blocks|key|1261649|text|一旦记录被删除，你的示例代码将开始表现不准确(它将不公平地偏爱ids较低的项)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261650|在数据库中使用随机方法可能会更好。根据您使用的数据库不同，它们会有所不同，但是:order+=>+"RAND()“适用于mysql，order+=>+"RANDOM()”适用于postgres|1261651|Model.first(:order+=>+"RANDOM()")+#+postgres+example|code-block|syntax|javascript|1261652|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|L|8|@]|9|@]|A|$]]|$1|D|3|E|5|F|7|M|8|@]|9|@]|A|$G|H]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

Your example code will start to behave inaccurately once records are deleted (it will unfairly favor items with lower ids)

You're probably better off using the random methods within your database. These vary depending on which DB you're using, but :order => "RAND()" works for mysql and :order => "RANDOM()" works for postgres

<pre><code>Model.first(:order =&gt; "RANDOM()") # postgres example
</code></pre>

blocks|key|1261842|text|这并不需要那么难。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261843|ids+=+Model.pluck(:id)
random_model+=+Model.find(ids.sample)|code-block|syntax|javascript|1261844|pluck返回表中所有id的数组。数组上的sample方法返回数组中的随机id。|offset|length|style|CODE|1261845|这应该表现良好，具有相同的选择概率和对删除行的表的支持。您甚至可以将其与约束相混合。|1261846|User.where(favorite_day:+"Friday").pluck(:id)|1261847|从而随机选择一个喜欢星期五的用户，而不是任何一个用户。|1261848|entityMap^0|0|0|0|5|L|6|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@$I|X|J|Y|K|L]|$I|Z|J|10|K|L]]|9|@]|A|$]]|$1|M|3|N|5|6|7|11|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|12|8|@]|9|@]|A|$E|F]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|T|$]]

It doesn't have to be that hard.

<pre><code>ids = Model.pluck(:id)
random_model = Model.find(ids.sample)
</code></pre>

<code>pluck</code> returns an array of all the id's in the table. The <code>sample</code> method on the array, returns a random id from the array.

This should perform well, with equal probability of selection and support for tables with deleted rows. You can even mix it with constraints.

<pre><code>User.where(favorite_day: "Friday").pluck(:id)
</code></pre>

And thereby pick a random user who likes fridays rather than just any user.

blocks|key|1046837|text|我做了一个rails+3+gem来处理这个问题：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1046838|https://github.com/spilliton/randumb|offset|length|1046839|它可以让你做这样的事情：|1046840|Model.where(:column+=>+"value").random(10)|code-block|syntax|javascript|1046841|entityMap|0|LINK|mutability|MUTABLE|url^0|0|0|10|0|0|0|0^^$0|@$1|2|3|4|5|6|7|T|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|U|8|@]|9|@$D|V|E|W|1|X]]|A|$]]|$1|F|3|G|5|6|7|Y|8|@]|9|@]|A|$]]|$1|H|3|I|5|J|7|Z|8|@]|9|@]|A|$K|L]]|$1|M|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|C]]]]

I made a rails 3 gem to handle this:

<a href="https://github.com/spilliton/randumb" rel="noreferrer">https://github.com/spilliton/randumb</a>

It allows you do do stuff like this:

<pre><code>Model.where(:column =&gt; "value").random(10)
</code></pre>

blocks|key|1047063|text|我经常在控制台中使用它，我在一个初始化器中扩展了ActiveRecord+--+Rails+4示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1047064|class+ActiveRecord::Base
++def+self.random
++++self.limit(1).offset(rand(self.count)).first
++end
end|code-block|syntax|javascript|1047065|然后，我可以调用Foo.random返回一个随机记录。|offset|length|style|CODE|1047066|entityMap^0|0|0|8|A|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@$I|R|J|S|K|L]]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

I use this so often from the console I extend ActiveRecord in an initializer - Rails 4 example:

<pre><code>class ActiveRecord::Base
 def self.random
 self.limit(1).offset(rand(self.count)).first
 end
end
</code></pre>

I can then call <code>Foo.random</code> to bring back a random record.

blocks|key|1046921|text|Postgres中的一个查询：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1046922|User.order('RANDOM()').limit(3).to_sql+#+Postgres+example
=>+"SELECT+"users".*+FROM+"users"+ORDER+BY+RANDOM()+LIMIT+3"|code-block|syntax|javascript|1046923|使用偏移量，有两个查询：|1046924|offset+=+rand(User.count)+#+returns+an+integer+between+0+and+(User.count+-+1)
Model.offset(offset).limit(1)|1046925|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

One query in Postgres:

<pre><code>User.order('RANDOM()').limit(3).to_sql # Postgres example
=&gt; "SELECT "users".* FROM "users" ORDER BY RANDOM() LIMIT 3"
</code></pre>

Using an offset, two queries:

<pre><code>offset = rand(User.count) # returns an integer between 0 and (User.count - 1)
Model.offset(offset).limit(1)
</code></pre>

blocks|key|1262307|text|您可以使用Array方法sample，sample方法从数组中返回一个随机对象，为了使用它，您只需在返回集合的简单ActiveRecord查询中执行，例如：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1262308|User.all.sample|code-block|syntax|javascript|1262309|将返回类似如下的内容：|1262310|#<User+id:+25,+name:+"John+Doe",+email:+"admin@example.info",+created_at:+"2018-04-16+19:31:12",+updated_at:+"2018-04-16+19:31:12">|1262311|entityMap^0|5|5|C|6|J|6|1L|C|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@$9|R|A|S|B|C]|$9|T|A|U|B|C]|$9|V|A|W|B|C]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|Z|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|10|8|@]|D|@]|E|$]]|$1|M|3|N|5|H|7|11|8|@]|D|@]|E|$I|J]]|$1|O|3|-4|5|6|7|12|8|@]|D|@]|E|$]]]|P|$]]

You can use the <code>Array</code> method <code>sample</code>, the method <code>sample</code> returns a random object from an array, in order to use it you just need to exec in a simple <code>ActiveRecord</code> query that return a collection, for example:

<pre><code>User.all.sample
</code></pre>

will return something like this:

<pre><code>#&lt;User id: 25, name: "John Doe", email: "admin@example.info", created_at: "2018-04-16 19:31:12", updated_at: "2018-04-16 19:31:12"&gt;
</code></pre>

blocks|key|1047607|text|这是一个非常古老的问题，但是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1047608|rand_record+=+Model.all.shuffle|code-block|syntax|javascript|1047609|你得到了一个记录数组，按随机顺序排序。不需要宝石或脚本。|1047610|如果您想要一条记录：|1047611|rand_record+=+Model.all.shuffle.first|1047612|entityMap^0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|O|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|R|8|@]|9|@]|A|$]]|$1|K|3|L|5|D|7|S|8|@]|9|@]|A|$E|F]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Very old question but with :

<pre><code>rand_record = Model.all.shuffle
</code></pre>

You got an Array of record, sort by random order.
No need gems or scripts.

If you want one record :

<pre><code>rand_record = Model.all.shuffle.first
</code></pre>

blocks|key|1261999|text|从列表中随机选择一项的Ruby方法是sample。想要为ActiveRecord创建一个高效的sample，并基于前面的答案，我使用：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1262000|module+ActiveRecord
++class+Base
++++def+self.sample
++++++offset(rand(size)).first
++++end
++end
end|code-block|syntax|javascript|1262001|我将它放在lib/ext/sample.rb中，然后在config/initializers/monkey_patches.rb中加载它|1262002|Dir[Rails.root.join('lib/ext/*.rb')].each+{+%7Cfile%7C+require+file+}|1262003|如果模型的大小已经缓存，这将是一个查询，否则是两个查询。|1262004|entityMap^0|I|6|1B|6|0|0|5|H|R|11|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|X|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|Y|8|@$9|Z|A|10|B|C]|$9|11|A|12|B|C]]|D|@]|E|$]]|$1|M|3|N|5|H|7|13|8|@]|D|@]|E|$I|J]]|$1|O|3|P|5|6|7|14|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|15|8|@]|D|@]|E|$]]]|R|$]]

The Ruby method for randomly picking an item from a list is <code>sample</code>. Wanting to create an efficient <code>sample</code> for ActiveRecord, and based on the previous answers, I used:

<pre><code>module ActiveRecord
 class Base
 def self.sample
 offset(rand(size)).first
 end
 end
end
</code></pre>

I put this in <code>lib/ext/sample.rb</code> and then load it with this in <code>config/initializers/monkey_patches.rb</code>:

<pre><code>Dir[Rails.root.join('lib/ext/*.rb')].each { |file| require file }
</code></pre>

This will be one query if the size of the model is already cached and two otherwise.

blocks|key|1047149|text|对于MySQL数据库，请尝试:+Model.order("RAND()").first|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1047150|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

For MySQL database try: Model.order("RAND()").first

blocks|key|1047213|text|如果您使用的是PostgreSQL+9.5%2B，则可以利用TABLESAMPLE选择随机记录。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1047214|两种默认采样方法(SYSTEM和BERNOULLI)要求您将返回的行数指定为表中总行数的百分比。|1047215|--+Fetch+10%25+of+the+rows+in+the+customers+table.
SELECT+*+FROM+customers+TABLESAMPLE+BERNOULLI(10);|code-block|syntax|javascript|1047216|这需要了解表中的记录量以选择适当的百分比，这可能不容易快速找到。幸运的是，有一个tsm_system_rows+module允许您指定要直接返回的行数。|1047217|CREATE+EXTENSION+tsm_system_rows;

--+Fetch+a+single+row+from+the+customers+table.
SELECT+*+FROM+customers+TABLESAMPLE+SYSTEM_ROWS(1);|1047218|要在ActiveRecord中使用此功能，请首先在迁移中启用该扩展：|1047219|class+EnableTsmSystemRowsExtension+<+ActiveRecord::Migration[5.0]
++def+change
++++enable_extension+"tsm_system_rows"
++end
end|1047220|然后修改查询的from子句：|1047221|customer+=+Customer.from("customers+TABLESAMPLE+SYSTEM_ROWS(1)").first|1047222|我不知道SYSTEM_ROWS抽样方法是完全随机的，还是只返回随机页面的第一行。|1047223|这些信息大部分来自2ndQuadrant+blog+post+written+by+Gulcin+Yildirim。|1047224|entityMap|0|LINK|mutability|MUTABLE|url|https://www.postgresql.org/docs/9.5/static/sql-select.html#SQL-FROM|1|https://www.postgresql.org/docs/9.5/static/tsm-system-rows.html|2|http://blog.2ndquadrant.com/tablesample-and-other-methods-for-getting-random-tuples/^0|S|B|S|B|0|0|9|6|G|9|0|0|14|F|14|M|1|0|0|0|0|7|4|0|0|4|B|0|9|1C|2|0^^$0|@$1|2|3|4|5|6|7|1E|8|@$9|1F|A|1G|B|C]]|D|@$9|1H|A|1I|1|1J]]|E|$]]|$1|F|3|G|5|6|7|1K|8|@$9|1L|A|1M|B|C]|$9|1N|A|1O|B|C]]|D|@]|E|$]]|$1|H|3|I|5|J|7|1P|8|@]|D|@]|E|$K|L]]|$1|M|3|N|5|6|7|1Q|8|@$9|1R|A|1S|B|C]]|D|@$9|1T|A|1U|1|1V]]|E|$]]|$1|O|3|P|5|J|7|1W|8|@]|D|@]|E|$K|L]]|$1|Q|3|R|5|6|7|1X|8|@]|D|@]|E|$]]|$1|S|3|T|5|J|7|1Y|8|@]|D|@]|E|$K|L]]|$1|U|3|V|5|6|7|1Z|8|@$9|20|A|21|B|C]]|D|@]|E|$]]|$1|W|3|X|5|J|7|22|8|@]|D|@]|E|$K|L]]|$1|Y|3|Z|5|6|7|23|8|@$9|24|A|25|B|C]]|D|@]|E|$]]|$1|10|3|11|5|6|7|26|8|@]|D|@$9|27|A|28|1|29]]|E|$]]|$1|12|3|-4|5|6|7|2A|8|@]|D|@]|E|$]]]|13|$14|$5|15|16|17|E|$18|19]]|1A|$5|15|16|17|E|$18|1B]]|1C|$5|15|16|17|E|$18|1D]]]]

If you're using PostgreSQL 9.5+, you can take advantage of <a href="https://www.postgresql.org/docs/9.5/static/sql-select.html#SQL-FROM" rel="nofollow noreferrer"><code>TABLESAMPLE</code></a> to select a random record.

The two default sampling methods (<code>SYSTEM</code> and <code>BERNOULLI</code>) require that you specify the number of rows to return as a percentage of the total number of rows in the table.

<pre><code>-- Fetch 10% of the rows in the customers table.
SELECT * FROM customers TABLESAMPLE BERNOULLI(10);
</code></pre>

This requires knowing the amount of records in the table to select the appropriate percentage, which may not be easy to find quickly. Fortunately, there is the <a href="https://www.postgresql.org/docs/9.5/static/tsm-system-rows.html" rel="nofollow noreferrer"><code>tsm_system_rows</code> module</a> that allows you to specify the number of rows to return directly.

<pre><code>CREATE EXTENSION tsm_system_rows;

-- Fetch a single row from the customers table.
SELECT * FROM customers TABLESAMPLE SYSTEM_ROWS(1);
</code></pre>

To use this within ActiveRecord, first enable the extension within a migration:

<pre><code>class EnableTsmSystemRowsExtension &lt; ActiveRecord::Migration[5.0]
 def change
 enable_extension "tsm_system_rows"
 end
end
</code></pre>

Then modify the <code>from</code> clause of the query:

<pre><code>customer = Customer.from("customers TABLESAMPLE SYSTEM_ROWS(1)").first
</code></pre>

I don't know if the <code>SYSTEM_ROWS</code> sampling method will be entirely random or if it just returns the first row from a random page.

Most of this information was taken from a <a href="http://blog.2ndquadrant.com/tablesample-and-other-methods-for-getting-random-tuples/" rel="nofollow noreferrer">2ndQuadrant blog post written by Gulcin Yildirim</a>.

blocks|key|1262168|text|在看到这么多答案后，我决定在我的PostgreSQL(9.6.3)数据库上对它们进行基准测试。我使用了一个更小的100,000表，去掉了随机(“Model.order()”).first，因为它已经慢了两个数量级。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1262169|使用一个有2,500,000个条目和10列的表格，获胜者是pluck方法，几乎比亚军快8倍(偏移量。我只在本地服务器上运行，所以这个数字可能会被夸大，但它足够大，所以我最终将使用pluck方法。同样值得注意的是，这可能会导致问题是您一次提取多个结果，因为每个结果都是唯一的，也就是较少的随机性。|1262170|Pluck在我的25,000,000行表格上运行100次Edit:实际上，这一次包括了循环中的pluck，如果我把它取出来，它的运行速度和对id的简单迭代一样快。但是，它确实占用了相当多的RAM。|1262171|RandomModel+++++++++++++++++user+++++system++++++total++++++++real
Model.find_by(id:+i)+++++++0.050000+++0.010000+++0.060000+(++0.059878)
Model.offset(rand(offset))+0.030000+++0.000000+++0.030000+(+55.282410)
Model.find(ids.sample)+++++6.450000+++0.050000+++6.500000+(++7.902458)|code-block|syntax|javascript|1262172|下面是在我的100,000行表上运行2000次的数据，以排除随机|1262173|RandomModel+++++++user+++++system++++++total++++++++real
find_by:iterate++0.010000+++0.000000+++0.010000+(++0.006973)
offset+++++++++++0.000000+++0.000000+++0.000000+(++0.132614)
"RANDOM()"+++++++0.000000+++0.000000+++0.000000+(+24.645371)
pluck++++++++++++0.110000+++0.020000+++0.130000+(++0.175932)|1262174|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|R|8|@]|9|@]|A|$]]|$1|D|3|E|5|6|7|S|8|@]|9|@]|A|$]]|$1|F|3|G|5|H|7|T|8|@]|9|@]|A|$I|J]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|H|7|V|8|@]|9|@]|A|$I|J]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

After seeing so many answers I decided to benchmark them all on my PostgreSQL(9.6.3) database. I use a smaller 100,000 table and got rid of the Model.order("RANDOM()").first since it was already two orders of magnitude slower.

Using a table with 2,500,000 entries with 10 columns the hands down winner was the pluck method being almost 8 times faster than the runner up(offset. I only ran this on a local server so that number might be inflated but its bigger enough that the pluck method is what I'll end up using. It's also worth noting that this might cause issues is you pluck more than 1 result at a time since each one of those will be unique aka less random.

Pluck wins running 100 time on my 25,000,000 row table
Edit: actually this time includes the pluck in the loop if I take it out it it runs about as fast as simple iteration on the id. However; it does take up a fair amount of RAM.

<pre><code>RandomModel user system total real
Model.find_by(id: i) 0.050000 0.010000 0.060000 ( 0.059878)
Model.offset(rand(offset)) 0.030000 0.000000 0.030000 ( 55.282410)
Model.find(ids.sample) 6.450000 0.050000 6.500000 ( 7.902458)
</code></pre>

Here is the data running 2000 times on my 100,000 row table to rule out random

<pre><code>RandomModel user system total real
find_by:iterate 0.010000 0.000000 0.010000 ( 0.006973)
offset 0.000000 0.000000 0.000000 ( 0.132614)
"RANDOM()" 0.000000 0.000000 0.000000 ( 24.645371)
pluck 0.110000 0.020000 0.130000 ( 0.175932)
</code></pre>

blocks|key|1261889|text|我是一个全新的RoR新手，但我得到了一个适合我的代码：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1261890|+def+random
++++@cards+=+Card.all.sort_by+{+rand+}
+end|code-block|syntax|javascript|1261891|它来自于：|1261892|How+to+randomly+sort+(scramble)+an+array+in+Ruby?|offset|length|1261893|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/1816378/how-to-randomly-sort-scramble-an-array-in-ruby^0|0|0|0|0|1D|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@]|9|@]|A|$]]|$1|I|3|J|5|6|7|X|8|@]|9|@$K|Y|L|Z|1|10]]|A|$]]|$1|M|3|-4|5|6|7|11|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

I'm brand new to RoR but I got this to work for me:

<pre><code> def random
 @cards = Card.all.sort_by { rand }
 end
</code></pre>

It came from:

<a href="https://stackoverflow.com/questions/1816378/how-to-randomly-sort-scramble-an-array-in-ruby">How to randomly sort (scramble) an array in Ruby?</a>

blocks|key|1047347|text|接下来要做什么：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1047348|rand_record+=+Model.find(Model.pluck(:id).sample)|code-block|syntax|javascript|1047349|对我来说很清楚|1047350|entityMap^0|0|0|0^^$0|@$1|2|3|4|5|6|7|K|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|L|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|M|8|@]|9|@]|A|$]]|$1|I|3|-4|5|6|7|N|8|@]|9|@]|A|$]]]|J|$]]

What about to do:

<pre><code>rand_record = Model.find(Model.pluck(:id).sample)
</code></pre>

For me is much clear

blocks|key|1047441|text|我在我的应用程序上使用基准测试的rails+4.2.8+(我将1..Category.count设置为随机，因为如果随机取值为0，它将产生一个错误(ActiveRecord：：RecordNotFound:+Category‘t+find+Category+with+'id'=0))在我的应用程序中尝试了这个例子，而我的想法是：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1047442|+def+random1
2.4.1+:071?>+++Category.find(rand(1..Category.count))
2.4.1+:072?>+++end
+=>+:random1
2.4.1+:073+>+def+random2
2.4.1+:074?>++++Category.offset(rand(1..Category.count))
2.4.1+:075?>+++end
+=>+:random2
2.4.1+:076+>+def+random3
2.4.1+:077?>+++Category.offset(rand(1..Category.count)).limit(rand(1..3))
2.4.1+:078?>+++end
+=>+:random3
2.4.1+:079+>+def+random4
2.4.1+:080?>++++Category.pluck(rand(1..Category.count))
2.4.1+:081?>
2.4.1+:082+>+++++end
+=>+:random4
2.4.1+:083+>+n+=+100
+=>+100
2.4.1+:084+>+Benchmark.bm(7)+do+%7Cx%7C
2.4.1+:085+>+++++x.report("find")+{+n.times+{%7Ci%7C+random1+}+}
2.4.1+:086?>+++x.report("offset")+{+n.times+{%7Ci%7C+random2+}+}
2.4.1+:087?>+++x.report("offset_limit")+{+n.times+{%7Ci%7C+random3+}+}
2.4.1+:088?>+++x.report("pluck")+{+n.times+{%7Ci%7C+random4+}+}
2.4.1+:089?>+++end

++++++++++++++++++user++++++system++++++total+++++real
find++++++++++++0.070000+++0.010000+++0.080000+(0.118553)
offset++++++++++0.040000+++0.010000+++0.050000+(0.059276)
offset_limit++++0.050000+++0.000000+++0.050000+(0.060849)
pluck+++++++++++0.070000+++0.020000+++0.090000+(0.099065)|code-block|syntax|javascript|1047443|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

I try this of Sam's example on my App using rails 4.2.8 of Benchmark( I put 1..Category.count for random, because if the random takes a 0 it will produce an error(ActiveRecord::RecordNotFound: Couldn't find Category with 'id'=0)) and the mine was:

<pre><code> def random1
2.4.1 :071?&gt; Category.find(rand(1..Category.count))
2.4.1 :072?&gt; end
 =&gt; :random1
2.4.1 :073 &gt; def random2
2.4.1 :074?&gt; Category.offset(rand(1..Category.count))
2.4.1 :075?&gt; end
 =&gt; :random2
2.4.1 :076 &gt; def random3
2.4.1 :077?&gt; Category.offset(rand(1..Category.count)).limit(rand(1..3))
2.4.1 :078?&gt; end
 =&gt; :random3
2.4.1 :079 &gt; def random4
2.4.1 :080?&gt; Category.pluck(rand(1..Category.count))
2.4.1 :081?&gt;
2.4.1 :082 &gt; end
 =&gt; :random4
2.4.1 :083 &gt; n = 100
 =&gt; 100
2.4.1 :084 &gt; Benchmark.bm(7) do |x|
2.4.1 :085 &gt; x.report("find") { n.times {|i| random1 } }
2.4.1 :086?&gt; x.report("offset") { n.times {|i| random2 } }
2.4.1 :087?&gt; x.report("offset_limit") { n.times {|i| random3 } }
2.4.1 :088?&gt; x.report("pluck") { n.times {|i| random4 } }
2.4.1 :089?&gt; end

 user system total real
find 0.070000 0.010000 0.080000 (0.118553)
offset 0.040000 0.010000 0.050000 (0.059276)
offset_limit 0.050000 0.000000 0.050000 (0.060849)
pluck 0.070000 0.020000 0.090000 (0.099065)
</code></pre>

blocks|key|1262389|text|.order('RANDOM()').limit(limit)看起来很整洁，但对于大型表来说很慢，因为即使limit为1，它也需要获取所有行并对其进行排序(在数据库内部，而不是在Rails中)。我对MySQL不是很确定，但是Postgres会发生这种情况。在here和here中有更多解释。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1262390|大型表的一种解决方案是.from("products+TABLESAMPLE+SYSTEM(0.5)")，其中0.5表示0.5%25。但是，如果您有过滤掉大量行的WHERE条件，我发现这个解决方案仍然很慢。我猜这是因为TABLESAMPLE+SYSTEM(0.5)在应用WHERE条件之前获取所有行。|1262391|针对大型表(但不是非常随机)的另一种解决方案是：|1262392|products_scope.limit(sample_size).sample(limit)|code-block|syntax|javascript|1262393|其中sample_size可以是100+(但不要太大，否则会很慢并消耗大量内存)，而limit可以是1。请注意，虽然这很快，但它不是真正的随机，它只在sample_size记录中是随机的。|1262394|PS:上面答案中的基准测试结果是不可靠的(至少在Postgres中是这样)，因为由于有了数据库缓存，一些第二次运行的数据库查询可能会比第一次运行的速度快得多。不幸的是，在Postgres中没有简单的方法来禁用缓存来保证这些基准测试的可靠性。|1262395|entityMap|0|LINK|mutability|MUTABLE|url|https://stackoverflow.com/questions/5297396/quick-random-row-selection-in-postgres|1|https://stackoverflow.com/questions/8674718/best-way-to-select-random-rows-postgresql/8674844#8674844^0|0|V|1H|5|3L|4|0|3Q|4|1|0|B|15|1J|3|1O|4|27|5|2Z|N|3P|5|0|0|0|2|B|G|3|16|5|1E|1|23|B|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@$9|13|A|14|B|C]|$9|15|A|16|B|C]]|D|@$9|17|A|18|1|19]|$9|1A|A|1B|1|1C]]|E|$]]|$1|F|3|G|5|6|7|1D|8|@$9|1E|A|1F|B|C]|$9|1G|A|1H|B|C]|$9|1I|A|1J|B|C]|$9|1K|A|1L|B|C]|$9|1M|A|1N|B|C]|$9|1O|A|1P|B|C]]|D|@]|E|$]]|$1|H|3|I|5|6|7|1Q|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|1R|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|1S|8|@$9|1T|A|1U|B|C]|$9|1V|A|1W|B|C]|$9|1X|A|1Y|B|C]|$9|1Z|A|20|B|C]|$9|21|A|22|B|C]]|D|@]|E|$]]|$1|Q|3|R|5|6|7|23|8|@]|D|@]|E|$]]|$1|S|3|-4|5|6|7|24|8|@]|D|@]|E|$]]]|T|$U|$5|V|W|X|E|$Y|Z]]|10|$5|V|W|X|E|$Y|11]]]]

<code>.order('RANDOM()').limit(limit)</code> looks neat but is slow for large tables because it needs to fetch and sort all rows even if <code>limit</code> is 1 (internally in database but not in Rails). I'm not sure about MySQL but this happens in Postgres. More explanation in <a href="https://stackoverflow.com/questions/5297396/quick-random-row-selection-in-postgres">here</a> and <a href="https://stackoverflow.com/questions/8674718/best-way-to-select-random-rows-postgresql/8674844#8674844">here</a>.

One solution for large tables is <code>.from("products TABLESAMPLE SYSTEM(0.5)")</code> where <code>0.5</code> means <code>0.5%</code>. However, I find this solution is still slow if you have <code>WHERE</code> conditions that filter out a lot of rows. I guess it's because <code>TABLESAMPLE SYSTEM(0.5)</code> fetch all rows before <code>WHERE</code> conditions apply.

Another solution for large tables (but not very random) is:

<pre><code>products_scope.limit(sample_size).sample(limit)
</code></pre>

where <code>sample_size</code> can be <code>100</code> (but not too large otherwise it's slow and consumes a lot of memory), and <code>limit</code> can be <code>1</code>. Note that although this is fast but it's not really random, it's random within <code>sample_size</code> records only.

PS: Benchmark results in answers above are not reliable (at least in Postgres) because some DB queries running at 2nd time can be significantly faster than running at 1st time, thanks to DB cache. And unfortunately there is no easy way to disable cache in Postgres to make these benchmarks reliable.

blocks|key|1262482|text|除了使用RANDOM()之外，您还可以将以下代码放入作用域：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1262483|class+Thing
++scope+:random,+->+(limit+=+1)+{
++++order('RANDOM()').
++++limit(limit)
++}
end|code-block|syntax|javascript|1262484|或者，如果你不想把它作为一个作用域，那就把它放到一个类方法中。现在，Thing.random与Thing.random(n)一起工作。|1262485|entityMap^0|4|8|0|0|Y|C|1B|F|0^^$0|@$1|2|3|4|5|6|7|O|8|@$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|L|5|6|7|S|8|@$9|T|A|U|B|C]|$9|V|A|W|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|X|8|@]|D|@]|E|$]]]|N|$]]

Along with using <code>RANDOM()</code>, you can also throw this into a scope:

<pre><code>class Thing
 scope :random, -&gt; (limit = 1) {
 order('RANDOM()').
 limit(limit)
 }
end
</code></pre>

Or, if you don't fancy that as a scope, just throw it into a class method. Now <code>Thing.random</code> works along with <code>Thing.random(n)</code>.

blocks|key|1262499|text|您可以获取所有ids的数组，然后使用sample方法返回随机元素。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1262500|Model.ids.sample|code-block|syntax|javascript|1262501|entityMap|0|LINK|mutability|MUTABLE|url|https://apidock.com/rails/Array/sample^0|I|6|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@$A|R|B|S|1|T]]|C|$]]|$1|D|3|E|5|F|7|U|8|@]|9|@]|C|$G|H]]|$1|I|3|-4|5|6|7|V|8|@]|9|@]|C|$]]]|J|$K|$5|L|M|N|C|$O|P]]]]

You can get array of all ids and then return random element with <a href="https://apidock.com/rails/Array/sample" rel="nofollow noreferrer">sample</a> method.
<pre><code>Model.ids.sample
</code></pre>

blocks|key|1047677|text|如果你想在你选择的数据库上运行基准测试，这里有一个模板：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1047678|gem+'activerecord',+git:+'https://github.com/rails/rails'
gem+'sqlite3'
gem+'benchmark'

require+'active_record'
require+'benchmark'

ActiveRecord::Base.establish_connection(adapter:+'sqlite3',+database:+':memory:')

ActiveRecord::Schema.define+do
++create_table+:users
end

class+User+<+ActiveRecord::Base
++def+self.sample_random
++++order('RANDOM()').first
++end

++def+self.sample_pluck_id_sample
++++find(pluck(:id).sample)
++end

++def+self.sample_all_sample
++++all.sample
++end

++def+self.sample_offset_rand_count
++++offset(rand(count)).first
++end
end

USERS_COUNTS+=+[1000,+10_000,+100_000,+1_000_000]
N+=+100

USERS_COUNTS.each+do+%7Ccount%7C
++puts+"Creating+#{count}+users"

++User.insert_all((1..count).map+{+%7Cid%7C+{+id:+id+}+})

++Benchmark.bm+do+%7Cx%7C
++++x.report("sample_random")+{+N.times+{+User.sample_random+}+}
++++x.report("sample_offset_rand_count")+{+N.times+{+User.sample_offset_rand_count+}+}
++++if+count+<+10_000
++++++x.report("sample_pluck_id_sample")+{+N.times+{+User.sample_pluck_id_sample+}+}
++++++x.report("sample_all_sample")+{+N.times+{+User.sample_all_sample+}+}
++++end
++end

++puts+"Deleting+#{User.count}+users"

++User.delete_all
end|code-block|syntax|javascript|1047679|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

If you want to run benchmarks on your database of choice, here is a template:
<pre class="lang-rb prettyprint-override"><code>gem 'activerecord', git: 'https://github.com/rails/rails'
gem 'sqlite3'
gem 'benchmark'

require 'active_record'
require 'benchmark'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

ActiveRecord::Schema.define do
 create_table :users
end

class User &lt; ActiveRecord::Base
 def self.sample_random
 order('RANDOM()').first
 end

 def self.sample_pluck_id_sample
 find(pluck(:id).sample)
 end

 def self.sample_all_sample
 all.sample
 end

 def self.sample_offset_rand_count
 offset(rand(count)).first
 end
end

USERS_COUNTS = [1000, 10_000, 100_000, 1_000_000]
N = 100

USERS_COUNTS.each do |count|
 puts &quot;Creating #{count} users&quot;

 User.insert_all((1..count).map { |id| { id: id } })

 Benchmark.bm do |x|
 x.report(&quot;sample_random&quot;) { N.times { User.sample_random } }
 x.report(&quot;sample_offset_rand_count&quot;) { N.times { User.sample_offset_rand_count } }
 if count &lt; 10_000
 x.report(&quot;sample_pluck_id_sample&quot;) { N.times { User.sample_pluck_id_sample } }
 x.report(&quot;sample_all_sample&quot;) { N.times { User.sample_all_sample } }
 end
 end

 puts &quot;Deleting #{User.count} users&quot;

 User.delete_all
end
</code></pre>

I'm in need of getting a random record from a table via ActiveRecord. I've followed the example from <a href="http://weblog.jamisbuck.org/2006/10/7/helping-activerecord-finders-help-you" rel="noreferrer">Jamis Buck from 2006</a>.

However, I've also come across another way via a Google search (can't attribute with a link due to new user restrictions):

<pre><code> rand_id = rand(Model.count)
 rand_record = Model.first(:conditions =&gt; ["id &gt;= ?", rand_id])
</code></pre>

I'm curious how others on here have done it or if anyone knows what way would be more efficient.

Random record in ActiveRecord

SQL数据库

PostgreSQL

数据库

服务器

MySQL

我需要通过ActiveRecord从表中获取随机记录。我遵循了中的示例。然而，我也通过谷歌搜索遇到了另一种方式(由于新的用户限制，无法使用链接)： rand_id = rand(Model.count) rand_record = Model.first(:conditions => ["id >= ?", rand_id])我很好奇这里的其他人是如何做到的，或者是否有人知道什么方法会更有效。

问ActiveRecord中的随机记录
EN

回答 18

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