title: Salesforce Cross Object SOQL Query
body: <p>I have a custom object called 'Billings'.  On that object there is a custom lookup field to the Opportunity.  I'm trying to query all the 'Billings' records associated with a particular Opportunity. I get "Unknown error parsing query".</p>
  
  <p>SELECT Id, StageName,
      ( SELECT Name, Email FROM Billings__C )
      FROM Opportunity WHERE Opportunity ID = '0011000002mfTil'</p>