How to test an Update class in apex - unit-testing

Question

Hello I am new to apex and soql, I would like some help on my test class, below is the code i am having trouble with,
public class UpdateMyCard {
#AuraEnabled(cacheable=false)
public static Card__c updateCard(Double Amount){
String userId = UserInfo.getUserId();
Card__c myCard = [SELECT Id, Card_no__c, total_spend__c From Card__c Where OwnerId =:userId];
myCard.total_spend__c = myCard.total_spend__c + Amount;
try{
update myCard;
}catch (Exception e) {
System.debug('unable to update the record due to'+e.getMessage());
}
return myCard;
}
}
Test Class
#isTest
public static void updatecard(){
UpdateMyCard.updateCard(200);
}
Error:
System.QueryException: List has no rows for assignment to SObject

Answer 1

Your code assumes there's already a Card record for this user. And it's exactly 1 record (if you have 2 your query will fail). I'm not sure what's your business requirement. Is it guaranteed there will always be such record? Should the code try to create one on the fly if none found? You might have to make that "updateCard" bit more error-resistant. Is the total spend field marked as required? Because "null + 5 = exception"
But anyway - to fix your problem you need something like that in the test.
#isTest
public static void updatecard(){
Card__c = new Card__c(total_spend__c = 100);
insert c;
UpdateMyCard.updateCard(200);
c = [SELECT total_spend__c FROM Card__c WHERE Id = :c.Id];
System.assertEquals(300, c.total_spend__c);
}