# Populating from existing tables with migration **URL:** \ **Category:** Rails\ **Created:** [October 8, 2007, 5:39pm UTC](https://www.ruby-forum.com/t/populating-from-existing-tables-with-migration/116671 "2007-10-08T17:39:54Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![kabish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/a88e57/32.png) [@kabish](https://www.ruby-forum.com/u/kabish)\ **Post date:** [October 8, 2007, 5:39pm UTC](https://www.ruby-forum.com/t/populating-from-existing-tables-with-migration/116671/1 "2007-10-08T17:39:54Z")
Hi all, I was wondering how to create a rails migration which, from one preexisting and populated table, creates two new tables, each populated with a subset of the columns of the old, big table. I suppose it is possible and easy, but I haven´t found any example out there. Thanks, Alvaro. ---
**Author:** ![kabish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/a88e57/32.png) [@kabish](https://www.ruby-forum.com/u/kabish)\ **Post date:** [October 8, 2007, 5:46pm UTC](https://www.ruby-forum.com/t/populating-from-existing-tables-with-migration/116671/2 "2007-10-08T17:46:58Z")
On Oct 8, 2007, at 10:39 , Alvaro P. wrote: > I was wondering how to create a rails migration which, from one > preexisting and populated table, creates two new tables, each > populated > with a subset of the columns of the old, big table. You can do this by executing raw SQL in the migration (via the execute method). I don’t know if you can do it using the migration DSL. Michael G. grzm seespotcode net ---
**Author:** ![kabish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/a88e57/32.png) [@kabish](https://www.ruby-forum.com/u/kabish)\ **Post date:** [October 8, 2007, 6:22pm UTC](https://www.ruby-forum.com/t/populating-from-existing-tables-with-migration/116671/3 "2007-10-08T18:22:48Z")
On 10/8/07, Alvaro P. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > Hi all, > > I was wondering how to create a rails migration which, from one > preexisting and populated table, creates two new tables, each populated > with a subset of the columns of the old, big table. I suppose it is > possible and easy, but I haven´t found any example out there. Rails doesn’t have a “create table as select” construct like some databases have. You can use ActiveRecord::Base.execute to run any arbitrary SQL you wish. However, this makes your migration very database-specific. If you want to stick with the migration DSL, you need to create the new tables, then load the data (you can use any of the AR methods like find, create, etc.), and finally drop the old table. If you’re going to use AR methods, you should define “empty” model classes for each of the three tables inside your migration class, so your migration doesn’t trip over validations, associations, etc. ---
**Author:** ![kabish](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/k/a88e57/32.png) [@kabish](https://www.ruby-forum.com/u/kabish)\ **Post date:** [October 8, 2007, 7:13pm UTC](https://www.ruby-forum.com/t/populating-from-existing-tables-with-migration/116671/4 "2007-10-08T19:13:53Z")
On 10/8/07, Bob S. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > tabase-specific. > If you want to stick with the migration DSL, you need to create the > new tables, then load the data (you can use any of the AR methods like > find, create, etc.), and finally drop the old table. > > If you’re going to use AR methods, you should define “empty” model > classes for each of the three tables inside your migration class, so > your migration doesn’t trip over validations, associations, etc. This also ensures that old migrations won’t break as the REAL models code evolves. The models don’t even need to be ‘empty’ I’ve found it useful to add methods to the models 're’defined in a migration for some manipulations. – Rick DeNatale My blog on Ruby [http://talklikeaduck.denhaven2.com/](http://talklikeaduck.denhaven2.com/)