Migration code to move to unique ids

gistfile1.rb

def generate_uuid(prefix, id)
  UUIDTools::UUID.sha1_create(UUIDTools::UUID_OID_NAMESPACE, "#{prefix}-#{id}")
end

class TransformToUuids < ActiveRecord::Migration
  def change
    models = [Contact, User, Version]

    models.each do |model|
      # Create new column for uuid
      add_column model.table_name, :uuid, :uuid



      # Create columns for associations
      model.reflect_on_all_associations(:belongs_to).each do |assoc|
        add_column model.table_name, "#{assoc.name}_uuid", :uuid # only works for associations where the column has not been renamed
      end
      model.reflect_on_all_associations(:has_and_belongs_to_many).each do |assoc|
        if assoc.class_name == model.to_s
          # self-referential, so add uuid for other side as well
          # assoc.table_name is not the name of the join table, that's assoc.options[:table_name]
          add_column assoc.options[:table_name], assoc.association_foreign_key.to_s.gsub("_id", "_uuid"), :uuid
        end
        # Add column to hold the foreign key to this object
        add_column assoc.options[:table_name], assoc.foreign_key.to_s.gsub("_id", "_uuid"), :uuid
      end



      # Update all values
      model.find_each do |obj|
        # Generate uuid values
        obj.update_column(:uuid, generate_uuid(model.to_s, obj.id))

        model.reflect_on_all_associations(:belongs_to).each do |assoc|
          # find correct classname to use
          if assoc.polymorphic?
            classname = obj.attributes["#{assoc.name}_type"]
          else
            classname = assoc.klass.to_s
          end
          obj.update_column("#{assoc.name}_uuid", generate_uuid(classname, obj.attributes["#{assoc.name}_id"]))
        end
        model.reflect_on_all_associations(:has_and_belongs_to_many).each do |assoc|
          if assoc.class_name == model.to_s
            # self-referential, update other side too
            execute("UPDATE #{assoc.options[:table_name]} SET #{assoc.association_foreign_key.to_s.gsub("_id", "_uuid")}=#{obj.uuid.quoted_id} WHERE #{assoc.association_foreign_key}=#{obj.id}")
          end
          execute("UPDATE #{assoc.options[:table_name]} SET #{assoc.foreign_key.to_s.gsub("_id", "_uuid")}=#{obj.uuid.quoted_id} WHERE #{assoc.foreign_key}=#{obj.id}")
        end
      end



      # Remove old columns and add index on these new foreign keys
      remove_column model.table_name, :id
      rename_column model.table_name, :uuid, :id
      # Set column as primary key
      # WARNING: is not reflected in db/schema.rb! Add there manually
      # Related to https://github.com/jashmenn/activeuuid/pull/22, but that probably wont work for rails 4
      execute("ALTER TABLE #{model.table_name} ADD PRIMARY KEY (id)")
      # change_column model.table_name, :id, :uuid, primary_key: true # This does not work :(


      model.reflect_on_all_associations(:belongs_to).each do |assoc|
        remove_column model.table_name, "#{assoc.name}_id"
        rename_column model.table_name, "#{assoc.name}_uuid", "#{assoc.name}_id"
        add_index model.table_name, "#{assoc.name}_id"
      end


      model.reflect_on_all_associations(:has_and_belongs_to_many).each do |assoc|
        # Drop the index because we'll get an error about duplicate values for the non-composite index after removing one of the component columns
        remove_index assoc.options[:table_name], [assoc.foreign_key, assoc.association_foreign_key]
        if assoc.class_name == model.to_s
          # self-referential
          remove_index assoc.options[:table_name], [assoc.association_foreign_key, assoc.foreign_key]
          remove_column assoc.options[:table_name], assoc.association_foreign_key
          rename_column assoc.options[:table_name], assoc.association_foreign_key.to_s.gsub("_id", "_uuid"), assoc.association_foreign_key
        end
        remove_column assoc.options[:table_name], assoc.foreign_key
        rename_column assoc.options[:table_name], assoc.foreign_key.to_s.gsub("_id", "_uuid"), assoc.foreign_key
        # Add composite index
        add_index assoc.options[:table_name], [assoc.foreign_key, assoc.association_foreign_key], name: "index_#{assoc.table_name}_on_#{assoc.foreign_key}_and_#{assoc.association_foreign_key}", unique: true, using: :btree
      end
                                                                                                                       
    end

  end
end