blocks|key|1321643|text|只需使用：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1321644|MyModel.objects.order_by('?').first()|code-block|syntax|javascript|1321645|它在QuerySet+API中有文档记录。|offset|length|1321646|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.order_by^0|0|0|2|C|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@$I|V|J|W|1|X]]|A|$]]|$1|K|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

Simply use:

<pre><code>MyModel.objects.order_by('?').first()
</code></pre>

It is documented in <a href="https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.order_by" rel="noreferrer">QuerySet API</a>.

blocks|key|1321700|text|使用order_by('?'):N的解决方案非常慢，即使对于中等大小的表，如果您使用MySQL+(不了解其他数据库)。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1321701|order_by('?')[:N]将被转换为SELECT+...+FROM+...+WHERE+...+ORDER+BY+RAND()+LIMIT+N查询。|offset|length|style|CODE|1321702|这意味着对于表中的每一行，都将执行RAND()函数，然后根据该函数的值对整个表进行排序，然后返回前N条记录。如果您的表很小，这是很好的。但在大多数情况下，这是一个非常慢的查询。|1321703|我写了一个简单的函数，即使id有空洞也能工作(删除了一些行)：|1321704|def+get_random_item(model,+max_id=None):
++++if+max_id+is+None:
++++++++max_id+=+model.objects.aggregate(Max('id')).values()[0]
++++min_id+=+math.ceil(max_id*random.random())
++++return+model.objects.filter(id__gte=min_id)[0]|code-block|syntax|javascript|1321705|它比order_by('?')更快几乎在所有的情况下。|1321706|entityMap^0|0|0|H|M|1H|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|V|8|@$D|W|E|X|F|G]|$D|Y|E|Z|F|G]]|9|@]|A|$]]|$1|H|3|I|5|6|7|10|8|@]|9|@]|A|$]]|$1|J|3|K|5|6|7|11|8|@]|9|@]|A|$]]|$1|L|3|M|5|N|7|12|8|@]|9|@]|A|$O|P]]|$1|Q|3|R|5|6|7|13|8|@]|9|@]|A|$]]|$1|S|3|-4|5|6|7|14|8|@]|9|@]|A|$]]]|T|$]]

The solutions with order_by('?')[:N] are extremely slow even for medium-sized tables if you use MySQL (don't know about other databases).

<code>order_by('?')[:N]</code> will be translated to <code>SELECT ... FROM ... WHERE ... ORDER BY RAND() LIMIT N</code> query.

It means that for every row in table the RAND() function will be executed, then the whole table will be sorted according to value of this function and then first N records will be returned. If your tables are small, this is fine. But in most cases this is a very slow query.

I wrote simple function that works even if id's have holes (some rows where deleted):

<pre><code>def get_random_item(model, max_id=None):
 if max_id is None:
 max_id = model.objects.aggregate(Max('id')).values()[0]
 min_id = math.ceil(max_id*random.random())
 return model.objects.filter(id__gte=min_id)[0]
</code></pre>

It is faster than order_by('?') in almost all cases.

blocks|key|1537005|text|使用order_by('?')将在生产的第二天杀死数据库服务器。一种更好的方法是类似于Getting+a+random+row+from+a+relational+database中描述的方法。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1537006|from+django.db.models.aggregates+import+Count
from+random+import+randint

class+PaintingManager(models.Manager):
++++def+random(self):
++++++++count+=+self.aggregate(count=Count('id'))['count']
++++++++random_index+=+randint(0,+count+-+1)
++++++++return+self.all()[random_index]|code-block|syntax|javascript|1537007|entityMap|0|LINK|mutability|MUTABLE|url|http://web.archive.org/web/20110802060451/http://bolddream.com/2010/01/22/getting-a-random-row-from-a-relational-database/^0|2|D|17|1B|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@$9|V|A|W|1|X]]|E|$]]|$1|F|3|G|5|H|7|Y|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|L|$M|$5|N|O|P|E|$Q|R]]]]

Using <code>order_by('?')</code> will kill the db server on the second day in production. A better way is something like what is described in <a href="http://web.archive.org/web/20110802060451/http://bolddream.com/2010/01/22/getting-a-random-row-from-a-relational-database/" rel="noreferrer">Getting a random row from a relational database</a>.

<pre><code>from django.db.models.aggregates import Count
from random import randint

class PaintingManager(models.Manager):
 def random(self):
 count = self.aggregate(count=Count('id'))['count']
 random_index = randint(0, count - 1)
 return self.all()[random_index]
</code></pre>

blocks|key|1537080|text|只需注意一个(相当常见的)特殊情况，如果表中有一个没有删除的索引自动增量列，执行随机选择的最佳方法是查询，如下所示：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1537081|SELECT+*+FROM+table+WHERE+id+=+RAND()+LIMIT+1|code-block|syntax|javascript|1537082|这假设表有一个名为id的列。在django中，您可以通过以下方式完成此操作：|1537083|Painting.objects.raw('SELECT+*+FROM+appname_painting+WHERE+id+=+RAND()+LIMIT+1')|1537084|其中必须将appname替换为您应用程序名称。|1537085|一般情况下，带有id列的order_by('?')使用以下命令可以更快地完成此操作：|1537086|Paiting.objects.raw(
++++++++'SELECT+*+FROM+auth_user+WHERE+id>=RAND()+*+(SELECT+MAX(id)+FROM+auth_user)+LIMIT+%25d'+
++++%25+needed_count)|1537087|entityMap^0|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|U|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|W|8|@]|9|@]|A|$]]|$1|M|3|N|5|6|7|X|8|@]|9|@]|A|$]]|$1|O|3|P|5|D|7|Y|8|@]|9|@]|A|$E|F]]|$1|Q|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]]|R|$]]

Just to note a (fairly common) special case, if there is a indexed auto-increment column in the table with no deletes, the optimum way to do a random select is a query like:

<pre><code>SELECT * FROM table WHERE id = RAND() LIMIT 1
</code></pre>

that assumes such a column named id for table. In django you can do this by:

<pre><code>Painting.objects.raw('SELECT * FROM appname_painting WHERE id = RAND() LIMIT 1')
</code></pre>

in which you must replace appname with your application name.

In General, with an id column, the order_by('?') can be done much faster with:

<pre><code>Paiting.objects.raw(
 'SELECT * FROM auth_user WHERE id&gt;=RAND() * (SELECT MAX(id) FROM auth_user) LIMIT %d' 
 % needed_count)
</code></pre>

blocks|key|1321861|text|这只是我如何做到的一个简单的想法：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1321862|def+_get_random_service(self,+professional):
++++services+=+Service.objects.filter(professional=professional)
++++i+=+randint(0,+services.count()-1)
++++return+services[i]|code-block|syntax|javascript|1321863|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|$]]

Just a simple idea how I do it:

<pre><code>def _get_random_service(self, professional):
 services = Service.objects.filter(professional=professional)
 i = randint(0, services.count()-1)
 return services[i]
</code></pre>

blocks|key|1537198|text|其他答案要么可能很慢(使用order_by('?'))，要么使用多个SQL查询。下面是一个示例解决方案，没有排序，只有一个查询(假设Postgres)：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1537199|random_instance_or_none+=+Model.objects.raw('''
++++select+*+from+{0}+limit+1
++++offset+floor(random()+*+(select+count(*)+from+{0}))
'''.format(Model._meta.db_table)).first()|code-block|syntax|javascript|1537200|请注意，如果表为空，这将引发索引错误。编写一个与模型无关的辅助函数来检查这一点。|1537201|entityMap^0|D|D|0|0|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]]|$1|K|3|L|5|6|7|S|8|@]|D|@]|E|$]]|$1|M|3|-4|5|6|7|T|8|@]|D|@]|E|$]]]|N|$]]

The other answers are either potentially slow (using <code>order_by('?')</code>) or use more than one SQL query. Here's a sample solution with no ordering and just one query (assuming Postgres):
<pre class="lang-py prettyprint-override"><code>random_instance_or_none = Model.objects.raw('''
 select * from {0} limit 1
 offset floor(random() * (select count(*) from {0}))
'''.format(Model._meta.db_table)).first()
</code></pre>
Be aware that this will raise an index error if the table is empty. Write yourself a model-agnostic helper function to check for that.

blocks|key|1537272|text|一种更简单的方法是，只需向下过滤到感兴趣的记录集，并使用random.sample选择所需的记录集：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1537273|from+myapp.models+import+MyModel
import+random

my_queryset+=+MyModel.objects.filter(criteria=True)++#+Returns+a+QuerySet
my_object+=+random.sample(my_queryset,+1)++#+get+a+single+random+element+from+my_queryset
my_objects+=+random.sample(my_queryset,+5)++#+get+five+random+elements+from+my_queryset|code-block|syntax|javascript|1537274|请注意，您应该准备了一些代码来验证my_queryset是否为空；如果第一个参数包含的元素太少，则random.sample返回ValueError:+sample+larger+than+population。|1537275|entityMap^0|S|D|0|0|H|B|1D|D|1S|15|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]|$9|X|A|Y|B|C]]|D|@]|E|$]]|$1|M|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|N|$]]

One much easier approach to this involves simply filtering down to the recordset of interest and using <code>random.sample</code> to select as many as you want:

<pre><code>from myapp.models import MyModel
import random

my_queryset = MyModel.objects.filter(criteria=True) # Returns a QuerySet
my_object = random.sample(my_queryset, 1) # get a single random element from my_queryset
my_objects = random.sample(my_queryset, 5) # get five random elements from my_queryset
</code></pre>

Note that you should have some code in place to verify that <code>my_queryset</code> is not empty; <code>random.sample</code> returns <code>ValueError: sample larger than population</code> if the first argument contains too few elements.

blocks|key|1537409|text|嗨，我需要从查询集中随机选择一条记录，我也需要报告谁的长度(例如，网页生成了描述的项目，并表示留下了记录)|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1537410|q+=+Entity.objects.filter(attribute_value='this+or+that')
item_count+=+q.count()
random_item+=+q[random.randomint(1,item_count%2B1)]|code-block|syntax|javascript|1537411|所需时间(0.7秒与1.7秒)的一半：|1537412|item_count+=+q.count()
random_item+=+random.choice(q)|1537413|我猜它避免了在选择随机条目之前拉下整个查询，并使我的系统对重复访问的页面具有足够的响应能力，因为用户希望看到item_count倒计时。|1537414|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]]|$1|G|3|H|5|6|7|Q|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|S|8|@]|9|@]|A|$]]|$1|M|3|-4|5|6|7|T|8|@]|9|@]|A|$]]]|N|$]]

Hi I needed to select a random record from a queryset who's length I also needed to report (ie web page produced described item and said records left)

<pre><code>q = Entity.objects.filter(attribute_value='this or that')
item_count = q.count()
random_item = q[random.randomint(1,item_count+1)]
</code></pre>

took half as long(0.7s vs 1.7s) as:

<pre><code>item_count = q.count()
random_item = random.choice(q)
</code></pre>

I'm guessing it avoids pulling down the whole query before selecting the random entry and made my system responsive enough for a page that is accessed repeatedly for a repetitive task where users want to see the item_count count down.

blocks|key|1322185|text|我有一个非常简单的解决方案，自定义管理器：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1322186|class+RandomManager(models.Manager):
++++def+random(self):
++++++++return+random.choice(self.all())|code-block|syntax|javascript|1322187|然后添加模型：|1322188|class+Example(models.Model):
++++name+=+models.CharField(max_length=128)
++++objects+=+RandomManager()|1322189|现在，您可以使用它：|1322190|Example.objects.random()|1322191|entityMap^0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|Q|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|R|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|S|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|T|8|@]|9|@]|A|$E|F]]|$1|K|3|L|5|6|7|U|8|@]|9|@]|A|$]]|$1|M|3|N|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|O|3|-4|5|6|7|W|8|@]|9|@]|A|$]]]|P|$]]

I got very simple solution, make custom manager:

<pre><code>class RandomManager(models.Manager):
 def random(self):
 return random.choice(self.all())
</code></pre>

and then add in model:

<pre><code>class Example(models.Model):
 name = models.CharField(max_length=128)
 objects = RandomManager()
</code></pre>

Now, you can use it:

<pre><code>Example.objects.random()
</code></pre>

I have a model that represents paintings I present on my site. On the main webpage I'd like to show some of them: newest, one that was not visited for most time, most popular one and a random one. 

I'm using Django 1.0.2.

While first 3 of them are easy to pull using django models, last one (random) causes me some trouble. I can ofc code it in my view, to something like this:

<pre><code>number_of_records = models.Painting.objects.count()
random_index = int(random.random()*number_of_records)+1
random_paint = models.Painting.get(pk = random_index)
</code></pre>

It doesn't look like something I'd like to have in my view tho - this is entirely part of database abstraction and should be in the model. Also, here I need to take care of removed records (then number of all records won't cover me all the possible key values) and probably lots of other things.

Any other options how I can do it, preferably somehow inside the model abstraction?

How to pull a random record using Django's ORM?

数据库

服务器

Django

MySQL

我有一个模型，代表我在我的网站上展示的绘画。在主网页上，我想展示其中的一些:最新的，大多数时间没有访问的，最受欢迎的和随机的。我使用的是Django 1.0.2。虽然前3个模型很容易使用django模型来提取，但最后一个(随机)给我带来了一些麻烦。我可以在我的视图中对其进行ofc编码，如下所示：number_of_re...

问如何使用Django的ORM拉取随机记录？
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