DataMapper：按关联计数（相关对象的数量）对结果进行排序

Question

给定的是 m:n 关系中的电影和演员。我想做的是检索演员列表，按他们出演的电影数量排序。

class Movie
  include DataMapper::Resource
  property :id,     Serial
  property :title,  String
  has n,   :actors, through: Resource
end

class Actor
  include DataMapper::Resource
  property :name,   String, key: true
  has n,   :movies, through: Resource
end

在伪 DM 中我想要的是这样的：

Actor.all order: [ :movies.count ]

我发现了另一个关于按关联的单个属性进行排序的问题，但这种方法仅适用于真实属性。任何可用的解决方案都会有所帮助。谢谢！

Answer 1

DataMapper 的文档有点过时，我努力尝试完成与您所做的相同的事情。

我改为使用直接 MySQL 查询：

records = repository(:default).adapter.select(“SELECT * FROM actor ORDER BY count(movies) desc;”) 

重要的是要注意，当您使用直接 MySQL 查询时，将返回一个结构，而不仅仅是数据的散列。如果您将此数据作为 JSON 返回，则必须手动将其转换为哈希。

您可以通过以下方式在 Ruby 1.8-1.9 中将结构转换为哈希：

actors = repository(:default).adapter.select( "SELECT actors.name, count(actor_movies.actor_name) AS count " + "FROM actors " + "JOIN actor_movies WHERE actors.name = actor_movies.actor_name " + "GROUP BY actors.name " + "ORDER BY count(actor_movies.actor_name) desc " + "LIMIT 5;" ).map{|struct| {:name => struct.name, :count => struct.count}}

在 Ruby 2.0 中，他们添加了 to_h，因此您可以使用它：

actors = repository(:default).adapter.select( "SELECT actors.name, count(actor_movies.actor_name) AS count " + "FROM actors " + "JOIN actor_movies WHERE actors.name = actor_movies.actor_name " + "GROUP BY actors.name " + "ORDER BY count(actor_movies.actor_name) desc " + "LIMIT 5;" ).map(&:to_h)

Answer 2

以 Sean Larkin 的回答为出发点，我最终得到了这样的结果：

actors = repository(:default).adapter.select(
  "SELECT actors.name, count(actor_movies.actor_name) AS count " +
  "FROM actors " +
  "JOIN actor_movies WHERE actors.name = actor_movies.actor_name " +
  "GROUP BY actors.name " +
  "ORDER BY count(actor_movies.actor_name) desc " +
  "LIMIT 5;"
)

=> [
  #<struct name="Samuel L. Jackson", count=66>,
  #<struct name="Michael Caine", count=64>,
  #<struct name="Robert De Niro", count=59>,
  #<struct name="Harvey Keitel", count=58>,
  #<struct name="Gene Hackman", count=57>
]