🤨 N + 1問題とは

💎 何ですか。

💎 例えば

国名を取得する

# SELECT * FROM country;
Country.all.each do |country|
  put country.name
    # Japan
    # Poland
end

別のテーブルをループで呼び出す

# SELECT * FROM country;
Country.all.each do |country|
  put country.name
    # Japan
    # Poland

  # SELECT * FROM food WHERE country_id=1;
  # SELECT * FROM food WHERE country_id=2;
  # 😫 😭 if country table had 200 records, 200 query would be executed!!! OMG
  country.foods.each do |food|
    put food.name
      # sushi
      # ramen
      # udon
      # pierogi
      # kotlet
      # ziemniaki
  end
end

💎 どのように解決するには？

結合

# SELECT * FROM country INNER JOIN food ON country.id=food.country_id
# 😀 😀 only 1 query !
Country.join(:food).all.each do |country|
  country.foods.each do |food|
  end
end

熱心な負荷

# SELECT * FROM country;
# SELECT * FROM food WHERE country_id IN(1,2)
# 😀 😀 only 2 queries !
Country.includes(:food).all.each do |country|
  country.foods.each do |food|
  end
end

💎 不利