Raised when an error occurred while doing a mass assignment to an attribute through the attributes= method. The exception has an attribute property that is the name of the offending attribute.

Methods
N
Attributes
[R] attribute
[R] exception
Class Public methods
new(message, exception, attribute)

Source: show | on GitHub 

# File activerecord/lib/active_record/errors.rb, line 166
def initialize(message, exception, attribute)
  @exception = exception
  @attribute = attribute
  @message = message
end