1s ===> 0.005s

app/controllers/school_controller.rb

@@ -47,10 +47,14 @@ class SchoolController < ApplicationController
       options << "<option value = '#{p.province}' >#{p.province}</option>"
     end
 
-    # 查询已添加用户的学校
-    schools_arr = UserExtensions.where("school_id IS NOT NULL").pluck(:school_id).uniq
     # 取id取学校名
-    school = School.where(id: schools_arr).select("id, name")
+    # 连接子表: 查询已添加用户的学校
+    school = School.select("id, name").
+              joins("RIGHT JOIN (
+                      SELECT DISTINCT school_id 
+                      FROM #{UserExtensions.table_name} 
+                      WHERE school_id IS NOT NULL) AS sids ON schools.id = sids.school_id").
+              where("#{School.table_name}.id IS NOT NULL")
 
     options_s = ""
     school.each do |s|