active_record_base_fix.rb

# Patch for rails 3.1 below
# For master s/connection_pool/connection.schema_cache/

module ActiveRecord
  class Base
    class << self
      ...
      def columns
        if defined?(@primary_key)
          if connection_pool.columns.has_key?(table_name) && connection_pool.primary_keys[table_name].nil?
            connection_pool.columns.delete table_name
            connection_pool.columns_hash.delete table_name
          end
          connection_pool.primary_keys[table_name] ||= primary_key
        end

        connection_pool.columns[table_name]
      end
      ...
    end
  end
end

failing_primary_key_test.rb

## must run activerecord's `rake postgresql:build_databases`
require 'active_record'

ActiveRecord::Base.establish_connection(:adapter => 'postgresql', :database => 'activerecord_unittest')

ActiveRecord::Schema.define do
  execute <<_SQL
    CREATE SEQUENCE foo_seq;
    CREATE TABLE posts(
      foo int NOT NULL DEFAULT nextval( 'foo_seq'::regclass )
    );
_SQL
end

class Post < ActiveRecord::Base
  self.primary_key = :foo
end

begin
  p Post.create
  p Post.primary_key
ensure
  ActiveRecord::Base.connection.execute <<_SQL
    DROP TABLE posts;
    DROP SEQUENCE foo_seq;
_SQL
end