Unit-Testing: Database set-up for tests

Question

I'm writing unit-tests for an app that uses a database, and I'd like to be able to run the app against some sample/test data - but I'm not sure of the best way to setup the initial test data for the tests.

What I'm looking for is a means to run the code-under-test against the same database (or schematically identical) that I currently use while debugging - and before each test, I'd like to ensure that the database is reset to a clean slate prior to inserting the test data.

I realize that using an IRepository pattern would allow me to remove the complexity of testing against an actual database, but I'm not sure that will be possible in my case.

Any suggestions or articles that could point me in the right direction?

Thanks!

--EDIT--

Thanks everyone, those are some great suggestions! I'll probably go the route of mocking my data access layer, combined with some simple set-up classes to generate exactly the data I need per test.

Answer 1

If you are thinking about a real database usage, then mostlikely we're talking integration tests here. I.e tests, which check app behavior as a composition of different components contrary to unit tests, where components are supposed to be tested in isolation.

Having the testing scope defined, I wouldn't recommend using things like in-memory databases or mocking libraries as the other authors suggested. The problem is that usually there is a slightly different behavior or reduced set of features for in-memory databases and there is no database at all with mocking, therefore you'll be testing some other application in general sense and not the one you'll be delivering to your customers.

I'd rather suggest to minimize the amount of integration tests by covering just a crucial parts of your logic leaving the rest for unit testing, while using a real database with the setup as close to the production one as possible. Test runs could be too slow and a real pain if there are a lot of integration ones.

Also you might use some tricks to optimize the speed of your tests execution:

• Split tests to Read and Write in regard to the data mutations they introduce and run the former ones in parallel and without any cleanup. (E.g HTTP GET requests are safe to be run in parallel if the system under test is a webapp and tests are more like end-to-end);
• Use the only insert/delete script for all the data and optimize as much as possible. You might find Reseed library I'm developing currently helpful. It's able to generate both insert and delete scripts for you. So basically what you asked for. Or check out Respawn which could be used for database cleanup;
• Use database snapshots for the restore, which might be faster than full insert/delete cycle;
• Wrap each test in transaction and revert it afterwards (this one is also not 100% honest and somewhat fragile);
• Parallelize your tests by using a pool of databases instead of the only. Docker and TestContainers could be suitable here;

Answer 2

Mocking is of cause the best way to unit test your code.

As far as integration tests go, I have had some issues using in-memory databases like SQLite, mainly because of small differences in behaviour and/or syntax.

I have been using a local instance of MySql for integration tests in several projects. A returning problem is the server setup and creation of test data. I have created a small Nuget package called Mysql.Server (see more at https://github.com/stumpdk/MySql.Server), that simply sets up a local instance of MySql every time you run your tests.

With this instance running you can easily set up table structures and sample data for your tests without being concerned of either your production environment or local server setup.

Answer 3

This code clears all data from all user's tables in MS SQL Server:

private DateTime _timeout;

public void ClearDatabase(SqlConnection connection)
{
    _timeout = DateTime.Now + TimeSpan.FromSeconds(30);
    do
    {
        SqlCommand command = connection.CreateCommand();
        command.CommandText = "exec sp_MSforeachtable 'DELETE FROM ?'";
        try
        {
            command.ExecuteNonQuery();
            return;
        }
        catch (SqlException)
        {
        }
    } while (!TimeOut());

    if (TimeOut())
        Assert.Fail("Fail to clear DB");
}

private bool TimeOut()
{
    return DateTime.Now > _timeout;
}

Answer 4

The best way I've found to handle this is to use a static test database with known data, and use transactions to ensure that your tests don't change anything.

In your test setup you would start a transaction, and in your test cleanup, you would roll the transaction back. This lets you modify data in your tests but also makes sure everything gets restored to its original state when the test completes.

Answer 5

Here's the general approach I try to use. I conceive of tests at about three or four levels:: unit-tests, interaction tests, integration tests, acceptance tests.

At the unit test level, it's just code. Any database interaction is mocked out, either manually or using one of the popular frameworks, so loading data is not an issue. They run quick, and make sure the objects work as expected. This allows for very quick write-test/write code/run test cycles. The mock objects serve up the data that is needed by each test.

Interaction tests test the interactions of non-trivial class interactions. Again, no database required, it's mocked out.

Now at the integration level, I'm testing integration of components, and that's where real databases, queues, services, yada yada, get thrown in. If I can, I'll use one of the popular in-memory databases, so initialization is not an issue. It always starts off empty, and I use utility classes to scrub the database and load exactly the data I want before each test, so that there's no coupling between the tests.

The problem I've hit using in-memory databases is that they often don't support all the features I need. For example, perhaps I require an outer join, and the in-memory DB doesn't support that. In that case, I'll typically test against a local conventional database such as MySQL, again, scrubbing it before each test. Since the app is deployed to production in a separate environment, that data is untouched by the testing cycle.

Answer 6

I found it best to have my tests go to a different db so I could wipe it clean and put in the data I wanted for the test.

You may want to have the database be something that can be set within the program, then your test can tell the classes to change the database.

Answer 7

I know you're using C# but in the Java World there's the Spring framework. It allows you to run database minipulations in a transaction and after this transaction, you roll this one back. This means that you operate against a real database without touching the state after the test finishes. Perhaps this could be a hint to further investigation in C#.

Answer 8

I don't think there is an easy way to finish this. You just have to create those Pre-Test sql setup scripts and post-test Tear-down scripts. Then you need trigger those scripts for each run. A lot of people suggest SQLLite for unit test setup.