Example migration migrating from integer IDs as a primary key to uuid IDs while maintaining existing associations.

migration_from_id_to_uuid.rb

class ChangeIdIntToUuid < ActiveRecord::Migration
  def up
    remove_foreign_key "business_debts", "loan_applications"
    remove_foreign_key "guarantors", "loan_applications"
    remove_foreign_key "guarantors", "users"

    table_names = [
      :loan_applications,
      :business_debts,
      :case_owners,
      :guarantors,
      :document_collections,
      :users,
      :legacy_users,
    ]

    update_primary_keys(table_names)
    update_has_many_associations(table_names)
    remove_temporary_columns(table_names)

    add_foreign_key "business_debts", "loan_applications"
    add_foreign_key "guarantors", "loan_applications"
    add_foreign_key "guarantors", "users"

    add_index :guarantors, :loan_application_id
    add_index :guarantors, :user_id
    add_index :business_debts, :loan_application_id
    add_index(
      :document_collections,
      [:documentable_type, :documentable_id, :name, :year],
      unique: true,
      name: "index_document_collections_by_type",
    )
  end

  def down
    raise ActiveRecord::IrreversibleMigration.new(
      "This cannot be undone because it's destructive of the original IDs."
    )
  end

  def update_primary_keys(table_names)
    table_names.each do |table|
      update_primary_key_to_uuid(table)
    end
  end

  def update_primary_key_to_uuid(table_name)
    add_column table_name, :temp_uuid, :uuid, default: "uuid_generate_v4()"

    execute "ALTER TABLE #{table_name} DROP CONSTRAINT #{table_name}_pkey;"

    rename_column table_name, :id, "#{original_prefix}id"
    rename_column table_name, :temp_uuid, :id

    execute "ALTER TABLE #{table_name} ADD PRIMARY KEY (id);"
  end

  def update_has_many_associations(table_names)
    has_many_associations = table_names.map do |table_name|
      model_class = constantize_table(table_name)
      model_class.reflect_on_all_associations(:has_many)
    end.flatten

    has_many_associations = has_many_associations.select do |association|
      association.instance_of?(ActiveRecord::Reflection::HasManyReflection)
    end

    has_many_associations = has_many_associations.uniq do |association|
      [association.class_name, association.foreign_key]
    end

    has_many_associations.each do |association|
      update_has_many_association(association)
    end
  end

  def update_has_many_association(association)
    parent_class = association.active_record
    model_class = association.class_name.constantize
    table_name = model_class.table_name
    foreign_key = association.foreign_key
    original_foreign_key = "#{original_prefix}#{foreign_key}"

    rename_column table_name, foreign_key, original_foreign_key
    add_column table_name, foreign_key, :uuid

    parent_class.reset_column_information
    model_class.reset_column_information

    update_all_model_associations(
      model_class: model_class,
      parent_class: parent_class,
      original_foreign_key: original_foreign_key,
      foreign_key: foreign_key
    )
  end

  def update_all_model_associations(
    model_class:,
    parent_class:,
    original_foreign_key:,
    foreign_key:
  )
    model_class.find_each do |instance|
      association_id = instance.send(original_foreign_key)
      parent_model = parent_class.find_by(
        "#{original_prefix}id": association_id
      )
      if parent_model
        instance.update_column(foreign_key, parent_model.id)
      end
    end
  end

  def remove_temporary_columns(table_names)
    table_names.each do |table_name|
      model_class = constantize_table(table_name)
      columns_to_remove = model_class.column_names.select do |column|
        column.start_with?(original_prefix)
      end

      remove_columns table_name, *columns_to_remove
    end
  end

  def constantize_table(table_name)
    table_name.to_s.singularize.camelize.constantize
  end

  def original_prefix
    "original_"
  end
end