Technical questions (a)

• n + 1 problem

n + 1 questions:
    What is: for example, list a (the primary table) is associated list b (from Table), performs n data once acquired primary table a, since the relationship needs to be performed n times a query from table B, total query the number n + 1. Multiple queries on the table, causing performance problems, this is the n + 1 problem.
    Why: The main table query only once, really need one by one to the associated query from a table, n times, causing performance problems.
    Solution: (1) lazily: the time with the re-query
    (2) rails recommend solutions: includes feeding from the table; joins are inner joins, on the intersection; (preload and includes eager_load and can be pre-loaded)   

#n+1问题
User.all.each do |user|
  p user.blogs
end

#查询user表时，把includes进blogs
User.includes(:blogs).each  do |user|
  p user.blogs
end

#使用joins,会产生重复记录，可以用uniq去除
User.joins(:blogs) 或Blog.joins(:user)
#select users.* from users inner joins blogs on users.id=blogs.user_id
#select blogs.* from blogs inner joins users ...
#另外：使用joins关联多个对象
User.joins(:blogs,:articles)  <===> User.joins(:blogs).joins(:articles) 
#select users.* from users 
#inner joins blogs on users.id=blogs.user_id
#inner joins articles on users.id=articles.user_id