One of our rails applications was built on top of a legacy schema, and
we
haven’t had any particular issues. Even if you run into minor problems,
migrations bring enough benefits that they’re worth a little bit of
annoyance.
On 9/7/06, linux user [email protected] wrote:
What I would like to know is
- Is it possible to migrate the data along with migrating the schema if
use Migrations? If it is possible can somebody point me to an example,
because in all the migrations tutorials that I see they have self.up (to
create tables) and self.down (to drop tables) but what about data?
Yes. You have a couple of options when updating data. If you are
updating
many rows at once, the simplest way can be to just execute the
appropriate
SQL directly using ‘execute’. For example:
def self.up
#make some table changes here…
execute ‘update mytable set …’
end
However, keep in mind a migration is just a normal ruby source file,
with
some convenient rails stuff already set up for you. This means you can
execute whatever ruby code want as part of the migration. For example:
def self.up
User.transaction do
User.find(:all).each {|u| u.new_attribute = ‘some default value’;
u.save!}
end
end
will work just as well. You’ll probably want to make sure that you do
all
your updates (or even the whole migration) in a transaction, in case
something goes wrong. You should probably also take a look at the
documentation for ActiveRecord::Migration for a discussion of issues
such as
using a model after you have changed its table in the migration.
- If I use migrations I will have to change my model classes, how can I
minimize the pains of keeping my model classes in sync with schema changes
specially when I am using A) ‘set_primary_key’ etc. B) rake migrate
VERSION=xx
- When I write my unit test for testing models, how do I keep my tests
in
sync with rake migrate VERSION=xx? Can I migrate my fixtures along with
schema changes?
What pains do you forsee in terms of keeping things in sync? As long as
you
have everything under version control, run your tests regularly, and
check
in regularly, you should be able to assume that whenever you do an
update
the models and the migrations match up. If you run tests without having
migrated, its usually pretty obvious - you’ll typically get a ton of
database errors in your unit tests, in which case you can just rake
db:migrate and then run the tests again. This has not been an issue for
us.
- I will need a to keep my test: database in sync with migrations and I
am
assuming that rake would take care of migrating the schema in test database
before running the tests. Is my assumption correct?
Yes, before you run your tests rake will automatically clone the
structure
of your development database to your test database. As long as you run
rake
db:migrate to get your development database to reflect the structure you
want, everything else should work nicely.
-Steve
). So my scenario is an