Why does activerecord not populate an auto-incrementing column in the item returned from a create?

Question

Why does rails not populate an auto-incrementing column in the item returned from a create? Is there a better way to do this?

In rails, when you do a = Foo.create then a.id is populated

But if you have a field that was created via

def up   
  execute "ALTER TABLE my_table ADD COLUMN my_auto_incrementing_column INTEGER AUTO_INCREMENT not null UNIQUE KEY;"
end

Then that field does not appear when you use create. You have to use a reload also.

a = Foo.create
a.id # not nil
a.my_auto_incrementing_column # nil
a.reload
a.my_auto_incrementing_column # is now populated

Version information:

$ ruby -v
ruby 1.9.3p484 (2013-11-22 revision 43786) [x86_64-darwin14.5.0]
$ bundle exec rails -v
Rails 3.2.12
$ mysql --version
mysql  Ver 14.14 Distrib 5.6.26, for osx10.10 (x86_64) using  EditLine wrapper

Some background:

This code is being applied to a large existing in-production rails codebase that requires that all id fields be UUIDs. The auto_increment column is not a primary key, because it was added after we had discovered that a new external integration partner could not handle using our existing long unique identifiers (UUIDs).

We are working hard to update our version of ruby but we don't want to wait for that as a solution to this problem. Also, after reading changelogs in activerecord, I still don't have proof that any future version of ruby/rails will contain a bugfix for this problem.

The code which I want to improve:

class Foo < ActiveRecord::Base
  has_one :object_containing_auto_incrementing_column

  def my_method
    if self.object_containing_auto_incrementing_column.nil?
      self.object_containing_auto_incrementing_column = ObjectContainingAutoIncrementingColumn.create(owner: self)
      self.object_containing_auto_incrementing_column.reload
    end
    self.object_containing_auto_incrementing_column.my_auto_incrementing_column
  end
end

Answer 1

After looking at the source code it does not appear that ActiveRecord tries to populate auto-incrementing columns. It only assigns the value that is returned by the INSERT statement to the #id attribute and nothing else.

 # ActiveRecord:: Persistence::ClassMethods
 def create
 # ...
   self.id ||= new_id if self.class.primary_key
 # ...
 end

If you want to populate my_auto_incrementing_column without hitting the DB twice, I think there is no way around patching ActiveRecord itself.

Have a look at how the insert method is implemented:

  # Returns the last auto-generated ID from the affected table.
  #
  # +id_value+ will be returned unless the value is nil, in
  # which case the database will attempt to calculate the last inserted
  # id and return that value.
  #
  # If the next id was calculated in advance (as in Oracle), it should be
  # passed in as +id_value+.
  def insert(arel, name = nil, pk = nil, id_value = nil, sequence_name = nil, binds = [])
    sql, binds = sql_for_insert(to_sql(arel, binds), pk, id_value, sequence_name, binds)
    value      = exec_insert(sql, name, binds)
    id_value || last_inserted_id(value)
  end

There might not be any trivial way to change the current API to populate your field in question.