anchor.diff:  ----
   
   Each Persistent backend needs to know how to translate the relevant values into
  -something the database can understand. However, it would be awkward do have to
  +something the database can understand. However, it would be awkward to have to
   express all of our data simply in terms of these basic types. The next layer is
   the +PersistField+ typeclass, which defines how an arbitrary Haskell datatype
   can be marshaled to and from a +PersistValue+. A +PersistField+ correlates to a
  
positive.code: 
  === Custom Fields
  
  Occassionally, you will want to define a custom field to be used in your
  datastore. The most common case is an enumeration, such as employment status.
  For this, Persistent provides a helper Template Haskell function:
  
  
negative.code:   The default type-safe persistent API does not support joins, allowing support for a
    wider number of storage layers.
    Joins and other SQL specific functionality can be achieved through using
    a raw SQL layer (with very little type safety).
    An additional library, link:http://hackage.haskell.org/package/esqueleto[Esqueleto],
    builds on top of the Persistent data model, adding type-safe joins and SQL functionality.
  
  