Rails 4 migration: how to reorder columns

Question

I learned that add_column has an :after option to set where the column gets inserted. Too bad I learned about it :after adding a bunch.

How can I write a migration to simply reorder columns?

Answer 1

When using MySQL, you can call change_column, but you have to repeat the column type (just copy and paste it from your other migration):

def up
  change_column :your_table, :some_column, :integer, after: :other_column
end

Or if you have to reorder multiple columns in one table:

def up
  change_table :your_table do |t|
    t.change :some_column, :integer, after: :other_column
    # ...
  end
end

change_column calls ALTER TABLE under the hood. From the MySQL documentation:

You can also use FIRST and AFTER in CHANGE or MODIFY operations to reorder columns within a table.

Note that this approach doesn't work with PostgreSQL. (see Alter column positions)