如何在rails中查询查询结果（使用rails查询'DISTINCT ON'的结果＆postgres

Question

简版： 我想查询另一个查询的结果，以便选择一个更有限的结果集但是，添加where子句重写第一个查询，而不是对结果的工作，所以我没有得到我想要的答案

的细节：。 我有两个型号，检查和ticks.Checks has_many ticks

第一个查询使用DISTINCT ON并收集所有'检查'和所有r兴高采烈的滴答，但只返回最近的滴答。我在模型中作为范围工作。

在我的控制器，

def checklist 
    #Filter the results by scope or return all checks with latest tick 
    case params[:filter] 
    when "duebylastresult" 
     @checks = Check.mostrecenttickonly.duebylastresult 
    when "duebydate" 
     @checks = Check.mostrecenttickonly.duebydate 
    else 
     @checks = Check.mostrecenttickonly 
    end 
    end 

在该模型中，第一个范围（工作）：

scope :mostrecenttickonly, -> { 
includes(:ticks) 
.order("checks.id, ticks.created_at DESC") 
.select("DISTINCT ON (checks.id) *").references(:ticks) 
} 

生成以下SQL：

Parameters: {"filter"=>""} 
    SQL (1.0ms) SELECT DISTINCT ON (checks.id) *, 
"checks"."id" AS t0_r0, 
"checks"."area" AS t0_r1, "checks"."frequency" AS t0_r2, 
"checks"."showinadvance" AS t0_r3, "checks"."category" AS t0_r4, 
"checks"."title" AS t0_r5, "checks"."description" AS t0_r6, 
"checks"."created_at" AS t0_r7, "checks"."updated_at" AS t0_r8, 
"ticks"."id" AS t1_r0, "ticks"."result" AS t1_r1, 
"ticks"."comments" AS t1_r2, "ticks"."created_at" AS t1_r3, 
"ticks"."updated_at" AS t1_r4, "ticks"."check_id" AS t1_r5 
FROM "checks" LEFT OUTER JOIN "ticks" 
ON "ticks"."check_id" = "checks"."id" 
ORDER BY checks.id, ticks.created_at DESC 

已经得到了结果，我只想显示具有等于或大于3的值的刻度，所以范围：

scope :duebylastresult, -> { where("ticks.result >= 3") } 

生成SQL

Parameters: {"filter"=>"duebylastresult"} 
    SQL (1.0ms) SELECT DISTINCT ON (checks.id) *, 
"checks"."id" AS t0_r0, 
"checks"."area" AS t0_r1, "checks"."frequency" AS t0_r2, 
"checks"."showinadvance" AS t0_r3, "checks"."category" AS t0_r4, 
"checks"."title" AS t0_r5, "checks"."description" AS t0_r6, 
"checks"."created_at" AS t0_r7, "checks"."updated_at" AS t0_r8, 
"ticks"."id" AS t1_r0, "ticks"."result" AS t1_r1, 
"ticks"."comments" AS t1_r2, "ticks"."created_at" AS t1_r3, 
"ticks"."updated_at" AS t1_r4, "ticks"."check_id" AS t1_r5 
FROM "checks" LEFT OUTER JOIN "ticks" 
ON "ticks"."check_id" = "checks"."id" 
WHERE (ticks.result >= 3) 
ORDER BY checks.id, ticks.created_at DESC 

尽我所知道的，WHERE语句在行动前的DISTINCT ON子句，所以我现在有“其结果为> = 3最新滴答”，而我正在寻找'最近的滴答，那么只有当结果是> = 3'时。

希望有道理&在此先感谢！

编辑 - 是我所得到什么，我需要的例子：

The Data: 
Table Checks: 
ID: 98 Title: Eire 
ID: 99 Title: Land 

Table Ticks: 
ID: 1 CheckID: 98 Result:1 Date: Jan12 
ID: 2 CheckID: 98 Result:5 Date: Feb12 
ID: 3 CheckID: 98 Result:1 Date: Mar12 
ID: 4 CheckID: 99 Result:4 Date: Apr12 

First query returns the most recent result, like; 
Check.ID: 98 Tick.ID: 3 Tick.Result: 1 Tick.Date: Mar12 
Check.ID: 99 Tick.ID: 4 Tick.Result: 4 Tick.Date: Apr12 

Second query currently returns the most recent result where the result is =>3, like; 
Check.ID: 98 Tick.ID: 2 Tick.Result: 5 Tick.Date: Feb12 
Check.ID: 99 Tick.ID: 4 Tick.Result: 5 Tick.Date: Apr12 

When I really want: 
Check.ID: 99 Tick.ID: 4 Tick.Result: 5 Tick.Date: Apr12 

(ID 98 doesn't show as the last Tick.Result is 1). 

Answer 1

你能尝试以下，看看它是否开始你在正确的方向：

scope :just_a_test, -> { 
    includes(:ticks) 
    .order("checks.id") 
    .where("ticks.created_at = (SELECT MAX(ticks.created_at) FROM ticks WHERE ticks.check_id = checks.id)") 
    .where("ticks.result >= 3") 
    .group("checks.id") 
    } 

Answer 2

我米不知道我真的明白:mostrecenttickonly范围的点，因为你只是加载检查。

话虽这么说，如果你想获得只有那些检查其最近的蜱有大于三的结果，我认为要做到这一点的最佳方式将是一个window function：

check.rb

... 
    scope :duebylastresult, -> { 
    find_by_sql(
     'SELECT * 
     FROM (SELECT checks.*, 
        ticks.id AS tick_ids, 
        ticks.date AS tick_date, 
        ticks.result AS tick_result, 
        dense_rank() OVER (
         PARTITION BY checks.id 
         ORDER BY ticks.date DESC 
        ) AS tick_rank 
      FROM checks 
      LEFT OUTER JOIN ticks ON checks.id = ticks.check_id) AS ranked_ticks 
     WHERE tick_rank = 1 AND tick_result >= 3;' 
    ) 
    } 
... 

基本上，我们只是在检查连接一切，蜱表，然后添加名为tick_rank是在结果根据其date相对于相同其他行集居各行的另一个属性值。

的方式SQL工作原理是，谓词（在WHERE子句中的条件）的SELECT领域的评估之前评估的，这意味着我们不能只写tick_rank = 1本声明。

所以我们必须去包装结果（我们别名为ranked_ticks）的额外步骤，然后选择所有内容并将我们想要的谓词应用于此外部选择语句。该tick_rank必须是1，这意味着它最近tick，其结果必须是> = 3

---

编辑：我用的是那篇文章我联系的复习，因为我常常忘记SQL语法，但看着它之后，我认为这将在一定程度上更好的性能（基本上只是迫不及待地加入checks直到划分完成后，这样，我相信它会做较少的完全扫描）：

scope :duebylastresult, -> { 
    find_by_sql(
     'SELECT * 
     FROM checks 
     LEFT OUTER JOIN 
      (SELECT id AS tick_id, 
        check_id AS check_id, 
        date AS tick_date, 
        result AS tick_result, 
        dense_rank() OVER (
         PARTITION BY ticks.check_id 
         ORDER BY ticks.date DESC 
        ) AS tick_rank 
      FROM ticks) AS ranked_ticks ON checks.id = ranked_ticks.check_id 
     WHERE tick_rank = 1 AND tick_result >= 3;' 
    ) 
    }