Unit testing a service method- how to assert

Question

Hello all, I'm trying to get my head around the idea of testing functionality and minimizing the number of asserts.

So lets say I have such a service method:

public void TransferFunds(int debitedId, int creditedId, Sum funds)
{
    Account debited = accountRepository.Get(debitedId);
    Account credited = accountRepository.Get(creditedId);
    debited.Debit(Sum);
    credited.Credit(Sum);
    accountRepository.Update(debited);
    accountRepository.Update(credited);
}

My unit tests I presume should then split into two, testing the crediting of one account and the debiting of the other. But should I assert on each account current state, to verify the sum has been added/deducted- or is this a test for my Account model? If so, what should I assert? that update was called? That doesn't verify the functionality...

Anyone has an idea?

Answer 1

If you test the outcome (i.e. the final balances) then you are effectively also testing the Account objects, which are not under test here, so you should try and avoid that if possible.

In this scenario, I would expect Account to be an interface which you could mock out. I'm not sure what language you are using (looks like maybe C#). In Java, I would do this with an interface and jMock.

Then, you assert that the TransferFunds method calls Debit with the right Sum on the right Account, and calls Credit with the right Sum on the right Account. You will also need to mock out your account repository and have your mock framework return your mock Accounts in response to AccountRepository.Get