Migrations

Rails
1

I’m trying to understand migrations and here is my problem, lets take
this example code from the API docs for migration. This code creates a
table then adds a new row to it, my problem is how can this add a row
to the table if the model doesn’t exist and how can you create a model
if the table doesn’t first exist. Can the migrations also be told to
create a model for a new table? Because right now if I run this code
it will fail to add the first row because the model does not exist
yet.

class AddSystemSettings < ActiveRecord::Migration
def self.up
create_table :system_settings do |t|
t.column :name, :string
t.column :label, :string
t.column :value, :text
t.column :type, :string
t.column :position, :integer
end

  SystemSetting.create :name => "notice", :label => "Use notice?",

:value => 1
end

def self.down
  drop_table :system_settings
end

end

2

On Dec 29, 2005, at 7:17 AM, Frank H. wrote:

I’m trying to understand migrations and here is my problem, lets take
this example code from the API docs for migration. This code creates a
table then adds a new row to it, my problem is how can this add a row
to the table if the model doesn’t exist and how can you create a model
if the table doesn’t first exist…

The table does not need to exist in order to define a new model–it
only needs to exist the first time you try to use the model. Thus,
you can use the generator to create a new model, and then reference
the model in the migration that creates the table.

  • Jamis
3

Jamis B. wrote:

The table does not need to exist in order to define a new model–it
only needs to exist the first time you try to use the model. Thus,
you can use the generator to create a new model, and then reference the
model in the migration that creates the table.

On a related note, is there a clean way to call the scaffold generator
(or any generator, for that matter) from within a migration? The
scaffold generator won’t fly without a table. I know it’s not what the
migrations are supposed to be for, but it’d be damn nice to have :slight_smile:

4

I was incorrectly assuming you couldn’t generate a model for a table
that did not exist in the DB yet. I was thinking of what Alex was
talking about. Sometimes these things can get confusing (well for at
least me, haha). Thanks for the help.