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Editorial Team
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Asked: 2026-06-12T00:17:58+00:00

The Rails migration guide , suggests you create a faux model inside the migration,

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The Rails migration guide, suggests you create a faux model inside the migration, if you need to operate on the data from the database, like:

class AddFuzzToProduct < ActiveRecord::Migration
  class Product < ActiveRecord::Base
  end

  def change
    add_column :products, :fuzz, :string
    Product.reset_column_information
    Product.all.each do |product|
      product.update_attributes!(:fuzz => 'fuzzy')
    end
  end
end

The thing is, inside the AddFuzzToProduct class, the name of the Product model will be AddFuzzToProduct::Product. I have the following situation:

class RemoveFirstNameFromStudentProfile < ActiveRecord::Migration

  class StudentProfile < ActiveRecord::Base
    has_one :user,:as => :profile
  end

  class User < ActiveRecord::Base
    belongs_to :profile,:polymorphic => true,:dependent => :destroy
  end

  def up
    StudentProfile.all.each do |student|
       # I need to do some work on the user object as well as on the student object
       user = student.user
       ... # do some stuff on the user object
    end
  end

end

The thing is, inside the each block for the student profile, user is nil. After I activated the logger, I can see that Rails is trying to do the following query:

 RemoveFirstNameFromStudentProfile::User Load (0.8ms)  SELECT "users".* FROM "users" WHERE "users"."profile_id" = 30 AND "users"."profile_type" = 'RemoveFirstNameFromStudentProfile::StudentProfile' LIMIT 1

Of course, this can be fixed by moving the User and StudentProfile up one level, like in:

  class StudentProfile < ActiveRecord::Base
    has_one :user,:as => :profile
  end

  class User < ActiveRecord::Base
    belongs_to :profile,:polymorphic => true,:dependent => :destroy
  end

  class RemoveFirstNameFromStudentProfile < ActiveRecord::Migration
    def up
      StudentProfile.all.each do |student|
        ...
      end
    end
  end

My question is: can moving the definitions of the faux models outside of the declaration of the migration cause any problems for me? Is there something I’m missing here? Why did the guys from the Rails team declare them inside the migration class?

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1 Answer

  1. Editorial Team

    No it will not cause any problem for you as you are not adding any new columns and updating it.

    Rails team has declared it inside the migration because they were adding a new column and then updating it but if Model is outside and it will try to validate that column which is not possible as it was not there, it was just added in migration. Due to that reason they have created local models in migration only for more read this Using Models in your migrations

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