Spring MongoRepository custom query - spring

Question

Hi I am new to using Spring with MongoRepository and I'm working on creating a custom query for MongoDB using Spring's MongoRepository.
What I would like to do is return a custom query for another variable in my model instead of the Object id.
for my model I have:
#Document(collection = "useraccount")
public class UserAccounts {
#Id
private String id;
private String accountNumber;
private String firstName;
private String lastName;
// getters and setters
}
inside of my repository I just extend the generic MongoRepository:
#Repository
public interface UserAccountsRepository extends MongoRepository<UserAccounts, String> {
}
I am trying to create a custom query that returns the accountNumber inside of my UserAccountsService:
#Service
public class UserAccountsService {
private final UserAccountsRepository userAccountsRepository;
public UserAccountsService(UserAccountsRepository userAccountsRepository) {
this.userAccountsRepository = userAccountsRepository;
}
// generic find by Object id
public UserAccounts findOne(String id) {
Optional<UserAccounts> userAccountsOptional =
userAccountsRepository.findById(id);
if(!userAccountsOptional.isPresent()) {
throw new RuntimeException("User Account Not Found");
}
return userAccountsOptional.get();
}
// would like to implement custom query to return UserAccount if
// found by accountNumber variable
public UserAccounts findOneByUserAccountNumber(String accountNumber) {
return dormantAccountsRepository.findOne(*need query here*);;
}
}
How would I go about creating a custom query to find a User Account by the accountNumber instead of the object id?
Any help would be great thanks!