Run old migrations in the new migrations

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:
- 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
- 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
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