find or create by - ruby-on-rails

Question

i have added the following setter/getter methods to my model, though whenever i try and save the form i am getting an error about mass assignment. from my understanding how this should work is, that if the opponent_name cant be found it will add a entry to the database
def opponent_name
opponent.try(:name)
end
def opponent_name(name)
self.opponent = Opponent.find_or_create_by_name(name) if name.present?
end
here is the error from the console log
Started POST "/events" for 127.0.0.1 at 2013-03-26 19:07:26 +1100
Processing by EventsController#create as JS
Parameters: {"utf8"=>"✓", "authenticity_token"=>"h7OrLKeDL/9KmZeGZeO+QTWHtlUdOlaMqnoMGhYaDUU=", "event"=>{"datetime(3i)"=>"2", "datetime(2i)"=>"3", "datetime(1i)"=>"2013", "datetime(4i)"=>"00", "datetime(5i)"=>"00", "event"=>"1", "location_id"=>"7", "duration"=>"30", "arrival_time"=>"30", "opponent_name"=>"Test", "home_or_away"=>"Home"}, "commit"=>"Create Event"}
User Load (0.9ms) SELECT "users".* FROM "users" WHERE "users"."id" = 4 LIMIT 1
Completed 500 Internal Server Error in 14ms
ActiveModel::MassAssignmentSecurity::Error (Can't mass-assign protected attributes: opponent_name):
app/controllers/application_controller.rb:22:in `catch_not_found'
opponent model
class Opponent < ActiveRecord::Base
has_many :events
belongs_to :team
attr_accessible :name, :team_id
validates :name, :presence => true
end
event model
class Event < ActiveRecord::Base
include PublicActivity::Model
tracked
belongs_to :location
belongs_to :opponent
belongs_to :team
belongs_to :result
has_many :availabilities, :dependent => :destroy
def opponent_name
opponent.try(:name)
end
def opponent_name(name)
self.opponent = Opponent.find_or_create_by_name(name) if name.present?
end
attr_accessible :location_id, :user_id, :datetime, :score_for, :score_against, :event,
:team_id, :home_or_away, :arrival_time, :duration, :selected_players, :date, :time, :result_id
validates :event, :location_id, :team_id, :presence => true
validates_numericality_of :team_id, :only_integer => true, :greater_than_or_equal_to =>0, :message => " needs to be set, please contact your Administrator"
#validates_numericality_of :arrival_time, :only_integer =>true, :greater_than_or_equal_to =>0, :message => " must be greater than 0 minutes", :allow_blank => true
validates :home_or_away, :presence => true, :if => :event == 1
validates :score_for, :presence => true, :if => :score_against
validates :score_against, :presence => true, :if => :score_for
EVENT_TYPES = [['Game', 1], ['Training', 2], ['Social Event', 3]]
HOME_OR_AWAY = [:Home, :Away]
end

Answer 1

Have a look at ActiveModel::MassAssignmentSecurity::ClassMethods.
I beleive you have to add following statement in your Opponent model
attr_accessible :opponent_name

Answer 2

Try to put in your Event model
attr_accessible :opponent_name
It should clear the error then
Edit:
Just updating an answer, but all credits goes to Mischa for this edit.
The problem may be that you defined your setter like def
opponent_name(name) while it should be def opponent_name=(name)

Answer 3

If opponent_name is a field in your model's database table, then Rails has already defined getters and setters for that attribute. All you need to do is add
attr_accessible :opponent_name

Answer 4

Reference #Mischa
The problem may be that you defined your setter like def opponent_name(name) while it should be def opponent_name=(name). When you do this and attr_accessible :opponent_name, it may work. Not sure why it errors out on that line. Seems unrelated to the error.