Currently I'm working on a CMS that allows developers to create and install
third-party extensions. These extensions can contain controllers, models,
helpers, the whole shebang.
In order to make the installation/development process easier migrations are
used for each extension. The basic structure of each extension is the same
as your average gem:
extensions
|
|__ my_extension
|
|__ lib
| |
| |__ my_extension
| |__ my_extension.rb
|
|__ migrations
|
|__ 1234567890_create_schema.rb
As you can see, nothing really special. There is however one big problem
when using Sequel migrations. By default Sequel uses a single column to keep
track of the last run migration.
While this is no problem for a single directory this can cause problems
pretty quickly when running migrations from multiple directories (with the
possibility of the same name being used N times).
To solve this problem I figured that it would be best to give each extension
it's own column. When installed this column would be created, the initial
migrations would be executed and you're good to go.
Well, turns out it's not that easy. The problem is that Sequel expects the
column to be there once the table is created, which makes sense and can be
solved quite easily. The bigger problem however
is that it seems to expect that at least 1 migration was executed before
creating the column, creating the column and not running any migration
(regardless of the column value) will trigger an error like
the following:
Applied migration files not in file system: 20000101_test.rb
Whether the column value is NULL, 0, "" or "cookies" Sequel will never run
the migration UNLESS the column value matches an existing migration. This is
fine...when you've actually ran any migrations.
When running an installation for the first time however Sequel will bail out
and the migration is never executed. The code I'm using to install
extensions is the following: http://scrp.at/pv Note that this Rake
task will work fine for the first migration executed (as no columns exist
yet) but any following migrations won't be executed.
So the question is, how to get around this. Personally I think this issue
could be solved by 1. finding out if Sequel requires some sort of default
value 2. Modifying Sequel itself to do the same process for N columns
rather than only the first one. The latter most likely produces the best
results but this will also cause the issue of possibly ending up with N
columns if you happen to have a few typos/errors in your code.
To make it a bit easier to understand I've listed the installation steps of
an extension below:
1. install the actual application
2. download an extension (either using Rubygems or something else)
3. require it in a config file (config/requires.rb)
4. run "rake extension:install["com.some-extension.identifier"]". This runs
the migration(s) for the extension
5. done
Here's a screenshot that shows a basic example of the installation process:
http://grab.by/8r0t
As you can see the initial installation works fine as there's no
"schema_migrations" table yet, the second migration however fails as the
column is there (created by the Rake task) but has no value. Sequel thinks
this is a migration, tries to execute/find it and fails as the file ""/NULL
doesn't exist.
Free E-beer for the one who can help me out!
Yorick
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