Unit-Testing Databases

Question

This past summer I was developing a basic ASP.NET/SQL Server CRUD app, and unit testing was one of the requirements. I ran into some trouble when I tried to test against the database. To my understanding, unit tests should be:

• stateless
• independent from each other
• repeatable with the same results i.e. no persisting changes

These requirements seem to be at odds with each other when developing for a database. For example, I can't test Insert() without making sure the rows to be inserted aren't there yet, thus I need to call the Delete() first. But, what if they aren't already there? Then I would need to call the Exists() function first.

My eventual solution involved very large setup functions (yuck!) and an empty test case which would run first and indicate that the setup ran without problems. This is sacrificing on the independence of the tests while maintaining their statelessness.

Another solution I found is to wrap the function calls in a transaction which can be easily rolled back, like Roy Osherove's XtUnit. This work, but it involves another library, another dependency, and it seems a little too heavy of a solution for the problem at hand.

So, what has the SO community done when confronted with this situation?

Answer 1

There's no real way to unit test a database other than asserting that the tables exist, contain the expected columns, and have the appropriate constraints. But that's usually not really worth doing.

You don't typically unit test the database. You usually involve the database in integration tests.

You typically use your favourite automated unit testing framework to perform integration tests, which is why some people get confused, but they don't follow the same rules. You are allowed to involve the concrete implementation of many of your classes (because they've been unit tested). You are testing how your concrete classes interact with each other and with the database.

Answer 2

DBunit

You can use this tool to export the state of a database at a given time, and then when you're unit testing, it can be rolled back to its previous state automatically at the beginning of the tests. We use it quite often where I work.

Answer 3

The usual solution to external dependencies in unit tests is to use mock objects - which is to say, libraries that mimic the behavior of the real ones against which you are testing. This is not always straightforward, and sometimes requires some ingenuity, but there are several good (freeware) mock libraries out there for .Net if you don't want to "roll your own". Two come to mind immediately:

Rhino Mocks is one that has a pretty good reputation.

NMock is another.

There are plenty of commercial mock libraries available, too. Part of writing good unit tests is actually desinging your code for them - for example, by using interfaces where it makes sense, so that you can "mock" a dependent object by implmenting a "fake" version of its interface that nonetheless behaves in a predictable way, for testing purposes.

In database mocks, this means "mocking" your own DB access layer with objects that return made up table, row, or dataset objects for your unit tests to deal with.

Where I work, we typically make our own mock libs from scratch, but that doesn't mean you have to.

Answer 4

What you should do is run your tests from a blank copy of the database that you generate from a script. You can run your tests and then analyze the data to make sure it has exactly what it should after your tests run. Then you just delete the database, since it's a throwaway. This can all be automated, and can be considered an atomic action.

Answer 5

tgmdbm said:

You typically use your favourite automated unit testing framework to perform integration tests, which is why some people get confused, but they don't follow the same rules. You are allowed to involve the concrete implementation of many of your classes (because they've been unit tested). You are testing how your concrete classes interact with each other and with the database.

So if I read this correctly, there is really no way to effectively unit-test a Data Access Layer. Or, would a "unit test" of a Data Access Layer involve testing, say, the SQL/commands generated by the classes, independent of actual interaction with the database?

Answer 6

Yeah, you should refactor your code to access Repositories and Services which access the database and you can then mock or stub those objects so that the object under test never touches the database. This is much faster than storing the state of the database and resetting it after every test!

I highly recommend Moq as your mocking framework. I've used Rhino Mocks and NMock. Moq was so simple and solved all the problems I had with the other frameworks.

Answer 7

pbh101 said:

So if I read this correctly, there is really no way to effectively unit-test a Data Access Layer. Or, would a "unit test" of a Data Access Layer involve testing, say, the SQL/commands generated by the classes, independent of actual interaction with the database?

Exactly. Parts of your data access layer will return Commands or query objects and you assert against those. You don't unit test the connection.ExecuteReader method because that doesn't belong to you.

Answer 8

If you're using LINQ to SQL as the ORM then you can generate the database on-the-fly (provided that you have enough access from the account used for the unit testing). See http://www.aaron-powell.com/blog.aspx?id=1125

Answer 9

I've had the same question and have come to the same basic conclusions as the other answerers here: Don't bother unit testing the actual db communication layer, but if you want to unit test your Model functions (to ensure they're pulling data properly, formatting it properly, etc.), use some kind of dummy data source and setup tests to verify the data being retrieved.

I too find the bare-bones definition of unit testing to be a poor fit for a lot of web development activities. But this page describes some more 'advanced' unit testing models and may help to inspire some ideas for applying unit testing in various situations:

Unit Test Patterns

Answer 10

I explained a technique that I have been using for this very situation here.

The basic idea is to exercise each method in your DAL - assert your results - and when each test is complete, rollback so your database is clean (no junk/test data).

The only issue that you might not find "great" is that i typically do an entire CRUD test (not pure from the unit testing perspective) but this integration test allows you to see your CRUD + mapping code in action. This way if it breaks you will know before you fire up the application (saves me a ton of work when I'm trying to go fast)