Moving data to another table - sql

Question

I have a question associated with moving data from one table to another. I am using Postgres as a database.
I have two models:
class User < ActiveRecord::Base
has_many :emails
end
class Email < ActiveRecord::Base
belongs_to :user
end
schema looks like this:
create_table "users", force: :cascade do |t|
t.string "first_name", limit: 255
t.string "last_name", limit: 255
t.string "email"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
create_table "emails", force: :cascade do |t|
t.string "email", null: false
t.integer "user_id"
t.datetime "created_at", null: false
t.datetime "updated_at", null: false
end
Now I want to move the latest email for the user and save it in users table email column. I can easily do it using Rails models in migration but then when I rename model or remove it. Migration will fail. Is there any easy way to do it with raw sql?

Answer 1

You can use window functions. The example below is a starting point.
UPDATE users u
SET u.email = (SELECT
MAX(FIRST_VALUE(email)) OVER (PARTITION BY user_id ORDER BY created_at DESC)
FROM emails e
WHERE e.user_id = u.id);

Answer 2

You can still use ActiveRecord with fake classes just for the purpose of migration. Something like:
class YourMigration < ActiveRecord::Migration
class FakeUser < ActiveRecord::Base
self.table_name = 'users'
has_many :fake_emails, foreign_key: 'user_id'
end
class FakeEmail < ActiveRecord::Base
self.table_name = 'emails'
belongs_to :fake_user, foreign_key: 'user_id'
end
def change
FakeUser.all.each do |user|
[...]
end
end
end