How to prevent duplicate records in a Join Table - ruby-on-rails

Question

I'm quite new to Ruby and Rails so please bear with me.
I have two models Player, and Reward joined via a has_many through relationship as below. My Player model has an attribute points. As a player accrues points they get rewards. What I want to do is put a method on the Player model that will run before update and give the appropriate reward(s) for the points they have like below.
However I want to do it in such a way that if the Player already has the reward it won't be duplicated, nor cause an error.
class Player < ActiveRecord::Base
has_many :earned_rewards, -> { extending FirstOrBuild }
has_many :rewards, :through => :earned_rewards
before_update :assign_rewards, :if => :points_changed?
def assign_rewards
case self.points
when 1000
self.rewards << Reward.find_by(:name => "Bronze")
when 2000
self.rewards << Reward.find_by(:name => "Silver")
end
end
class Reward < ActiveRecord::Base
has_many :earned_rewards
has_many :players, :through => :earned_rewards
end
class EarnedReward < ActiveRecord::Base
belongs_to :player
belongs_to :reward
validates_uniqueness_of :reward_id, :scope => [:reward_id, :player_id]
end
module FirstOrBuild
def first_or_build(attributes = nil, options = {}, &block)
first || scoping{ proxy_association.build(attributes, &block) }
end
end

Answer 1

You should validate it in db also
Add follwing in migrate file-
add_index :earnedrewards, [:reward_id, :player_id], unique: true

Answer 2

EDIT:
I've realised that my previous answer wouldn't work, as the new Reward is not associated to the parent Player model.
In order to correctly associate the two, you need to use build.
See https://stackoverflow.com/a/18724458/4073431
In short, we only want to build if it doesn't already exist, so we call first || build
Specifically:
class Player < ActiveRecord::Base
has_many :earned_rewards
has_many :rewards, -> { extending FirstOrBuild }, :through => :earned_rewards
before_update :assign_rewards, :if => :points_changed?
def assign_rewards
case self.points
when 1000...2000
self.rewards.where(:name => "Bronze").first_or_build
when 2000...3000
self.rewards.where(:name => "Silver").first_or_build
end
end
class Reward < ActiveRecord::Base
has_many :earned_rewards
has_many :players, :through => :earned_rewards
end
class EarnedReward < ActiveRecord::Base
belongs_to :player
belongs_to :reward
validates_uniqueness_of :reward_id, :scope => [:reward_id, :player_id]
end
module FirstOrBuild
def first_or_build(attributes = nil, options = {}, &block)
first || scoping{ proxy_association.build(attributes, &block) }
end
end
When you build an association, it adds it to the parent so that when the parent is saved, the child is also saved. E.g.
pry(main)> company.customers.where(:fname => "Bob")
Customer Load (0.1ms) SELECT "customers".* FROM "customers"
=> [] # No customer named Bob
pry(main)> company.customers.where(:fname => "Bob").first_or_build
=> #<Customer id: nil, fname: "Bob"> # returns you an unsaved Customer
pry(main)> company.save
=> true
pry(main)> company.reload.customers
=> [#<Customer id: 1035, fname: "Bob">] # Bob gets created when the company gets saved
pry(main)> company.customers.where(:fname => "Bob").first_or_build
=> #<Customer id: 1035, fname: "Bob"> # Calling first_or_build again will return the first Customer with name Bob
Since our code is running in a before_update hook, the Player will be saved as well as any newly built Rewards as well.