Query for many-to-many association - sql

Question

There are 3 models in my project: Users, Games, GameVisits
class User < ActiveRecord::Base
has_many :game_visits
has_many :games, through: :game_visits
end
class Game < ActiveRecord::Base
has_many :game_visits
has_many :users, through: :game_visits
end
class GameVisit < ActiveRecord::Base
self.table_name = :users_games
enum status: [:visited, :not_visited, :unknown]
belongs_to :user
belongs_to :game
end
My frontend is written in angularjs, so I want a function that for a list of games returns array of hashes each containing name of user and status or nil. Data is presented as table, cols - players, rows - games, cell - presence of player on game.
This is example of such function:
def team_json(game_ids = nil)
game_ids ||= Game.pluck(:id)
games = Game.find(game_ids)
users = self.users
result = []
games.each do |game|
record = {name: game.name, date: game.date }
users_array = []
users.each do |user|
users_array << {
name: user.name,
status: user.game_visits.find_by_game_id(game.id)
}
end
record[:users] = users_array
result << record
end
result
end
Output:
[{:name=>"game 0", :date=>Fri, 19 Dec 2014 11:16:20 UTC +00:00, :users=>[{:name=>"Bob", :status=>nil}]},
{:name=>"game 1", :date=>Sat, 20 Dec 2014 11:16:20 UTC +00:00, :users=>[{:name=>"Bob", :status=>nil}]},
{:name=>"game 2", :date=>Fri, 19 Dec 2014 11:16:20 UTC +00:00, :users=>[{:name=>"Bob", :status=>nil}]}]
But this function produces tons of sql queries for GameVisits:
(1.0ms) SELECT "games"."id" FROM "games"
Game Load (1.3ms) SELECT "games".* FROM "games" WHERE "games"."id" IN (1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 88, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100, 101)
User Load (0.6ms) SELECT "users".* FROM "users" WHERE "users"."team_id" = $1 [["team_id", 1]]
GameVisit Load (0.6ms) SELECT "users_games".* FROM "users_games" WHERE "users_games"."user_id" = $1 AND "users_games"."game_id" = $2 LIMIT 1 [["user_id", 7], ["game_id", 1]]
GameVisit Load (0.6ms) SELECT "users_games".* FROM "users_games" WHERE "users_games"."user_id" = $1 AND "users_games"."game_id" = $2 LIMIT 1 [["user_id", 7], ["game_id", 2]]
GameVisit Load (0.5ms) SELECT "users_games".* FROM "users_games" WHERE "users_games"."user_id" = $1 AND "users_games"."game_id" = $2 LIMIT 1 [["user_id", 7], ["game_id", 3]]
..............
How can I optimise it?

Answer 1

Your user.game_visits.find_by_game_id(game.id) is what's firing all those queries. You can use eager-loading to get around this problem.
Change your assignment of users to the following:
users = self.users.includes(:game_visits)
Then change the value assignment of status in users_array to this:
status: user.game_visits.find{ |gv| gv.game_id == game.id }
The above line is using the enumerable's find, not ActiveRelation's. If you stick with find_by_game_id or some other find of AR, it'll still fire a query.
An additional optimization could be creating a hash for game_visits so that you can get around find altogether to do something like this:
user_game_visits_hash = \
user.game_visits.each_with_object({}) do |gv, hash|
hash[gv.game_id] = gv
end
# more code here
status: user_game_visits_hash[game.id]
And a minor point: In case your game_ids in the method's arguments are nil, the first two lines of your method will unnecessarily fire two queries, because you pluck all ids from games just to fetch all games. You can change that to this:
games = game_ids ? Game.find(game_ids) : Game.all