Ruby on Rails

This is the stereotypical Rails approach to multi-tenancy.  The database contains, in the case of Shopify, the products of many stores (logan is a store).  The key to multi-tenant apps is to ensure that all requests are scoped to a specific store.  In order to accomplish this in a Rails app a before_filter is used in application_controller.rb.  It typically looks like this.

def current_store
  @current_store ||= Store.where("subdomain = ?", request.subdomains.last)
end

The request to retrieve products (current_store.products) will retrieve only those records that belong to the correct (per the url) store.

 

I'm assuming that you mean you want the user to be able to choose to use one or the other.  You'll probably get more readable code by adding a domain field to the stores table and have it default to your app's domain.  The current_store method could end up looking something like...

def current_store
  unless @current_store
     if request.domain == 'shopify.com'
       @current_store = Store.where("subdomain = ?", request.subdomains.last)
     else
        @current_store = Store.where("domain = ?", request.domain)
     end
  end
  @current_store
end

HTH,
Bill