I am working on a Ruby on Rails app. We are using a PostgreSQL database.
There is a table named scores
with the following columns:
Column | Type
--------------+-----------------------
id | integer
value | double precision
ran_at | timestamp
active | boolean
build_id | bigint
metric_id | integer
platform_id | integer
mode_id | integer
machine_id | integer
higher_better | boolean
job_id | integer
variation_id | integer
step | character varying(255)
I need to add a sequence to job_id
(note: there is no model for job
).
How do I create this sequence?
Use CREATE SEQUENCE
:
CREATE SEQUENCE scores_job_id_seq; -- = default name for plain a serial
Then add a column default to scores.job_id
:
ALTER TABLE scores ALTER COLUMN job_id SET DEFAULT nextval('scores_job_id_seq');
If you want to bind the sequence to the column (so it is deleted when the column is deleted), also run:
ALTER SEQUENCE scores_job_id_seq OWNED BY scores.job_id;
All of this can be replaced with using the pseudo data type serial
for the column job_id
to begin with:
If your table already has rows, you may want to set the SEQUENCE
to the next highest value and fill in missing serial values in the table:
SELECT setval('scores_job_id_seq', COALESCE(max(job_id), 1)) FROM scores;
Optionally:
UPDATE scores
SET job_id = nextval('scores_job_id_seq')
WHERE job_id IS NULL;
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The only remaining difference, a serial
column is also set to NOT NULL
. You may or may not want that, too:
ALTER TABLE scores ALTER COLUMN job_id SET NOT NULL;
But you cannot just alter the type of an existing integer
:
ALTER TABLE scores ALTER job_id TYPE serial;
serial
is not an actual data type. It's just a notational convenience feature for CREATE TABLE
.
In Postgres 10 or later consider an IDENTITY
column:
So I figured out how to do this using ActiveRecord migrations on Ruby on Rails. I basically used Erwin's commands and help from this page and put them in the migration files. These are the steps:
1. In the terminal, type:
rails g migration CreateJobIdSequence
rails g migration AddJobIdSequenceToScores
2. Edit the migration files as follows:
20140709181616_create_job_id_sequence.rb :
class CreateJobIdSequence < ActiveRecord::Migration
def up
execute <<-SQL
CREATE SEQUENCE job_id_seq;
SQL
end
def down
execute <<-SQL
DROP SEQUENCE job_id_seq;
SQL
end
end
20140709182313_add_job_id_sequence_to_scores.rb :
class AddJobIdSequenceToScores < ActiveRecord::Migration
def up
execute <<-SQL
ALTER SEQUENCE job_id_seq OWNED BY scores.job_id;
ALTER TABLE scores ALTER COLUMN job_id SET DEFAULT nextval('job_id_seq');
SQL
end
def down
execute <<-SQL
ALTER SEQUENCE job_id_seq OWNED BY NONE;
ALTER TABLE scores ALTER COLUMN job_id SET NOT NULL;
SQL
end
end
3. Migrate the database. In the terminal type:
rake db:migrate
来源:https://stackoverflow.com/questions/24657693/creating-a-postgresql-sequence-to-a-field-which-is-not-the-id-of-the-record