Question

I have found the “pure SQL” answers to this question. Is there a way, in Rails, to reset the id field for a specific table?
Why do I want to do this? Because I have tables with constantly moving data – rarely more than 100 rows, but always different. It is up to 25k now, and there’s just no point in that. I intend on using a scheduler internal to the Rails app (rufus-scheduler) to run the id field reset monthly or so.

Answer 1

I came out with a solution based on hgimenez’s answer and this other one.

Since I usually work with either Sqlite or PostgreSQL, I’ve only developed for those; but extending it to, say MySQL, shouldn’t be too troublesome.

Put this inside lib/ and require it on an initializer:

# lib/active_record/add_reset_pk_sequence_to_base.rb
module ActiveRecord
  class Base
    def self.reset_pk_sequence
      case ActiveRecord::Base.connection.adapter_name
      when 'SQLite'
        new_max = maximum(primary_key) || 0
        update_seq_sql = "update sqlite_sequence set seq = #{new_max} where name = '#{table_name}';"
        ActiveRecord::Base.connection.execute(update_seq_sql)
      when 'PostgreSQL'
        ActiveRecord::Base.connection.reset_pk_sequence!(table_name)
      else
        raise "Task not implemented for this DB adapter"
      end
    end     
  end
end

Usage:

Client.count # 10
Client.destroy_all
Client.reset_pk_sequence
Client.create(:name => 'Peter') # this client will have id=1

EDIT: Since the most usual case in which you will want to do this is after clearing a database table, I recommend giving a look to database_cleaner. It handles the ID resetting automatically. You can tell it to delete just selected tables like this:

DatabaseCleaner.clean_with(:truncation, :only => %w[clients employees])