Update one column to value of another in Rails migration

I would create a migration

rails g migration set_updated_at_values

and inside it write something like:

class SetUpdatedAt < ActiveRecord::Migration
  def self.up
    Yourmodel.update_all("updated_at=created_at")
  end

  def self.down
  end
end

This way you achieve two things

this is a repeatable process, with each possible deploy (where needed) it is executed

this is efficient. I can't think of a more rubyesque solution (that is as efficient).

Note: you could also run raw sql inside a migration, if the query gets too hard to write using activerecord. Just write the following:

Yourmodel.connection.execute("update your_models set ... <complicated query> ...")

You can use update_all which works very similar to raw SQL. That's all options you have.

BTW personally I do not pay that much attention to migrations. Sometimes raw SQL is really best solution. Generally migrations code isn't reused. This is one time action so I don't bother about code purity.

As gregdan wrote, you can use update_all. You can do something like this:

Model.where(...).update_all('updated_at = created_at')

The first portion is your typical set of conditions. The last portion says how to make the assignments. This will yield an UPDATE statement, at least in Rails 4.

You can directly run following command to your rails console ActiveRecord::Base.connection.execute("UPDATE TABLE_NAME SET COL2 = COL1")

For example: I want to update sku of my items table with remote_id of items tables. the command will be as following:
ActiveRecord::Base.connection.execute("UPDATE items SET sku = remote_id")

Do not use application models in migrations unless you redefine them inside migration. If you use application model tahat you later change or delete your migration might fail.

Of course you can also use full power of SQL inside migrations.

Read https://makandracards.com/makandra/15575-how-to-write-complex-migrations-in-rails

This is a General way of solving, without the need for Writing Query, as queries are subjected to risk.

  class Demo < ActiveRecord::Migration
    def change
     add_column :events, :time_zone, :string
     Test.all.each do |p|
       p.update_attributes(time_zone: p.check.last.time_zone)
     end
     remove_column :sessions, :time_zone
    end
  end

You can also add updated_at column and update its values in one migration:

class AddUpdatedAtToTableName < ActiveRecord::Migration
  def change
    add_column :table_name, :updated_at, :datetime

    reversible do |dir|
      dir.up do
        update "UPDATE table_name SET updated_at=created_at"
      end
    end
  end
end

As a one time operation, I would just do it in the rails console. Will it really take hours? Maybe if there are millions of records…

records = ModelName.all; records do |r|; r.update_attributes(:updated_at => r.created_at); r.save!; end;`