Imagine you have to write new migration with a lot of code and this migration should have a lot of code, code which you have already wrote in older migration and this required code is exactly up/down/change part of those migration. Another words you would like to run migration part (up, down or change) in the new migration. Pay attention - you have to write or copy paste a lot of code You have 2 variants how to overcome this issue:

  1. The simplest and boring solution - it is copy and paste this code, or write it with scratch. This is not our case. Old code and new code will mix and you will have a huge amount of disgusting code - you won’t able to detect which is new code and which is old
  2. Use oldest migration in DRY way. Just use those code as you use another class, for instance User model which you probably have

Input data

In one project I had to rollback my old migration which has a lot of code. Check out this code:

db/migrate/20121107173946_add_search_content_to_products.rb

class AddSearchContentToProducts < ActiveRecord::Migration
def up
add_column :products, :tsvector_content_tsearch, :tsvector
add_column :products, :tsvector_content_dmetaphone, :tsvector

execute <<-EOS
CREATE INDEX products_tsvector_content_tsearch_idx ON products USING gin(tsvector_content_tsearch);
CREATE INDEX products_tsvector_content_dmetaphone_idx ON products USING gin(tsvector_content_dmetaphone);
EOS

execute <<-EOS
CREATE OR REPLACE FUNCTION get_tsvector(c text, w "char", lang regconfig DEFAULT 'pg_catalog.russian') RETURNS tsvector LANGUAGE plpgsql AS $$
begin
return setweight(to_tsvector(lang, coalesce(c, '')), w);
end
$$;
EOS

execute <<-EOS
CREATE OR REPLACE FUNCTION products_trigger() RETURNS trigger LANGUAGE plpgsql AS $$
declare
product_model record;
product_part record;
product_brand record;
product_seller record;

begin
select name, aliases, brand_id into product_model from models where id = new.model_id;
select name, aliases into product_brand from brands where id = product_model.brand_id;
select name, aliases into product_part from parts where id = new.part_id;
select email, name into product_seller from sellers where id = new.seller_id;

new.tsvector_content_tsearch :=
get_tsvector(new.note, 'C') ||

get_tsvector(product_model.name, 'A') ||
get_tsvector(product_model.aliases, 'A') ||

get_tsvector(product_part.name, 'A') ||
get_tsvector(product_part.aliases, 'A') ||

get_tsvector(product_brand.name, 'A') ||
get_tsvector(product_brand.aliases, 'A') ||

get_tsvector(product_seller.name, 'D') ||
get_tsvector(product_seller.email, 'D');

new.tsvector_content_dmetaphone :=
get_tsvector(new.note, 'C', 'simple') ||

get_tsvector(product_model.name, 'A', 'simple') ||
get_tsvector(product_model.aliases, 'A', 'simple') ||

get_tsvector(product_part.name, 'A', 'simple') ||
get_tsvector(product_part.aliases, 'A', 'simple') ||

get_tsvector(product_brand.name, 'A', 'simple') ||
get_tsvector(product_brand.aliases, 'A', 'simple') ||

get_tsvector(product_seller.name, 'D', 'simple') ||
get_tsvector(product_seller.email, 'D', 'simple');

return new;
end
$$;
EOS

execute <<-EOS
CREATE TRIGGER products_content_to_search_trigger BEFORE INSERT OR UPDATE
ON products FOR EACH ROW EXECUTE PROCEDURE products_trigger();
EOS

Product.all.each(&:touch)
end

def down
remove_column :products, :tsvector_content_tsearch
remove_column :products, :tsvector_content_dmetaphone

execute <<-EOS
DROP TRIGGER products_content_to_search_trigger ON products;
DROP FUNCTION products_trigger();
DROP FUNCTION get_tsvector(c text, w "char", lang regconfig);
EOS
end
end

As I have already said I would like to rollback it in my new migration on up. Write new migration and paste there this code is not right solution. I think you are agreed with me.

Solution

I think the best solution will be to include this migration in my new migration then use this migration class as usual code (yes - migration is a class too and we are able to do it). So the final code below:

require File.join(Rails.root, 'db/migrate/20121107173946_add_search_content_to_products.rb') # (1)

class ChangeSearchFunctions < ActiveRecord::Migration
def up
AddSearchContentToProducts.new.down # (2)

add_column :products, :search_vector, :tsvector

execute <<-EOS
CREATE INDEX products_search_vector_idx ON products USING gin(search_vector);

CREATE FUNCTION get_tsvector(c text, lang regconfig DEFAULT 'russian'::regconfig) RETURNS tsvector
LANGUAGE plpgsql
AS $$
begin
return to_tsvector(lang, coalesce(c, ''));
end
$$;

CREATE FUNCTION products_trigger() RETURNS trigger
LANGUAGE plpgsql
AS $$
declare
product_model record;
product_part record;
product_brand record;
product_seller record;

begin
select name, aliases, brand_id into product_model from models where id = new.model_id;
select name, aliases into product_brand from brands where id = product_model.brand_id;
select name, aliases into product_part from parts where id = new.part_id;
select email, name into product_seller from sellers where id = new.seller_id;

new.search_vector :=
get_tsvector(new.note) ||

get_tsvector(product_model.name) ||
get_tsvector(product_model.aliases) ||

get_tsvector(product_part.name) ||
get_tsvector(product_part.aliases) ||

get_tsvector(product_brand.name) ||
get_tsvector(product_brand.aliases) ||

get_tsvector(product_seller.name) ||
get_tsvector(product_seller.email);

return new;
end
$$;

CREATE TRIGGER products_content_to_search_trigger BEFORE INSERT OR UPDATE
ON products FOR EACH ROW EXECUTE PROCEDURE products_trigger();
EOS
Product.find_each(&:touch)
end

def down
remove_column :products, :search_vector

execute <<-EOS
DROP TRIGGER products_content_to_search_trigger ON products;
DROP FUNCTION products_trigger();
DROP FUNCTION get_tsvector(c text, lang regconfig);
EOS

AddSearchContentToProducts.new.up # (3)
Product.find_each(&:touch)
end
end

See the first line (1): require File.join(Rails.root, 'db/migrate/20121107173946_add_search_content_to_products.rb'). With this line I include migration file and after this we are able to use old migration’s class. On the fifth line (2) we I just use it: AddSearchContentToProducts.new.down to rollback old migration. On down I have to up this migration, see mark (3): AddSearchContentToProducts.new.up.

In this example I had to rewrite stored procedure for postgresql database and I’ve got clean and DRY solution as you can see. Imagine how many code I would have if I just pasted all entire migration in the new! I hope you will find this article useful and if you have issues like I had you won’t have problems now to solve them.

UPDATE 26.06.2013: In Rails 4 new method is appeared which allows to revert all migrations. Check it out:

require_relative '2012121212_example_migration'

class FixupExampleMigration < ActiveRecord::Migration
def change
revert ExampleMigration

create_table(:apples) do |t|
t.string :variety
end
end
end