[Ruby on Rails] 解析 active model 的 find_in_batches

Rails 中的 find_in_batches 是為了減低 db 的 memory loading.
def find_in_batches(options = {}) options.assert_valid_keys(:start, :batch_size) relation = self if logger && (arel.orders.present? || arel.taken.present?) logger.warn("Scoped order and limit are ignored, it's forced to be batch order and batch size") end start = options.delete(:start) batch_size = options.delete(:batch_size) || 1000 relation = relation.reorder(batch_order).limit(batch_size) records = start ? relation.where(table[primary_key].gteq(start)).to_a : relation.to_a while records.any? records_size = records.size primary_key_offset = records.last.id raise "Primary key not included in the custom select clause" unless primary_key_offset yield records break if records_size < batch_size records = relation.where(table[primary_key].gt(primary_key_offset)).to_a end end

可以這樣使用: Post.find_in_batches(:batch_size => 100, :include => [:user, :category]) do |post| u = post.user c = post.category # do stuff end
batch_size : 每次撈取的數量.
start : 從哪個 primary_id 開始. (ps: 因為 find_in_batches 是用 id 跟 limit 來 算出 sql 語法的)
範例 : start => 100  Mysql 會變成  select * from table where id >= 100 limt batch_size ;
conditions : 設定其他條件.

官網:http://api.rubyonrails.org/classes/ActiveRecord/Batches.html