Question

我有三个模型，有很多关联：

class School < ApplicationRecord
    has_many :school_teacher_subjects
    has_many :teachers, through: :school_teacher_subjects
    has_many :subjects, through: :school_teacher_subjects   
end


class Teacher < ApplicationRecord
    has_many :school_teacher_subjects
    has_many :schools, through: :school_teacher_subjects
    has_many :subjects, through: :school_teacher_subjects
end

class Subject < ApplicationRecord   
    has_many :school_teacher_subjects
    has_many :teachers, through: :school_teacher_subjects
    has_many :schools, through: :school_teacher_subjects
end


class SchoolTeacherSubject < ApplicationRecord
  belongs_to :teacher
  belongs_to :school
  belongs_to :subject
end

我需要知道一个学校有多少老师，在一个优化的查询中，但我不知道我该怎么做

这是school_teacher_subjects表：

+----+------------+-----------+------------+
| id | teacher_id | school_id | subject_id |
+----+------------+-----------+------------+
|  1 |          1 |         1 |          1 |
|  2 |          2 |         1 |          1 |
|  3 |          2 |         1 |          2 |
|  4 |          3 |         2 |          1 |
|  5 |          3 |         2 |          2 |
+----+------------+-----------+------------+

我需要这样的东西：

+-------------+-------------+---------------+
| School name | school city | Teacher count |
+-------------+-------------+---------------+
| School A    | New York    |             2 |
| School B    | Orlando     |             1 |
+-------------+-------------+---------------+

姓名和城市数据存储在学校表

中

有人帮助我吗？

Answer 1

有几种方法：
1）使用 rails查询，您需要将组与选择 + 计数结合使用，将它们合并为一个查询：

要获得只有教师的学校，请使用加入

School.joins(:school_teacher_subjects => [:teacher]).select("schools.id, schools.name, schools.city, count(DISTINCT teachers.id) as teacher_count ").group("schools.id")

要让所有有或没有老师的学校，请使用 left_outer_joins ：

School.left_outer_joins(:school_teacher_subjects => [:teacher]).select("schools.id, schools.name, schools.city, count(DISTINCT teachers.id) as teacher_count ").group("schools.id")

2）另一种方法是通过在学校类中附加as_json来获取数据时依赖学校模型，如下所示（如果你没有大量获取）：

def as_json(options = {})
    if options[:index]
      {
          id: id,
          school_name: name,
          city_name: city_name, # in case you have the city name in school class
          teachers: teachers.count
      }
    else
      super
    end
  end

然后按如下方式调用模型：

Teacher.as_json(index: true)

有多少老师通过协会在一所学校里

1 个答案: