If I'm adding a column via MySQL, I can specify where in the table that column will be using the AFTER modifier. But if I do the add_column via a Rails migration, the column will be created at the end of the table.
Is there any functionality for rails migrations to specify the position of an added column?
I do not think there is now but more to the point, why do you need to specify the exact place?
There does not seem to be a position option for the add_column method in migrations. But migrations support executing literal SQL. I'm no Rails developer, but something like the following:
class AddColumnAfterOtherColumn < ActiveRecord::Migration
def self.up
execute "ALTER TABLE table_name ADD COLUMN column_name INTEGER
AFTER other_column"
end
def self.down
remove_column :table_name, :column_name
end
end
There is no way within Rails to specify the position of a column. In fact, I think it's only coincidental (and therefore not to be relied on) that columns are created in the order they are named in a migration.
The order of columns within a table is almost relevant and should be so: the common "reason" given is to be able to see a particular subset when executing a "SELECT *", but that's really not a good reason.
Any other reason is probably a design smell, but I'd love to know a valid reason why I'm wrong!
On some platforms, there is a (miniscule) space and performance saving to be obtained by putting the columns with the highest probability of being NULL to the end (because the DMBS will not use any disk space for "trailing" NULL values, but I think you'd have to be running on 1980's hardware to notice.
I created a patch that adds this additional functionality to the ActiveRecord Mysql adapter. It works for master and 2-3-stable.
It might be mysql specific, but it doesn't make your migrations any less portable (other adapters would just ignore the extra positioning options).
A well-defined column order is also useful when you have database dumps (e.g. backups) and create a new database using your migrations and then want to fill the new database with data from the dumps.
This is now possible in Rails 2.3.6+ by passing the :after parameter
To everyone that doesn't see the advantage in having this feature: do you never look at your database outside of the ORM? If I'm viewing in any sort of UI, I like having things like foreign keys, status columns, flags, etc, all grouped together. This doesn't impact the application, but definitely speeds up my ability to review data.