apartment.rb

Apartment.configure do |config|
  config.excluded_models = ['Company']
  config.persistent_schemas = ['hstore']
  config.database_names = lambda{ Company.select(:name).map(&:name) }
end

create_employees.rb

class CreateEmployees < ActiveRecord::Migration
  def change
    create_table :employees do |t|
      t.integer :company_id
      t.string :name
      t.hstore :data

      t.timestamps
    end
  end
end

setup_hstore.rb

class SetupHstore < ActiveRecord::Migration
  def self.up
    execute "CREATE SCHEMA hstore"
    execute "CREATE EXTENSION IF NOT EXISTS hstore SCHEMA hstore"
  end

  def self.down
    execute "DROP EXTENSION IF EXISTS hstore"
    execute "DROP SCHEMA hstore"
  end
end

stack trace.txt

$ rake db:create
$ rake db:migrate --trace
** Invoke db:migrate (first_time)
** Invoke environment (first_time)
** Execute environment
** Invoke db:load_config (first_time)
** Execute db:load_config
** Execute db:migrate
==  SetupHstore: migrating ====================================================
-- execute("CREATE SCHEMA hstore")
   -> 0.0008s
-- execute("CREATE EXTENSION IF NOT EXISTS hstore SCHEMA hstore")
WARNING:  => is deprecated as an operator name
DETAIL:  This name may be disallowed altogether in future versions of PostgreSQL.
   -> 0.0209s
==  SetupHstore: migrated (0.0220s) ===========================================

==  CreateCompanies: migrating ================================================
-- create_table(:companies)
NOTICE:  CREATE TABLE will create implicit sequence "companies_id_seq" for serial column "companies.id"
NOTICE:  CREATE TABLE / PRIMARY KEY will create implicit index "companies_pkey" for table "companies"
   -> 0.0715s
==  CreateCompanies: migrated (0.0717s) =======================================

==  CreateEmployees: migrating ================================================
-- create_table(:employees)
NOTICE:  CREATE TABLE will create implicit sequence "employees_id_seq" for serial column "employees.id"
rake aborted!
An error has occurred, this and all later migrations canceled:

PG::Error: ERROR:  type "hstore" does not exist
LINE 1: ...d" integer, "name" character varying(255), "data" hstore, "c...
                                                             ^
: CREATE TABLE "employees" ("id" serial primary key, "company_id" integer, "name" character varying(255), "data" hstore, "created_at" timestamp NOT NULL, "updated_at" timestamp NOT NULL)