title: How can I prevent arbitrary _id insertion while using a collection transform?
body: <p>The meteor docs specifically call out using collection.allow's insert property as a means of preventing users from adding arbitrary _id fields to docs.  I recently added a transformation to one of my collections and discovered (after a frustrating hour of debugging) that transforms automatically add an _id field to a document.</p>
  
  <p>This creates a conflict with my insert callback, because now brand new documents will have an _id when the insert callback is run.  As far as I can tell, there's no way to tell whether the _id was set by transform or by the user, and the transform _id does not override the user supplied _id.</p>
  
  <p>Am I stuck with allowing users to supply arbitrary _id values if I want to use a transform on a Collection?</p>