Name='My test account', Foreign_Key__c='test123');
insert testAccount;
Test.startTest();
Survey__c testSurvey = new Survey__c(Account_Key__c='test123');
insert testSurvey;
Test.stopTest();
Survey__c result = [select Id, Account__c, Account__r.Name, Account__r.Foreign_Key__c from Survey__c where Account_Key__c = 'test123' limit 1];
system.assertEquals(testAccount.Id, result.Account__c);
}
}

Hope this helps!!