MongoDB的外键关联处理
用Mongodb本身的API需要硬编码才能实现外键关联,不够直观且难度较大,这种情况下可以用集算器来实现,下面用例子说明。
Collection UserCourseProgress记录着用户和课程的关系,其courseid字段是外键,指向Collection Course的_id字段。需要统计出每门课的人数,其中课程名称需要使用Course的title字段。
UserCourseProgress |
Course |
{"userId":"u01", "courseid":"c01", "timespent":6000, score:99} {"userId":"u02", "courseid":"c01", "timespent":6000, score:99} {"userId":"u03", "courseid":"c01", "timespent":6000, score:99} {"userId":"u04", "courseid":"c01", "timespent":6000, score:99} {"userId":"u05", "courseid":"c01", "timespent":6000, score:99} {"userId":"u01", "courseid":"c02", "timespent":6000, score:99} {"userId":"u02", "courseid":"c02", "timespent":6000, score:99} {"userId":"u03", "courseid":"c03", "timespent":6000, score:99} |
{"_id":"c01" "title":"Japanese159", "description":"Japanese base", "category":"language"} {"_id":"c02" "title":"Chinese200", "description":"Chinese middle", "category":"language"} {"_id":"c03" "title":"Political science 280", "description":"Political middle", "category":"politics"} {"_id":"c04" "title":"EE490", "description":"electronic engineering hign", "category":"Electronic"} |
集算器代码:
A1:连接MongoDB,连接字格式为mongo://ip:port/db?arg=value&…。
A2:统计出每门课的人数。这里使用aggregate函数从MongoDB中取数,该函数继承自mongdb,第1个参数是collection名,第2个参数是汇总表达式,遵循mongodb规范。计算结果是内存数据,如下:
A3:从Course中取出码表。这里使用find函数从MongoDB中取数,该函数继承自mongdb,第2个参数是过滤条件,写法遵循mongodb规范。Find的结果是游标,由于课程数量较少,因此用fetch函数将游标读入内存,结果如下:
A4:使用switch函数将A3中的外键切换为A2中的记录,结果如下:
A5:按对象方式访问内存,形成新的二维表,结果如下:
A6:关闭mongodb连接。