Ruby on rails 通过关系计数的轨道

我的大脑在这方面有问题

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我有四种模式：Account has many->List has many->ListItem您可以尝试以下方式：

top_team_lead_categories = Category.all(
  :joins => {:list_item => {:list => :account}},
  :select => "categories.*, sum(if(list_items.category_id = categories.id, 1, 0)) as list_items_count",
  :conditions => ['accounts.role =?', 'team lead'],
  :group => 'categories.id',
  :order => 'list_items_count desc'
)

将此添加到您的帐户模型：

然后你可以这样做：

a = Account.first
a.list_items.joins(:category).group("categories.name").count
# => {"foo"=>1, "bar"=>2}

编辑

您必须将类别字段添加到select中，查询如下所示：

SELECT categories.name, accounts.role, count(list_items.id) 
FROM list_items INNER JOIN lists ON lists.id = list_items.list_id 
INNER JOIN categories ON categories.id = list_items.category_id 
INNER JOIN accounts ON lists.account_id = accounts.id 
GROUP BY categories.name, accounts.role;

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如果我将第二行保留为：joins=>{：list\u item…，我发现名为“list\u item”的错误关联未找到；可能是您拼错了？。如果我将其更改为list\u items，我会在Postgres和SQLite中得到类似的错误：错误：函数ifboolean，integer，integer不存在。感谢您的回答-我认为这非常接近。该联接假设存在多对一关联在category和list_item之间，但是由于您的错误，我认为我的关联是错误的。第二个错误可能是因为我给了您一个MySQL函数。我认为Postgres版本的sum if是sumcase，当list_items.category_id=categories.id然后是1，否则是0 endRight，但是我必须检查有数千个帐户的所有帐户，并总结计算结果。我们的想法不是针对单个帐户，而是将前5名作为整个系统的整体想法。谢谢你的答案！好的，我明白你的意思。SQL查询对于很多记录来说都非常激烈。我编辑了我的答案-也许这会有所帮助。

a = Account.first
a.list_items.joins(:category).group("categories.name").count
# => {"foo"=>1, "bar"=>2}

@categories = Category.select("categories.name AS name, accounts.role AS role, count(list_items.id) AS count")
.joins(:list_items => {:list => :account})
.group("categories.name, accounts.role")
.order("count desc")
.limit(5)

@categories[0].role
# => Team Leader
@categories[0].count
# => 5

SELECT categories.name, accounts.role, count(list_items.id) 
FROM list_items INNER JOIN lists ON lists.id = list_items.list_id 
INNER JOIN categories ON categories.id = list_items.category_id 
INNER JOIN accounts ON lists.account_id = accounts.id 
GROUP BY categories.name, accounts.role;