Discovering Pending Rails Migrations With Git
At MyDrive, Gavin and I came up with what I think is a pretty novel solution to discovering new Rails migrations at deployment time. We deploy often but not on every commit. We usually know about new migrations from reading the commits—but no one is perfect and sometimes you miss a commit or three. The standard checks for pending migrations involve deploying the new code and verifying the schema version against the database before reloading the app. This works but it’s slow and you don’t find out about the migrations until the code has been pushed to at least one system. Automating this is still a good idea. But it’s often nice to find out about pending migrations right up front. Sometimes you just don’t want to deploy right now if there are pending migrations.
But, do we need to talk to the DB to find this out? Doesn’t git tell us what has changed in our code? Why yes, it does.
We tag each deployed ref with a git tag representing the deployment
environment (Capistrano stage) and the time and date. This is tells
us what code was running in any environment at any time. We now
also have a moving git tag that identifies the last deployment to
any environment. After each deployment we move the tag to the current
HEAD ref. Finding new migrations is now as simple as
git diff on the
db/migrate tree against
the previous deployment tag for this environment. Capistrano does
this as part of the deployment and lets us choose to abort if we
don’t want to deploy with migrations. It’s not perfect: it will
generate false positives on any change to any file in the migrations
tree, but old migrations are rarely changed. Here’s what it looks
Notably this needs to be done from a clean tree because you will be changing git tags on the local installation. So it should either be done from a deployment host or from a clean checkout of your code. It’s best to detect this in your deployment scripts as well so that you don’t get into trouble. If there is interest, we could probably release this as a Capistrano plugin. If you decide to implement it yourself, here are some notes on helpful git commands for doing so.
Update: I have released two gems to do this for you. The first is called capistrano-deploytags and handles all of the tagging for you at deployment time. The second, capistrano-detect-migrations does what I described above. It requires the functionality from capistrano-deploytags.
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