Ruby on Rails Migration - Create New Database Schema

Question

Hi

I have a migration that runs an SQL script to create a new Postgres schema. When creating a new database in Postgres by default it creates a schema called 'public', which is the main schema we use. The migration to create the new database schema seems to be working fine, however the problem occurs after the migration has run, when rails tries to update the 'schema_info' table that it relies on it says that it does not exist, as if it is looking for it in the new database schema and not the default 'public' schema where the table actually is.

Does anybody know how I can tell rails to look at the 'public' schema for this table?

Example of SQL being executed: ~

CREATE SCHEMA new_schema;
COMMENT ON SCHEMA new_schema IS 'this is the new Postgres database schema to sit along side the "public" schema';
-- various tables, triggers and functions created in new_schema

Error being thrown: ~

RuntimeError: ERROR C42P01  Mrelation "schema_info" does not exist
L221    RRangeVarGetRelid: UPDATE schema_info SET version = ??

Thanks for your help

Chris Knight

Answer 1

I'm not sure I understand what you're asking exactly, but, rake will be expecting to update the version of the Rails schema into the schema_info table. Check your database.yml config file, this is where rake will be looking to find the table to update.

Is it a possibility that you are migrating to a new Postgres schema and rake is still pointing to the old one? I'm not sure then that a standard Rails migration is what you need. It might be best to create your own rake task instead.

Edit: If you're referencing two different databases or Postgres schemas, Rails doesn't support this in standard migrations. Rails assumes one database, so migrations from one database to another is usually not possible. When you run "rake db:migrate" it actually looks at the RAILS_ENV environment variable to find the correct entry in database.yml. If rake starts the migration looking at the "development" environment and database config from database.yml, it will expect to update to this environment at the end of the migration.

So, you'll probably need to do this from outside the Rails stack as you can't reference two databases at the same time within Rails. There are attempts at plugins to allow this, but they're majorly hacky and don't work properly.

Answer 2

Well that depends what your migration looks like, what your database.yml looks like and what exactly you are trying to attempt. Anyway more information is needed change the names if you have to and post an example database.yml and the migration. does the migration change the search_path for the adapter for example ?

But know that in general rails and postgresql schemas don't work well together (yet?).

There are a few places which have problems. Try and build and app that uses only one pg database with 2 non-default schemas one for dev and one for test and tell me about it. (from thefollowing I can already tell you that you will get burned)

Maybe it was fixed since the last time I played with it but when I see http://rails.lighthouseapp.com/projects/8994/tickets/390-postgres-adapter-quotes-table-name-breaks-when-non-default-schema-is-used or this http://rails.lighthouseapp.com/projects/8994/tickets/918-postgresql-tables-not-generating-correct-schema-list or this in postgresql_adapter.rb

  # Drops a PostgreSQL database
  #
  # Example:
  #   drop_database 'matt_development'
  def drop_database(name) #:nodoc:
    execute "DROP DATABASE IF EXISTS #{name}"
  end

(yes this is wrong if you use the same database with different schemas for both dev and test, this would drop both databases each time you run the unit tests !)

I actually started writing patches. the first one was for the indexes methods in the adapter which didn't care about the search_path ending up with duplicated indexes in some conditions, then I started getting hurt by the rest and ended up abandonning the idea of using schemas: I wanted to get my app done and I didn't have the extra time needed to fix the problems I had using schemas.