In a Rails Migration (MySQL), can you specify what position a new column should be?

Sure you can.

• Short answer:

  add_column :users, :gender, :string, :after => :column_name
  

• Long answer:

Here is an example, let's say you want to add a column called "gender" after column "username" to "users" table.

1. Type rails g migration AddGenderToUser gender:string

2. Add "after => :username" in migration that was created so it looks like this:

   class AddSlugToDictionary < ActiveRecord::Migration
     def change
       add_column :users, :gender, :string, :after => :username
     end
   end
   

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.