How to do "special queries" ala The Rails Way on "associated" conditions?


#1

I have the following in a model class:

class Game < ActiveRecord::Base
has_many :player_stats
has_many :players, :through => :player_stats

def visitor_stats
stats = []
player_stats.each do |stat|
stats << stat if stat.player.team.team_code ==
self.visiting_team_code
end
stats
end

def home_stats
stats = []
player_stats.each do |stat|
stats << stat if stat.player.team.team_code ==
self.home_team_code
end
stats
end
end

I understand that this isn’t the most efficient or elegant way to do
things. I gleaned this most recently from the posting at (see his
Case 3):

http://www.therailsway.com/tags/has_many

So, I would like to do something like the following, but I’m having a
problem with the conditions:

has_many :visitor_stats, :class_name=>“PlayerStat”, :conditions=>
<what???>

the condition here should be something matching “player.team.team_code
== self.visiting_team_code” in the code I have now.

has_many :home_stats, :class_name=>“PlayerStat”, :conditions=>
<what???>

and here it should be something matching “player.team.team_code ==
self.home_team_code” in the existing code.

The conditions specified need to be something that could go in a SQL
statement, right? How would I make the conditions work thru the
associations of a PlayerStat belonging to a Player belonging to a Team
which has a team_code. And matching it to this Game’s
visiting_team_code or home_team_code


#2

lunaclaire wrote:

self.visiting_team_code
end

self.home_team_code" in the existing code.

The conditions specified need to be something that could go in a SQL
statement, right? How would I make the conditions work thru the
associations of a PlayerStat belonging to a Player belonging to a Team
which has a team_code. And matching it to this Game’s
visiting_team_code or home_team_code

I don’t think what you need can be formulated into normal has_manys.
You’ll either have to use the has_many :finder_sql option that
includes joins to the players and teams tables, allowing you to
add the appropriate SQL conditions, or you can keep visitor_stats
and home_stats as methods, but have them call find:

def home_stats
player_stats.find :all, :select => ‘player_stats.*’,
:joins => {:player => :team},
:conditions => “teams.team_code =
#{home_team_code}”
end


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#3

Thanks for the reply, Mark.

One of the assertions in that RailsWay posting was that by using the
has_many approach, I’d gain efficiency because of caching. I don’t
understand enough about caching yet to see why, but let me ask this
about using your approach… would the code you show with the :joins
clause be more or less efficient than the code I have above in my
current home_stats method?


#4

lunaclaire wrote:

Thanks for the reply, Mark.

One of the assertions in that RailsWay posting was that by using the
has_many approach, I’d gain efficiency because of caching. I don’t
understand enough about caching yet to see why, but let me ask this
about using your approach… would the code you show with the :joins
clause be more or less efficient than the code I have above in my
current home_stats method?

Your original code becomes more efficient if:

  • Both visitor and home stats are often used together in the same
    request
  • The collections are small

But because you were not pre-loading the the player and team
associations using :include, what you had is very inefficient.
Each time round each loop a DB fetch is being made for the
player of a stat record and the team of a player record.

You could add caching to the method I posted:

def home_stats(reload = false)
@home_stats = nil if reload
@home_stats ||= player_stats.find …
end


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