Test with Multiple Records

@isTest
private class LeaveRequestOverlapTest {
    @isTest
    static void overlappingRequestAgainstAnExistingApprovedRequestIsBlocked() {
        insert new Leave_Request__c(
            Employee_Name__c = 'Amara', Start_Date__c = Date.newInstance(2026, 4, 1),
            End_Date__c = Date.newInstance(2026, 4, 5), Status__c = 'Approved'
        );

        Test.startTest();
        try {
            insert new Leave_Request__c(
                Employee_Name__c = 'Amara', Start_Date__c = Date.newInstance(2026, 4, 3),
                End_Date__c = Date.newInstance(2026, 4, 10), Status__c = 'Pending'
            );
            System.assert(false, 'Expected this overlapping request to be blocked');
        } catch (DmlException e) {
            System.assert(e.getMessage().contains('overlap'), 'Expected the error to mention the overlap');
        }
        Test.stopTest();
    }
}

1. @isTest — marks the test class.
2. private class LeaveRequestOverlapTest { — the class.
3. @isTest — marks the test method.
4. static void overlappingRequestAgainstAnExistingApprovedRequestIsBlocked() { — a clearly-named test.
5. insert new Leave_Request__c( — an already-approved existing request.
6. Employee_Name__c = 'Amara', Start_Date__c = Date.newInstance(2026, 4, 1), — the employee and start date.
7. End_Date__c = Date.newInstance(2026, 4, 5), Status__c = 'Approved' — the end date and status.
8. ); — closes the constructor call.
9. (blank line)
10. Test.startTest(); — marks the start of the action being tested.
11. try { — this is exactly Module 26's "Test the Feature" pattern — plus a try/catch around the expected DmlException.
12. insert new Leave_Request__c( — a new, overlapping request.
13. Employee_Name__c = 'Amara', Start_Date__c = Date.newInstance(2026, 4, 3), — the same employee, overlapping dates.
14. End_Date__c = Date.newInstance(2026, 4, 10), Status__c = 'Pending' — the rest of the fields.
15. ); — closes the constructor call.
16. System.assert(false, 'Expected this overlapping request to be blocked'); — if this line is ever reached, the insert should have thrown but didn't — fail the test.
17. } catch (DmlException e) { — catches the expected failure.
18. System.assert(e.getMessage().contains('overlap'), 'Expected the error to mention the overlap'); — verifies it's the right error, not some unrelated failure.
19. } — closes the catch.
20. Test.stopTest(); — marks the end.
21. } — closes the test method.
22. } — closes the class.

Wrapped in @isTest, this verifies the new-vs-existing overlap check automatically, every time, instead of by hand.

@isTest
private class LeaveRequestOverlapTest {
    // ... overlappingRequestAgainstAnExistingApprovedRequestIsBlocked() from above ...

    @isTest
    static void overlappingRequestsInTheSameBatchAreBlocked() {
        List<Leave_Request__c> batch = new List<Leave_Request__c>{
            new Leave_Request__c(Employee_Name__c = 'Ben', Start_Date__c = Date.newInstance(2026, 5, 1), End_Date__c = Date.newInstance(2026, 5, 5), Status__c = 'Pending'),
            new Leave_Request__c(Employee_Name__c = 'Ben', Start_Date__c = Date.newInstance(2026, 5, 3), End_Date__c = Date.newInstance(2026, 5, 8), Status__c = 'Pending')
        };

        Test.startTest();
        try {
            insert batch;
            System.assert(false, 'Expected this overlapping batch to be blocked');
        } catch (DmlException e) {
            System.assert(e.getMessage().contains('overlap'), 'Expected the error to mention the overlap');
        }
        Test.stopTest();
    }
}

1. @isTest — marks the class.
2. private class LeaveRequestOverlapTest { — the same test class as before.
3. // ... overlappingRequestAgainstAnExistingApprovedRequestIsBlocked() from above ... — the first test method still lives here too.
4. (blank line)
5. @isTest — marks a second test method.
6. static void overlappingRequestsInTheSameBatchAreBlocked() { — a clearly-named test for the batch scenario.
7. List<Leave_Request__c> batch = new List<Leave_Request__c>{ — two requests submitted together.
8. new Leave_Request__c(Employee_Name__c = 'Ben', Start_Date__c = Date.newInstance(2026, 5, 1), End_Date__c = Date.newInstance(2026, 5, 5), Status__c = 'Pending'), — the first.
9. new Leave_Request__c(Employee_Name__c = 'Ben', Start_Date__c = Date.newInstance(2026, 5, 3), End_Date__c = Date.newInstance(2026, 5, 8), Status__c = 'Pending') — the second, overlapping the first.
10. }; — closes the list literal.
11. (blank line)
12. Test.startTest(); — marks the start.
13. try { — testing only single-record inserts (one at a time) would never exercise Lesson 4's new-vs-new comparison at all — that logic only ever runs when multiple records are inserted together in the same transaction.
14. insert batch; — inserts both at once.
15. System.assert(false, 'Expected this overlapping batch to be blocked'); — should never be reached.
16. } catch (DmlException e) { — catches the expected failure.
17. System.assert(e.getMessage().contains('overlap'), 'Expected the error to mention the overlap'); — verifies the right error.
18. } — closes the catch.
19. Test.stopTest(); — marks the end.
20. } — closes the test method.
21. } — closes the class.

This is exactly why "test with multiple records" earns its own dedicated test method rather than being folded into the single-record test.