title: Retrieve a Customer's Subscription ID from Stripe
body: <p>In Rails, I'm attempting to implement a controller action which will allow a customer to change the plan they are subscribed to. Per the Stripe documentation, this can be done as follows:</p>
  
  <pre><code>customer = Stripe::Customer.retrieve("CUSTOMER_ID")
  subscription = customer.subscriptions.retrieve("SUBSCRIPTION_ID")
  subscription.plan = "premium_monthly"
  subscription.save
  </code></pre>
  
  <p>I have each customer's ID saved in the user table in my Database, but I'm stuck on how to retrieve a customer's subscription ID given their customer ID. Each customer in this project will only have one subscription, how could I go about pulling this given their customer ID?</p>