# About doctrine odm questions

**URL:** https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238
**Category:** 3rd-Party Tools & Libraries
**Created:** [November 3, 2019, 1:01am UTC](https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238 "2019-11-03T01:01:57Z")
**Posts on this page:** 5
**Page:** 1

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### Author: ![jobsfan](https://yyz2.discourse-cdn.com/flex032/user_avatar/discourse.laminas.dev/jobsfan/32/399_2.png) [@jobsfan](https://discourse.laminas.dev/u/jobsfan)
#### Post date: [November 3, 2019, 1:01am UTC](https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238/1 "2019-11-03T01:01:57Z")

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In my article detail page require a random 3 records similar to this article.

Before I use mysql as database, and query by “order by rand() limit 3”, however my application crashed because mysql exausted the vps memery when a rogue search-engine scrapying my website by more than 4 concurrency. I am so disappoint!!

First of all, the vps I used the aliyun’s ecs, the ability is very weak. Some colleague suggest me to create the html file to solve the problem. I don’t want to do that. I want a dynamic application rather than a static one.

So I turn to mongodb instead of mysql.

I use [https://www.doctrine-project.org/projects/doctrine-mongodb-odm/en/stable/reference/aggregation-builder.html#aggregation-builder](https://www.doctrine-project.org/projects/doctrine-mongodb-odm/en/stable/reference/aggregation-builder.html#aggregation-builder)  
It is called odm. I am not familar to this.

 ![33333](https://canada1.discourse-cdn.com/flex032/uploads/zendframework/original/1X/748307f1badc9789f4c0bc0863c4f720062707a0.png)

 ![22222](https://canada1.discourse-cdn.com/flex032/uploads/zendframework/original/1X/c50d7aacc2a78221f637a29cbc7d4f34b3a3ea97.png)

 ![1111](https://canada1.discourse-cdn.com/flex032/uploads/zendframework/original/1X/f378e0e947f6457e2db253313639cb46df58b8c4.png)

I just wanna 3 random recoards which has the same class with the current detail.  
Any big gun can help on this?

I followed and changed a little according to the demo as the above linke. However what I got is not the record data, I don’t know what I got, it seems a mess of codes, and when I print\_r it, it keep outputting for a long time and almost cause a web browser crash.

thank you in advance.

ps: getPrev and getNext work well, just the getSimilar works not as I expected.  
I run `db.article.aggregate([{ $sample: { size: 3 }}])` in local (I mean on my work computer of windows 10 with a mongodb server) envirment, it takes several micro second. However when I run it in the online vps(very cheap one, ubuntu 16.04 server with mongod) it takes almost 6 seconds!! no matter the time it takes, first of all I wanna to make the code correct and get it run the right way.

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### Author: ![ocramius](https://yyz2.discourse-cdn.com/flex032/user_avatar/discourse.laminas.dev/ocramius/32/23_2.png) [@ocramius](https://discourse.laminas.dev/u/ocramius)
#### Post date: [November 3, 2019, 8:09am UTC](https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238/2 "2019-11-03T08:09:15Z")

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`ORDER BY RAND()` is potentially `O(N!)` (factorial!!!) in complexity: _do not use_!

This has nothing to do with ORM/ODM, but solely with the sorting of your records, since sorting is computed based on a routine +`RAND()`) evaluated per-scanned-row in your resultset.

Depending on the SQL engine, the `RAND()` routine may be called only once (scanning your entire dataset) or once per sorting comparison during quicksort/merge-sort operations.

Consider that I managed to bring down a production system due to the fact that I did not use `array_shuffle()`, but instead something similar to `usort(..., function () : int { return random_int(-1, 1);})` - that was with an array with 6 elements. Yes, you read it right: shuffling an array of 6 elements brought down a production system.

The correct way to deal with picking a random record is:

1. `SELECT COUNT(1) FROM ... WHERE <criteria>`

2. `'SELECT * FROM ... WHERE <criteria> OFFSET' . random_int(0, $theCountYouGotAbove - 1);`

That will give you one random record, incurring in a much lower computational complexity.

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### Author: ![jobsfan](https://yyz2.discourse-cdn.com/flex032/user_avatar/discourse.laminas.dev/jobsfan/32/399_2.png) [@jobsfan](https://discourse.laminas.dev/u/jobsfan)
#### Post date: [November 3, 2019, 11:00am UTC](https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238/3 "2019-11-03T11:00:04Z")

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Hi @ocramius, thanks so much for your reply!

I have got more than 120000 records in my database table. I got your suggestion, that’s good. I will following your suggestion. And offset 3 times for each time to fetch one record.

And also I wanna to learning something about mongodb doctrine odm stuff. Can you teach me about this? use aggrate for select the same class and sample 3 records?

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### Author: ![ocramius](https://yyz2.discourse-cdn.com/flex032/user_avatar/discourse.laminas.dev/ocramius/32/23_2.png) [@ocramius](https://discourse.laminas.dev/u/ocramius)
#### Post date: [November 3, 2019, 11:14am UTC](https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238/4 "2019-11-03T11:14:41Z")

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I don’t really use MongoDB, sorry, and tbh still fail to see a reason to (personally) ever use it 🤷‍♀️

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### Author: ![jobsfan](https://yyz2.discourse-cdn.com/flex032/user_avatar/discourse.laminas.dev/jobsfan/32/399_2.png) [@jobsfan](https://discourse.laminas.dev/u/jobsfan)
#### Post date: [November 3, 2019, 12:12pm UTC](https://discourse.laminas.dev/t/about-doctrine-odm-questions/1238/5 "2019-11-03T12:12:07Z")

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ok, thank you again.