September 25, 2012

[RAILS] PG::Error: ERROR: relation does not exist

I recently decided to try and migrate one of my rails apps to Heroku. It was my first Heroku app, and I'm learning a lot in the process. One of the first gotchas I ran into was that I needed to convert my app to use postgres since that's what Heroku seems to work best with and has the most documentation around.

I figured I would try and get this working locally before messing around with getting it to work on the Heroku server. First I had to modify my local config/database.yml file to connect to the postgres server. (Hint: Heroku will overwrite this file so your local copy doesn't matter over on the Heroku server)

  adapter: postgresql
  encoding: unicode
  database: pg_tcj
  username: postgres
  pool: 5
  host: localhost

Next I wanted to load my database schema into the new postgres database, something I've done a thousand times with other database connectors:

rake db:schema:load

To my surprise, I got the following error:

PG::Error: ERROR:  relation "pages" does not exist
LINE 4:              WHERE a.attrelid = '"pages"'::regclass
:             SELECT a.attname, format_type(a.atttypid, a.atttypmod), d.adsrc, a.attnotnull
              FROM pg_attribute a LEFT JOIN pg_attrdef d
                ON a.attrelid = d.adrelid AND a.attnum = d.adnum
             WHERE a.attrelid = '"pages"'::regclass
               AND a.attnum > 0 AND NOT a.attisdropped
             ORDER BY a.attnum

I spent more time than I'd like to admin trying to figure out what was going on. In my web searches I consistently found the same two suggestions:

  1. Simply run rake db:migrate:reset (or similar). Obviously this didn't work since I was getting the error while trying to run rake db:migrate or rake db:schema:load!

  2. The other suggestion was that something in the rails environment was trying to use a model before it was created. ActiveAdmin is a gem that came up frequently. This is was probably the more likely scenario for my code.

Armed with that knowledge I tried to figure out what could possibly be trying to use my pages model before it was created. One line in the stack trace I got from rake db:schema:load --trace stuck out to me:

/home/eric/dev/rails/tcj/config/routes.rb:56:in `block in <top (required)>'
/home/eric/.rvm/gems/ruby-1.9.3-p194/gems/actionpack-3.2.1/lib/action_dispatch/routing/route_set.rb:272:in `instance_exec'
/home/eric/.rvm/gems/ruby-1.9.3-p194/gems/actionpack-3.2.1/lib/action_dispatch/routing/route_set.rb:272:in `eval_block'
/home/eric/.rvm/gems/ruby-1.9.3-p194/gems/actionpack-3.2.1/lib/action_dispatch/routing/route_set.rb:249:in `draw'

Ah ha! Routes is a pretty good suspect for something that runs early in the rails loading process. In my routes.rb file, I have the following:

Page.all.reject{|x| x.slug.blank? }.each do |page|
  match page.slug => 'pages#show', :id =>

Bingo! In my application, which is a sort of pseudo CMS, I'm setting up routes for custom slugs on the pages that are added. Here I was trying to reference the page model before it was ever created.

Simply commenting out the three lines above temporarily solved the problem.

Hopefully this saves someone some time someday!