Question

Background: This is for a Ruby on Rails web application. I have a background job which downloads recent posts from Facebook and inserts them into the database. I’m using hand-coded SQL for performance. The RDBMS is PostgreSQL (on Heroku).

The table is called “posts”. I have a unique index on the combination of posts.uid and posts.contact_id. In the SQL, I use a WHERE condition to filter out uid–contact_id combinations which are already in the table, but even so, I am getting the following error:

ActiveRecord::RecordNotUnique: PGError: ERROR: duplicate key value violates unique constraint "index_posts_on_uid_and_contact_id"

Without further ado, here is the (dynamic) SQL:

INSERT INTO posts 
(message,contact_id,date,uid,created_at,updated_at,source,is_event)
SELECT  
    t.msg,
    contacts.id, 
    t.date,
    t.uid,
    CURRENT_TIMESTAMP,
    CURRENT_TIMESTAMP,'facebook',
    FALSE
FROM contacts, 
(VALUES #{posts.map { |post| "(E'#{post['message'].escape_singles}','#
 {post['uid']}',DATE '#{format_date(post['time'])}',#{post['status_id']})" }.join(", ")}) AS      
 t (msg,fb_id,date,uid)
WHERE contacts.fb_id = t.fb_id 
AND (NOT EXISTS (
           SELECT * FROM posts 
           WHERE posts.uid = t.uid 
           AND posts.contact_id = contacts.id));

Shouldn’t the NOT EXISTS condition prevent this from happening?

Answer 1

Your select query is returning duplicate rows.