Here’s an interesting scenario: I have a Linq-to-Entities query that is giving me no results when performing a system test, but when I look in the database, logically there should be results. Better yet, the unit test passes. How can that be?

The query includes a step where it is excluding any MyObject that is linked a particular SomeOtherObject. MyObject has a conceptual, but not foreign, key that links to SomeOtherObject (call it “SomeGUID”). In this scenario, a null is meaningful — it is a 1 to 0 relationship. That turns out to be the root cause of the problem above.

I was just telling someone a few days ago that a query on a nullable column must take the null into account, which surprised that person. And now I find that I’ve overlooked that myself! Two problems then:

1. A T-SQL WHERE clause like SomeGUID != 'SomeValue' will not return a row where SomeGUID is NULL. You need SomeGUID != 'SomeValue' OR SomeGUID IS NULL. But a similar Linq WHERE clause that doesn’t hit a database will return the null row.
2. Unit tests of Linq-to-Entities, that use a fake database, may not expose some problems with searches that encounter NULL values. Integration tests are needed for that. Hence, don’t forget to also throw together at least a few integration tests for your queries.

To prove this, I wrote a new unit test that uses the fake database (there are other tests for this method, but I wanted to focus on this particular problem by creating a new one). There is one MyObject and it is not connected to any SomeOtherObject. The query in question should return this one MyObject. The unit test does indeed pass.

I then slightly rewrite the test so that the query is hitting the real database, but with the same data: an integration test. The test fails — no results are returned. At this point I was expecting this.

Modify the Linq query so that it include the OR ... IS NULL clause. From

query = query.Where(x => x.SomeGUID != theGuidIDontWant);

To

query = query.Where(x => x.SomeGUID != theGuidIDontWant || x.SomeGUID == null);

Now both tests are passing.

Posted with : Tech, General Programming, Microsoft .NET Framework, Software Testing