Raised when unknown attributes are supplied via mass assignment.

class Person
  include ActiveModel::AttributeAssignment
  include ActiveModel::Validations
end

person = Person.new
person.assign_attributes(name: 'Gorby')
# => ActiveModel::UnknownAttributeError: unknown attribute 'name' for Person.
Methods
N
Attributes
[R] attribute
[R] record
Class Public methods
new(record, attribute)

Source: show | on GitHub 

# File activemodel/lib/active_model/errors.rb, line 511
def initialize(record, attribute)
  @record = record
  @attribute = attribute
  super("unknown attribute '#{attribute}' for #{@record.class}.")
end