SFDC AccountManager

クラスとそのテストクラスを下記のように実装し
確認するたびにエラーが発生
List has no rows for assignment to SObject

最後解決した方法が
@RestResource(urlMapping='/Account//contacts')
↓
@RestResource(urlMapping='/Accounts//contacts')

要は複数に変更し、確認すれば、sfdc trailheadが通してくれた。
ご参考まで

java

@RestResource(urlMapping='/Accounts/*/contacts')
global with sharing class AccountManager {

@HttpGet
global static Account getAccount() {
    RestRequest req = RestContext.request;
    RestResponse res = RestContext.response;
    String accountId = req.requestURI.substringBetween('Accounts/', '/contacts');

    Account acc = [SELECT Id, Name, (SELECT Id, Name FROM Contacts) FROM Account WHERE Id = :accountId];
    return acc;
}

}

java

@istest
public class AccountManagerTest {

@isTest static void testGetAccount() {
    Id recordId = createTestRecord();
    // Set up a test request
    RestRequest request = new RestRequest();

    request.requestUri =
        'https://ap5.salesforce.com/services/apexrest/Accounts/'+recordId+'/contacts';
    request.httpMethod = 'GET';
    RestContext.request = request;
    // Call the method to test
    Account thisAcc = AccountManager.getAccount();
    // Verify results
    System.assert(thisAcc != null);
    System.assertEquals('Test record', thisAcc.Name);
}

// Helper method
static Id createTestRecord() {
    // Create test record
    Account accTest = new Account(
        Name='Test record');
    insert accTest;

    Contact con = new Contact();
    con.LastName = 'test con';
    con.Account = accTest;
    insert con;
    return accTest.Id;
}

}