Imagine you have to write a new migration with a lot of code that you have already written in an older migration. The required code is exactly the up/down/change part of that migration. In other words, you would like to run a migration part (up, down or change) in the new migration. Pay attention — you would have to write or copy and paste a lot of code. You have 2 ways to overcome this issue:
- The simplest and most boring solution is to copy and paste this code or write it from scratch. This is not our choice. Old code and new code will mix, and you will have a huge amount of disgusting code — you won’t be able to tell which code is new and which is old.
- Use the older migration in a DRY way. Just use that code as you use another class, for instance the
Usermodel you probably have.
Input data
In one project, I had to roll back my old migration, which had a lot of code. Check it out:
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
endAs I have already said, I would like to roll it back in the up part of my new migration. Writing a new migration and pasting this code there is not the right solution. I think you agree with me.
Solution
I think the best solution is to include this migration in my new one, then use its class as usual code (yes — a migration is a class too, and we can do this). So here is the final code:
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
endSee the first line (1): require File.join(Rails.root, 'db/migrate/20121107173946_add_search_content_to_products.rb'). With this line, I include the migration file, and after this we can use the old migration’s class. On the fifth line (2), I use it: AddSearchContentToProducts.new.down to roll back the old migration. In down, I have to run that migration’s up method; see mark (3): AddSearchContentToProducts.new.up.
In this example, I had to rewrite a stored procedure for a PostgreSQL database, and I’ve got a clean, DRY solution, as you can see. Imagine how much code I would have if I pasted the entire migration into the new one! I hope you will find this article useful, and if you have issues like mine, you won’t have problems solving them now.
UPDATE 26.06.2013: In Rails 4, a new method appeared that allows you 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
