如何在查询中用数据填充外键字段而不是外键ID?

时间:2019-05-05 15:02:54

标签: javascript sql node.js postgresql knex.js

我有2个有班级和主题的桌子。

  • class(id,name,student_count)
  • 主题(id,名称,class_id)

我想在选择查询中用名称,学生人数和ID填充class_id。如何使用SQL或Knex做到这一点?

我尝试使用join,但是我只得到一个字段,而不是一行中的所有字段。

预期结果将是:

subject{
   name: "data"
   id: "data"
   class: {
     id: "data"
     name: "data"
     student_count: "data"
  }
}

顺便说一句,用Knex或普通SQL回答会有所帮助!

1 个答案:

答案 0 :(得分:0)

您必须使用内部联接或左联接。

在SQL中,它看起来像这样:

SELECT
      s.id            AS subject_id
    , s.name          AS subject_name
    , c.id            AS class_id
    , c.name          AS class_name
    , c.student_count AS class_student_count
FROM subjects s
INNER JOIN class ON s.class_id = c.id
WHERE s.id = 1

在Knex中看起来像这样:

getOne: () => knex
    .select(   's.id AS subject_id'
             , 's.name AS subject_name'
             , 'c.id AS class_id'
             , 'c.name AS class_name'
             , 'c.student_count AS class_student_count'
           )
    .from('subjects AS s')
    .innerJoin('class AS c', 's.class_id', 'c.id')
    .where('s.id': 1)

您将在AS之后获得一个变量名称为对象的对象。要使其成为您想要的格式,您必须创建一个映射器函数:

function mapToWantedFormat(subject) {
  return {
    name: subject.subject_name,
    id: subject.subject_id
    class: {
      id: subject.class_id,
      name: subject.class_name,
      student_count: subject.class_student_count
    }
  };
};

例如,这样使用它:

module.exports.getOne = async (req, res, next) => {
  try {
    const subject = await getOne();
    if (subject) {
      res.status(200).json(mapToWantedFormat(subject));
    } else {
      res.status(404).json();
    }
  } catch (error) {
    next(error);
  }
};