mysql - 不使用 find_by_sql 将 Mysql 查询转换为 Rails ActiveRecord 查询

Question

我有如下表命名的问题

+----+---------------------------------------------------------+----------+
| id | title                                                   | category |
+----+---------------------------------------------------------+----------+
| 89 | Tinker or work with your hands?                         |        2 |
| 54 | Sketch, draw, paint?                                    |        3 |
| 53 | Express yourself clearly?                               |        4 |
| 77 | Keep accurate records?                                  |        6 |
| 32 | Efficient?                                              |        6 |
| 52 | Make original crafts, dinners, school or work projects? |        3 |
| 70 | Be elected to office or make your opinions heard?       |        5 |
| 78 | Take photographs?                                       |        3 |
| 84 | Start your own political campaign?                      |        5 |
|  9 | Free spirit or a rebel?                                 |        3 |
| 38 | Lead a group?                                           |        5 |
| 71 | Work in groups?                                         |        4 |
|  2 | Helpful?                                                |        4 |
|  4 | Mechanical?                                             |        6 |
| 14 | Responsible?                                            |        6 |
| 66 | Pitch a tent, an idea?                                  |        1 |
| 62 | Write useful business letters?                          |        5 |
| 28 | Creative?                                               |        3 |
| 68 | Perform experiments?                                    |        2 |
| 10 | Like to figure things out?                              |        2 |
+----+---------------------------------------------------------+----------+

I have a sql query to get one random record from each category.Can any one convert the mysql query to rails activerecord query(with out using Question.find_by_sql).This mysql query is working absolutely fine but I need only active record query because of my dependency in further steps.

Here is mysql query

             SELECT t.id, title as question, category
                FROM
              (
                SELECT 
                (
                  SELECT id
                    FROM questions
                   WHERE category = t.category
                   ORDER BY RAND()
                   LIMIT 1
                ) id
                  FROM questions t
                 GROUP BY category
              ) q JOIN questions t
                  ON q.id = t.id

感谢您的考虑!

Answer 1

当事情变得疯狂时，我们必须伸出援手 Arel :

It is intended to be a framework framework; that is, you can build your own ORM with it, focusing on innovative object and collection modeling as opposed to database compatibility and query generation.

所以我们要做的是让 Arel 为我们创建查询。此外，方法here将被使用:questions 表与它自身的随机版本相连:

q_normal = Arel::Table.new("questions")
q_random = Arel::Table.new("questions").project(Arel.sql("*")).order("RAND()").as("q2")

离开加入的时间

query = q_normal.join(q_random, Arel::Nodes::OuterJoin).on(q_normal[:category].eq(q_random[:category])).group(q_normal[:category]).order(q_random[:category])

现在您可以使用 project 使用您想要的列，例如:

query.project(q_normal[:id])