Calling variables from custom class method in rails

Question

Beginner here. Have a custom class method that calculates the win probability of team_a and team_b if they were to play each other. In my view, I want to display those calculations. Trying to put this together with my limited understanding of how it works.

# team.rb
class Team
  attr_reader :home_team_win_probability, :away_team_win_probability
  def self.matchup(home_team, away_team)
    home_team_win_probability = (home_team.rating - home_team.rating * away_team.rating) / (home_team.rating) + away_team.rating - 2 * home_team.rating * away_team.rating)
    away_team_win_probability = 1 - home_team_win_probability
  end
end

# teams_controller.rb
class TeamsController < ApplicationController
  def matchups
    @team_a = Team.find_by(rank: 2)
    @team_b = Team.find_by(rank: 1)
    @matchup = Team.matchup(@team_a, @team_b)
  end
end

# matchups.html.erb
<h4>Matchup: <%= @team_a.name %> @ <%= @team_a.name %></h4>
# want to do something like this: 
<h5>Home Team Win Probability: <%= @matchup.home_team_win_probability %> </h5>
<h5>Away Team Win Probability: <%= @matchup.away_team_win_probability %> </h5>

@matchup.home_team_win_probability yields error because @matchup just returns the last line (the away team probability in this case) @matchup.home_team_win_probability => undefined method home_team_win_probability for #<BigDecimal:0x007f860872b3f8

How do I call home_team_win_probability and away_team_win_probability as calculated in my custom class method? Thanks.

Jason Brodie · Accepted Answer · 2015-12-22 21:41:42Z

1

Return a hash instead:

def self.matchup(home_team, away_team)
    home_team_win_probability = (home_team.rating - home_team.rating * away_team.rating) / (home_team.rating) + away_team.rating - 2 * home_team.rating * away_team.rating)
    away_team_win_probability = 1 - home_team_win_probability
    {home: home_team_win_probability, away: away_team_win_probability}
end

Then access as:

@matchup[:home]
@matchup[:away]

answered Dec 22, 2015 at 21:41

Jason Brodie

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1 Comment

iamse7en Over a year ago

This worked really well and is very simple. While it may make sense to move my matchup logic elsewhere, this was a very convenient answer given what I already have. Thank you!

SteveTurczyn · Accepted Answer · 2015-12-22 21:51:56Z

1

There's no particular reason the matchup logic needs to be in the Team model. I would create a new model...

class Matchup
  attr_accessor :home_team, :away_team
  def initialize(home_team, away_team)
    @home_team = home_team
    @away_team = away_team
  end
  def home_team_win_probability
    (home_team.rating - home_team.rating * away_team.rating) / (home_team.rating) + away_team.rating - 2 * home_team.rating * away_team.rating)
  end
  def away_team_win_probability 
    1 - home_team_win_probability
  end
end

Now you can do this in your TeamsController...

@matchup = Matchup.new(@team_a, @team_b)

This gives you the methods @matchup.home_team_probability and @matchup.away_team_probability

answered Dec 22, 2015 at 21:51

SteveTurczyn

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