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

Question

Stuff I've already figured out

I'm learning how to create a multi-tenant application in Rails that serves data from different schemas based on what domain or subdomain is used to view the application.

I already have a few concerns answered:

1. How can you get subdomain-fu to work with domains as well? Here's someone that asked the same question which leads you to this blog .
2. What database, and how will it be structured? Here's an excellent talk by Guy Naor , and good question about PostgreSQL and schemas .
3. I already know my schemas will all have the same structure. They will differ in the data they hold. So, how can you run migrations for all schemas? Here's an answer .

Those three points cover a lot of the general stuff I need to know. However, in the next steps I seem to have many ways of implementing things. I'm hoping that there's a better, easier way.

Finally, to my question

When a new user signs up, I can easily create the schema. However, what would be the best and easiest way to load the structure that the rest of the schemas already have? Here are some questions/scenarios that might give you a better idea.

1. Should I pass it on to a shell script that dumps the public schema into a temporary one, and imports it back to my main database (pretty much like what Guy Naor says in his video)? Here's a quick summary/script I got from the helpful #postgres on freenode . While this will probably work, I'm gonna have to do a lot of stuff outside of Rails, which makes me a bit uncomfortable.. which also brings me to the next question.
2. Is there a way to do this straight from Ruby on Rails ? Like create a PostgreSQL schema, then just load the Rails database schema (schema.rb - I know, it's confusing) into that PostgreSQL schema.
3. Is there a gem/plugin that has these things already? Methods like "create_pg_schema_and_load_rails_schema(the_new_schema_name)". If there's none, I'll probably work at making one, but I'm doubtful about how well tested it'll be with all the moving parts (especially if I end up using a shell script to create and manage new PostgreSQL schemas).

Thanks, and I hope that wasn't too long!

Answer 1

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

Answer 2

Change line 38 to:

conn.schema_search_path = "#{schema_name}, #{old_search_path}"

I presume that postgres is trying to lookup existing table names when loading schema.rb and since you've set the search_path to only contain the new schema, it fails. This of course, is presuming you still have the public schema in your database.

Hope that helps.

Answer 3

Is there a gem/plugin that has these things already?

pg_power provides this functionality to create/drop PostgreSQL schemas in migration, like this:

def change
  # Create schema
  create_schema 'demography'

  # Create new table in specific schema
  create_table "countries", :schema => "demography" do |t|
    # columns goes here
  end

  # Drop schema
  drop_schema 'politics'
end

Also it takes care about correctly dumping schemas into schema.rb file.