Data Migrations using Model Classes vs. SQL

I’m working on my first official migration that inserts data rather
than changing the structure of the database; there’s some data we’d
like to be present in any database, so it seems sensible to put it in
a migration.

However, I’m realizing that it’s not all that simple. The obvious way
to do a data migration is to use the model class.
Imagine that I have an organization with a mandatory name and an
optional description:

e.g. create_table :organizations do |t|
t.column :name, :string, :null => false
t.column :description, :string
end

So, for instance, if I want an organization called “Us”, i might do:

def self.up
Organization.new( :name => ‘Us’ ).save!
end

This works ok for now, but I’m realizing that it couples the migration
to the ‘current’ version of the model class. For instance, if I later
make description mandatory, and validate that (validates_presence_of),
this migration will cease to work. That means that a new developer
won’t be able to get the database by running the migration, because
the migration will complain that the organization does not have a
description.

So – does that mean I should do all data migrations using SQL
statements? How are other people approaching this?

  • Geoffrey

I think there’s a recipe In the “Rails Recipes” book for this

On 9/12/07, Geoffrey W. [email protected] wrote:

e.g. create_table :organizations do |t|
t.column :name, :string, :null => false
t.column :description, :string
end

So, for instance, if I want an organization called “Us”, i might do:

def self.up
Organization.new( :name => ‘Us’ ).save!
end

You can simplify that to Organization.create :name => ‘Us’

statements? How are other people approaching this?
One thing you can do is to pre-declare your model class at the top of
your migration file:

class Organization < ActiveRecord::Base
end

class MyMigration < ActiveRecord::Migration
def self.up

Since the Organization class exists, Rails won’t try to load your
model file. Without the model file, you don’t get validations,
associations, etc.

Two important comments on this…

On Sep 12, 2007, at 10:36 PM, Bob S. wrote:

Imagine that I have an organization with a mandatory name and an
Organization.new( :name => ‘Us’ ).save!
end

You can simplify that to Organization.create :name => ‘Us’

If you create or change the table, then you must call
Organization.reset_column_information before trying to do anything
through the model.

  def self.up
     ...

Since the Organization class exists, Rails won’t try to load your
model file. Without the model file, you don’t get validations,
associations, etc.

Actually, you want to declare a minimal version of your model
INSIDE the migration class like so:

class MyMigration < ActiveRecord::Migration
class Organization < ActiveRecord::Base
end

 def self.up
      ...
 end

end

This way, you can have several migrations that use “the” model. You
can even define little helper methods that might only make sense
during the migration.

It’s nice to use ‘say’ or ‘say_with_time’ in the migration if you’re
doing a lot of data shuffling so you (or the other developers) don’t
assume that the migration is stuck.

-Rob

Rob B. http://agileconsultingllc.com
[email protected]

On Sep 12, 9:24 pm, dailer [email protected] wrote:

I think there’s a recipe In the “Rails Recipes” book for this

Ah, so there is; just for anyone who finds this thread later, this is
mostly the same information that’s being delivered by other people on
this thread, so if you don’t have the book handy, just read on.

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