Creating a multi-tenant application using PostgreSQL’s schemas and Rails

Update Dec 5, 2011

Thanks to Brad Robertson and his team, there’s the Apartment gem. It’s very useful and does a lot of the heavy lifting.

However, if you’ll be tinkering with schemas, I strongly suggest knowing how it actually works. Familiarize yourself with Jerod Santo’s walkthrough , so you’ll know what the Apartment gem is more or less doing.

Update Aug 20, 2011 11:23 GMT+8

Someone created a blog post and walks though this whole process pretty well.

Update May 11, 2010 11:26 GMT+8

Since last night I’ve been able to get a method to work that creates a new schema and loads schema.rb into it. Not sure if what I’m doing is correct (seems to work fine, so far) but it’s a step closer at least. If there’s a better way please let me know.

module SchemaUtils
  def self.add_schema_to_path(schema)
    conn = ActiveRecord::Base.connection
    conn.execute "SET search_path TO #{schema}, #{conn.schema_search_path}"
  end

  def self.reset_search_path
    conn = ActiveRecord::Base.connection
    conn.execute "SET search_path TO #{conn.schema_search_path}"
  end

  def self.create_and_migrate_schema(schema_name)
    conn = ActiveRecord::Base.connection

    schemas = conn.select_values("select * from pg_namespace where nspname != 'information_schema' AND nspname NOT LIKE 'pg%'")

    if schemas.include?(schema_name)
      tables = conn.tables
      Rails.logger.info "#{schema_name} exists already with these tables #{tables.inspect}"
    else
      Rails.logger.info "About to create #{schema_name}"
      conn.execute "create schema #{schema_name}"
    end

    # Save the old search path so we can set it back at the end of this method
    old_search_path = conn.schema_search_path

    # Tried to set the search path like in the methods above (from Guy Naor)
    # [METHOD 1]: conn.execute "SET search_path TO #{schema_name}"
    # But the connection itself seems to remember the old search path.
    # When Rails executes a schema it first asks if the table it will load in already exists and if :force => true. 
    # If both true, it will drop the table and then load it. 
    # The problem is that in the METHOD 1 way of setting things, ActiveRecord::Base.connection.schema_search_path still returns $user,public.
    # That means that when Rails tries to load the schema, and asks if the tables exist, it searches for these tables in the public schema.
    # See line 655 in Rails 2.3.5 activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb
    # That's why I kept running into this error of the table existing when it didn't (in the newly created schema).
    # If used this way [METHOD 2], it works. ActiveRecord::Base.connection.schema_search_path returns the string we pass it.
    conn.schema_search_path = schema_name

    # Directly from databases.rake. 
    # In Rails 2.3.5 databases.rake can be found in railties/lib/tasks/databases.rake
    file = "#{Rails.root}/db/schema.rb"
    if File.exists?(file)
      Rails.logger.info "About to load the schema #{file}"
      load(file)
    else
      abort %{#{file} doesn't exist yet. It's possible that you just ran a migration!}
    end

    Rails.logger.info "About to set search path back to #{old_search_path}."
    conn.schema_search_path = old_search_path
  end
end