code: function hasFields(model, fields, exists)
  {
    if ( isArray( fields ) )
    {
      for (var i = 0; i < fields.length; i++)
      {
        if ( !exists( model[ fields[ i ] ] ) )
        {
          return false;
        }
      }
  
      return true;
    }
    else // isString( fields )
    {
      return exists( model[ fields ] );
    }
  }
docs: Determines whether the given model has the given fields