# Migrations and production databases **URL:** \ **Category:** Rails\ **Created:** [June 12, 2009, 8:03pm UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140 "2009-06-12T20:03:18Z")\ **Posts on this page:** 6\ **Page:** 1
**Author:** ![plambert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9d8465/32.png) [@plambert](https://www.ruby-forum.com/u/plambert)\ **Post date:** [June 12, 2009, 8:03pm UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140/1 "2009-06-12T20:03:18Z")
I was searching around to see how other people handle the issue of updating a production database during a release of new version of an application, and all I could find is that (according to one post) it was typically done via migrations. Migrations have a problem, though. After a while, the code changes to a point where earlier migrations stop working. If you are doing simple schema changes in your migrations, it will probably keep working, but if you are modifying application data in your migrations, then sooner or later someone will do something like add a new column and write validation code for it, which causes earlier migrations that worked with that table to fail. Usually this is only a problem if a few weeks have elapsed since the last update. Have other people run into this issue? How are you working around it (or avoiding it)? Thanks, –Paul ---
**Author:** ![plambert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9d8465/32.png) [@plambert](https://www.ruby-forum.com/u/plambert)\ **Post date:** [June 12, 2009, 8:28pm UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140/2 "2009-06-12T20:28:10Z")
Paul L. wrote: […] > Migrations have a problem, though. After a while, the code changes to > a point where earlier migrations stop working. > […] > Usually this is only a problem if a > few weeks have elapsed since the last update. > > Have other people run into this issue? How are you working around it > (or avoiding it)? You shouldn’t be running lots of old migrations. In general, new installations should be done with rake db:schema:load, not by running every migration since the beginning. Use migrations properly and this problem will go away. > Thanks, > –Paul ## Best, Marnen Laibow-Koser [http://www.marnen.org](http://www.marnen.org) removed\_email\_address@domain.invalid ---
**Author:** ![plambert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9d8465/32.png) [@plambert](https://www.ruby-forum.com/u/plambert)\ **Post date:** [June 12, 2009, 8:48pm UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140/3 "2009-06-12T20:48:32Z")
I was actually talking about updating an existing production database. In such a case, the migrations which would be run would be just the ones between the previous release and the current one. I have not tried your suggestion of rake db:schema:load but, judging by the contents of schema.rb, this would drop all of the tables and recreate them. We would lose both our application data and our user data. Perhaps I misunderstood how you were thinking to use that? On Jun 12, 2:28 pm, Marnen Laibow-Koser \ **Author:** ![plambert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9d8465/32.png) [@plambert](https://www.ruby-forum.com/u/plambert)\ **Post date:** [June 12, 2009, 11:59pm UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140/4 "2009-06-12T23:59:49Z") Thanks, Marnen, for mentioning seed\_fu-- that is an intriguing plugin, and I have been thinking about whether we could use it. I think we have a situation in which it would not work well for us-- for at least some of our tables. As a concrete example, suppose you have a large table of product information. If the table is large, loading it from a fixture (or seed\_fu file) could be slow, and anyway some of that information will probably be edited through a separate administrative web application, which would not be updating seed/fixture files. Also, you might want occasionally to add a new field to the table (another product code field) and compute initial values via a migration. In this case it is convenient to use a migration because: - you want to try out the change with the code that uses it in your development area before checking in - computing initial values for the new field might be very quick and easy with ruby code - you want the other developers to get the data change when they do an update of the project files If it is very important that the new product code field have a value for all records, you might put some validation code into the model class to prevent new records from being created without it. Doing so would break any earlier migration that created product records. I don’t really see any solution to this problem except identifying which tables are like this and directly copying them (from a dump file) into the production database. The migrations that modify them can be coded so that they don’t do anything when run on the production system, and that way the migrations can still be run to update other tables (e.g. user data tables) which cannot be copied. –Paul On Jun 12, 3:26 pm, Marnen Laibow-Koser \ **Author:** ![plambert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9d8465/32.png) [@plambert](https://www.ruby-forum.com/u/plambert)\ **Post date:** [June 13, 2009, 1:09am UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140/5 "2009-06-13T01:09:39Z") Paul L. wrote: […] > As a concrete example, suppose you have a large table of product > information. If the table is large, loading it from a fixture (or > seed\_fu file) could be slow, and anyway some of that information will > probably be edited through a separate administrative web application, > which would not be updating seed/fixture files. Right, although I suppose you could always dump the data to YAML or something if you needed to. > Also, you might want occasionally to add a new field to the table > (another product code field) and compute initial values via a > migration. Yes. This is entirely appropriate (although for myself, I like to set a default value on the DB side if things lend themselves to that). Migrations are about state transitions, and this recalculation is part of getting the DB to the new consistent state. […] > If it is very important that the new product code field have a value > for all records, you might put some validation code into the model > class to prevent new records from being created without it. Doing so > would break any earlier migration that created product records. Generally, you wouldn’t be running the earlier migration with the later codebase. I confess that I’m having trouble coming up with a scenario in which you’d ever do this. > I don’t really see any solution to this problem except identifying > which tables are like this and directly copying them (from a dump > file) into the production database. No need. As I implied above, just don’t run old migrations with new model files. In general, you should be releasing often enough that this won’t happen, and in any case, if problems arise, you can always apply one commit, run migrations, and then apply the next commit (assuming your developers are committing as frequently as they should be). > The migrations that modify them > can be coded so that they don’t do anything when run on the production > system, and that way the migrations can still be run to update other > tables (e.g. user data tables) which cannot be copied. No! Migrations are a fabulous tool for managing DB state. Work _with_ them in the ways I’ve been describing, not against them. Disabling migrations in the way you are proposing suggests that something is wrong with your release process. Fix the problem rather than breaking your tools. 🙂 > –Paul ## Best, Marnen Laibow-Koser [http://www.marnen.org](http://www.marnen.org) removed\_email\_address@domain.invalid ---
**Author:** ![plambert](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/p/9d8465/32.png) [@plambert](https://www.ruby-forum.com/u/plambert)\ **Post date:** [June 12, 2009, 9:26pm UTC](https://www.ruby-forum.com/t/migrations-and-production-databases/170140/6 "2009-06-12T21:26:40Z")
Paul L. wrote: > I was actually talking about updating an existing production > database. In such a case, the migrations which would be run would be > just the ones between the previous release and the current one. This should not cause problems in most cases. You mentioned that some of the problems involved application data; either that sort of thing should be removed from migrations or the migrations should provide for it. (If you have more concrete examples, I’ll try to see how I’d handle them.) If by “application data” you mean things like tables of country codes or shipping rates, these absolutely do not belong in migrations. Try something like seed\_fu. Basically, migrations should provide all the changes necessary to get the DB from one consistent state to the next. If you are having problems with migrations, then they probably do not adhere to this guideline. > I have not tried your suggestion of rake db:schema:load but, judging > by the contents of schema.rb, this would drop all of the tables and > recreate them. We would lose both our application data and our user > data. Right. > Perhaps I misunderstood how you were thinking to use that? You didn’t misunderstand; I thought you were talking about a different situation. ## Best, Marnen Laibow-Koser [http://www.marnen.org](http://www.marnen.org) removed\_email\_address@domain.invalid