问题描述
我有一个连接表的两个表 - 这只是伪代码:
I have a two tables joined with a join table - this is just pseudo code:
Library
Book
LibraryBooks
我需要做的是，如果我有图书馆的 id，我想获取该图书馆拥有的所有书籍所在的所有图书馆.
What I need to do is if i have the id of a library, i want to get all the libraries that all the books that this library has are in.
因此，如果我有图书馆 1，图书馆 1 中有书籍 A 和 B，而书籍 A 和 B 在图书馆 1、2 和 3 中，是否有一种优雅的(单行)方式在 rails 中做到这一点?
So if i have Library 1, and Library 1 has books A and B in them, and books A and B are in Libraries 1, 2, and 3, is there an elegant (one line) way todo this in rails?
我在想:
l = Library.find(1)
allLibraries = l.books.libraries
但这似乎不起作用.建议?
But that doesn't seem to work. Suggestions?
推荐答案
l = Library.find(:all, :include => :books)
l.books.map { |b| b.library_ids }.flatten.uniq
注意 map(&:library_ids) 比 map { |b| 慢b.library_ids } 在 Ruby 1.8.6 中，在 1.9.0 中更快.
Note that map(&:library_ids) is slower than map { |b| b.library_ids } in Ruby 1.8.6, and faster in 1.9.0.
我还应该提到，如果你在那里使用 :joins 而不是 include，它会在同一个查询中找到图书馆和相关书籍，从而加快数据库时间.:joins 仅在图书馆有书籍时才有效.
I should also mention that if you used :joins instead of include there, it would find the library and related books all in the same query speeding up the database time. :joins will only work however if a library has books.
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