Creating custom Primary Keys in Rails
Published on: 10 Mar 2012
Having recently discovered Ruby/Rails, I decided to explore Rail's integration with legacy databases so that I could write one-off scripts to manipulate our databases with it (while learning ruby/rails at the same time!). One of the first things I ran into was the need to generate custom primary keys (can't use auto-incrementing keys!). A little bit of googling around led me to this simple solution
require 'active_record/connection_adapters/abstract_mysql_adapter'
require 'mysql2'
module ActiveRecord
module ConnectionAdapters
class Mysql2Adapter < AbstractMysqlAdapter
@tn
def prefetch_primary_key?(table_name)
@tn = table_name
true
end
def next_sequence_value(sequence_name)
id = select_value("SELECT id from #{@tn} ORDER BY id DESC LIMIT 1",'id')
id+1;
end
end
end
end
Defining the prefetch_primary_key function tells the rails framework to call next_sequence_value to get the primary key. You can set the value of sequence_name in your models using set_sequence_name. Since I was essentially 'implementing' auto-increment I simply decided to store the table_name and use that instead of setting the sequence_name in all my models. Obviously this is not a very robust/production ready solution/thread safe etc., but for those who are just 'hacking' around, this is a handy technique if you need to generate your own primary key ids.
And a quick tip to see what the ORM is generating if you are simply exploring your models -
logger = Logger.new(STDOUT) ActiveRecord::Base.logger = logger ActiveResource::Base.logger = logger
This will conviniently log all the SQL statements that your ORM is issuing.Happy Hacking!
Ref: http://ar.rubyonrails.org/classes/ActiveRecord/Base.html
