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

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.

Answer 2

Apparently it is still not possible to get in create() a field (not id) generated in autoincrement at database level by SEQUENCE.
I worked around the problem
in my case: PostgreSQL AUTOINCREMENT by SEQUENCE
with an after_create callback
Migration
class CreateFoo < ActiveRecord::Migration[7.0]
def change
create_table :foo do |t|
t.integer :autoinc_field
end
execute "CREATE SEQUENCE table_name_seq OWNED BY table_name.autoinc_field INCREMENT BY 1 START WITH 100000"
execute "ALTER TABLE table_name ALTER COLUMN autoinc_field SET DEFAULT nextval('table_name_seq');"
end
end
Model
class Foo < ApplicationRecord
after_create :reload
end
Result
>> Foo.create!.autoinc_field
=> 100000