Rails Update Test Fixtures Upon Migration

Ever get the nagging feeling that everyone knows how to do some obvious thing except for you? This is the feeling I’ve long had with updating my Rails test fixtures. All self-respecting Rails developers know that a strong test infrastructure is a key aspect of any Rails application. And test fixtures are generally seen as the key component that drives test cases. But no Google query I’ve figured out yet has shown me a good way to keep my fixtures updated as my database changes. I’ll going to discuss our current working solution here.

What are fixtures? (aka: newb background information)

For newbs that might not have started their testing yet: test fixtures represent each table in your database as a single file (usually in YAML format) that contain specific, known records you can refer to in your tests.

For example, we have an items fixture that loads in a bunch of sample items records our tests to which our refer. A single record in this items YAML file looks something like:

title: "dummy item"
id: "71"
seller_id: "3"

Fixtures are usually created initially by exporting the data from one of your real database (development or production) running rake db:extract_fixtures. This will create a fixture file for every table, with the records in every table labeled something like “item_001”, “item_002”, etc. As you can see in the example above, my tendency is to rename these records so that they are more semantic. This makes it easier to remember which fixture record is which when loading them in my tests, by having syntax like item = items(‘items_not_valid_not_committed_missing_price‘). A lot easier to remember what that item represents later on, then having it named item = items(‘item_001’)… and then six months later asking “What was the item number 1 record again? Oh yeah! The not valid item because it was missing price! Of course.”

Sounds fine to me. So what’s the problem?

The problem is what happens when your database changes. Especially for an application in the midst of development, the database might change weekly. However, if you re-export your fixtures from the database, you’ll lose all of your custom-named fixture records. You’ll also lose any special cases you might have setup. In our case, we have records where we set stuff like “item_expires_at: <%= 1.day.from_now %>”, and that ain’t gonna fly if you re-export the database.

If you don’t re-export the database, though, then you are left with hundreds of records that are missing (or have extra) fields after your migration. What’s a developer to do?

The partial answer: The Rails Fixture Migration Plugin

This plugin, developed by Josh and Jake is a good start. After installing it, you can run “rake db:fixtures:migrate” and automatically have the fields that were added or removed in your migrations correspondingly added or removed from your fixtures. But there are a number of caveats:

* The code breaks if it finds empty fixtures. This can be fixed by wrapping lines 14-16 in migrate_fixtures.rb with an if so they only try to load non-empty fixtures
* The code evaluates and replaces any inline Ruby in your fixtures. So the previously mentioned “item_expires_at: <%= 1.day.from_now %>” will become “”item_expires_at: March 29, 2008” after migration. I’ve just worked around this by manually replacing those substitutions after running the migration. A bit more annoying is if you have code that does any loops in your fixtures. For example, we have a loop that creates 50 similar items in one of our fixtures, and the fixture migrator simply doesn’t understand what this code is doing, giving an exception when it tries to run it. For the time being, I just remove this loop before migration and re-add it after migration. A pain, surely, but less of a pain than adding new fields to the other 100 records in our items fixture.
* The fixture migration fails with some ambiguous error code if any of the fixtures it wants to migrate already exist. The fixture migration uses a schema_info.yml file in your /test/fixtures directory to keep track of the fixture migration. If you, for example, create a new scaffolding that creates both a migration file and a new fixture, the fixture migration tool will break when it gets to the migration to create the new table, because it seems the new table is already a fixture in your directory.

All in all, they are surmountable obstacles for us at this point, given the lack of other options.

The “better” answer: Uhh…

I’ll be curious to hear if other developers have a more elegant way to deal with the problem of writing tests that stay relevant as your database changes? One school of thought is that one could just write methods that create the records you need programmatically, avoiding fixtures altogether. This has crossed my mind, but fixtures are nice since they come pre-generated, and I think they are generally easier to read through than a hash that creates a new record. A bigger benefit of fixtures is that they are automatically placed in the test database, so you get all the interconnections between your tables loaded at once. It seems to me like it would be quite a headache to create an entire database of data programatically.

So, slightly painful fixture migrations it is, so far.

You can get your own copy of the slightly pain Rails migration plugin here.

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