Rails Data Migration Involving Two Records

Joalro Source

So I understand that in general data migrations are not best practice, so I would be happy to implement a rake task instead.

We previously had two models, User and Office without a formal relationship. The office a user belonged to was stored as a string with just the office name. In order to look up an office we had to use:


This clearly makes no sense, so I'm trying to run a migration to fix this. I'm adding the field "office_id" to the user so that we can use a proper belongs_to/has_one relationship.

I want to ensure that every user's office is copied over into the new format, and so in this case I'm using a data migration in the middle of the schema migration. We must also remove the old "office" field as otherwise it interferes with the belongs_to method that is generated.

This is what I have:

if not column_exists? :users, :office_id
  add_column :users, :office_id, :integer

  if defined? User
    User.all.each do |user|
      results = ActiveRecord::Base.connection.execute("select office FROM users where id = #{user.id}")
      office = Office.find_by_name(results.getvalue(0,0))
      if office.present?
        user.office_id = office.id

  if column_exists? :users, :office
    remove_column :users, :office

I had to use

ActiveRecord::Base.connection.execute("select office FROM users where id = #{user.id}")

Because the user.office was pulling from the belongs_to method generated, so this ensured we got the data from the database.

I saw this as one solution that would clean the code up a decent amount, except that I'm not sure how to pull the data from the office and set it to the user with sql.



answered 3 months ago Foo Bar Zoo #1

Why you are not creating a relationship based on string column?

class User
  belongs_to :office, foreign_key: :office_name, primary_key: :name

To be able to do this, you should rename the office column to something else or if you don't want to rename it, you can pick a different name for your association and keep using the office column like so;

class User
  belongs_to :user_office, foreign_key: :office, primary_key: :name

If you want to migrate the data anyway, I recommend you to write a rake task to run only once like below;

task set_users_office_id: :environment do
    UPDATE users
    SET office_id = (SELECT id FROM offices WHERE offices.name = users.office)

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