Update: The following incorporates the ISNULL aspect of the original query:

    from django.db.models import Sum
    
    ModelName.objects.filter(field_name__isnull=True).aggregate(Sum(&#39;field_name&#39;))
    # returns {&#39;field_name__sum&#39;: 1000} for example

You&#39;re looking for the Sum aggregation function, which works as follows:

    ModelName.objects.aggregate(Sum(&#39;field_name&#39;))

See: https://docs.djangoproject.com/en/dev/ref/models/querysets/#sum


How can i get the name of the class

    String.class.getName()  returns java.lang.String

&lt;br&gt;
I am only interested in getting last part  ie only String
&lt;br&gt;
Any Api can do that?



getting only name of the class Class.getName()

How do I compare values of generic types?

I have reduced it to a minimal sample:

    public class Foo&lt;T&gt; where T : IComparable
    {
        private T _minimumValue = default(T);

        public bool IsInRange(T value) 
        {
            return (value &gt;= _minimumValue); // &lt;-- Error here
        }
    }

The error is:

&gt;Operator &#39;&gt;=&#39; cannot be applied to operands of type &#39;T&#39; and &#39;T&#39;.

What on earth!? `T` is already constrained to `IComparable`, and even when constraining it to value types (`where T: struct`), we still can&#39;t apply any of the operators `&lt;`, `&gt;`, `&lt;=`, `&gt;=`, `==` or `!=`. (I know that workarounds involving `Equals()` exist for `==` and `!=`, but it doesn&#39;t help for the relational operators).

So, two questions:

 1. **Why** do we observe this weird behaviour? What keeps us from comparing the values of generic types which are *known* to be `IComparable`? Doesn&#39;t it somehow defeat the entire purpose of generic constraints?
 2. How do I resolve this, or at least work around it?

(I realize there are already a handful of questions related to this seemingly simple problem - but none of the threads gives an exhaustive or workable answer, so here.)

How to compare values of generic types?

I have a query akin to the following:

```sql
SELECT SUM(ISNULL(table.name)) FROM table
```

How does that `SUM` translate into a `QuerySet` in Django? i.e. What operation `xyz` does it translate to, in something like `MyModel.objects.xyz()`?

Django SUM Query?

I have a query akin to the following:
<pre><code class="hljs language-sql">SELECT SUM(ISNULL(table.name)) FROM table
</code></pre>
How does that <code>SUM</code> translate into a <code>QuerySet</code> in Django? i.e. What operation <code>xyz</code> does it translate to, in something like <code>MyModel.objects.xyz()</code>?

I have a sorted list, let say: (its not really just numbers, its a list of objects that are sorted with a complicated time consuming algorithm)

    mylist = [ 1 , 2 , 3 , 4 , 5 , 6 , 7 , 8 ,9  , 10 ]

Is there some python function that will give me N of the items, but will keep the order?

Example:

    randomList = getRandom(mylist,4)
    # randomList = [ 3 , 6 ,7 , 9 ]
    randomList = getRandom(mylist,4)
    # randomList = [ 1 , 2 , 4 , 8 ]

etc...

Get random sample from list while maintaining ordering of items?

I am using Django to build a website with MySQL. Now as I am learning so I need to change the Model very often so I want that all tables get cleared and new table get created.

But `syncdb` doesn&#39;t touch existing tables. Is there any  better way to handle this problem?


What is the easiest way to clear a database from the CLI with manage.py in Django?

In Python, what is the best way to compute the difference between two lists?

example

    A = [1,2,3,4]
    B = [2,5]
    
    A - B = [1,3,4]
    B - A = [5]

Compute list difference

In the [Wikipedia page describing short-circuit evaluation][1], `&amp;` and `|` are listed as eager operators in Python. What does this mean and when are they used in the language? 


  [1]: http://en.wikipedia.org/wiki/Short-circuit_evaluation

Where are the ampersand and vertical bar characters used in Python?

I need to pick out &quot;x&quot; number of non-repeating, random numbers out of a list. For example:

    all_data = [1, 2, 2, 3, 4, 5, 6, 7, 8, 8, 9, 10, 11, 11, 12, 13, 14, 15, 15]

How do I pick out a list like `[2, 11, 15]` and not `[3, 8, 8]`?



How do you pick &quot;x&quot; number of unique numbers from a list in Python?

I am trying to find out the best way for testing `admin.ModelAdmin` in `admin.py`. Specifically I am overriding the `save_model()` function which I want to test. From the research I have done, the only solution I have found was writing a request/response test and then query the database.

testing admin.ModelAdmin in django

I try to post parameter like 

     jQuery.ajax(
			{
				&#39;type&#39;: &#39;POST&#39;,
        		&#39;url&#39;: url,
        		&#39;contentType&#39;: &#39;application/json&#39;,
        		&#39;data&#39;: &quot;{content:&#39;xxx&#39;}&quot;,
        		&#39;dataType&#39;: &#39;json&#39;,
        		&#39;success&#39;: rateReviewResult	
			}
		);

However, Django return ` Forbidden 403. CSRF verification failed. Request aborted.`
I am using `&#39;django.middleware.csrf.CsrfViewMiddleware&#39;` and couldn&#39;t find how I can prevent this problem without compromising security.





&quot;CSRF token missing or incorrect&quot; while post parameter via AJAX in Django

I am using SQlAlchemy in my web project. What should I use - `scoped_session(sessionmaker())` or plain `sessionmaker()` - and why? Or should I use something else? 

    ## model.py
    from sqlalchemy import *
    from sqlalchemy.orm import *
    
    engine = create_engine(&#39;mysql://dbUser:dbPassword@dbServer:dbPort/dbName&#39;,
    pool_recycle=3600, echo=False)
    metadata = MetaData(engine)
    Session = scoped_session(sessionmaker())
    Session.configure(bind=engine)
    user = Table(&#39;user&#39;, metadata, autoload=True)

    class User(object):
	pass

    usermapper = mapper(User, user)

    ## some other python file called abc.py
    from models import *

    def getalluser():
       session = Session()	
       session.query(User).all()
       session.flush()
       session.close()

    ## onemore file defg.py
    from models import *

    def updateuser():
       session = Session()	
       session.query(User).filter(User.user_id == &#39;4&#39;).update({User.user_lname: &#39;villkoo&#39;})
       session.commit()
       session.flush()
       session.close()

I create a `session = Session()` object for each request and I close it. Am I doing the right thing or is there a better way to do it?



scoped_session(sessionmaker()) or plain sessionmaker() in sqlalchemy?

I&#39;ve got an app where users create pages. I want to run a simple DB query that returns how many users have created more than 2 pages.

This is essentially what I want to do, but of course it&#39;s not the right method:

    User.objects.select_related(&#39;page__gte=2&#39;).count()

What am I missing?

Django Query Related Field Count

I want to write a Django query equivalent to this SQL query:

    SELECT * from user where income &gt;= 5000 or income is NULL.

How to construct the Django queryset filter?

    User.objects.filter(income__gte=5000, income=0)

This doesn&#39;t work, because it `AND`s the filters. I want to `OR` the filters to get union of individual querysets.

How to perform OR condition in django queryset?

Hey i have looked around through some simliar posts here on SO but havent found anything that has solved my problem. I have the following models,

    from django.db import models
    
    class Areas(models.Model):
    
    	name =  models.CharField(max_length = 120)
    	order_in_sidebar_network = models.IntegerField(blank=True, null=True)
    	order_in_section_network = models.IntegerField(blank=True, null=True)
    
    
    	def __unicode__ (self):
    		return self.area_name
    
    	class Meta:
    		verbose_name_plural = &quot;Areas&quot;
    		verbose_name = &quot;Area&quot;
    
    class Countries(models.Model):
    	name = models.CharField(max_length = 120, help_text = &quot;The name of the country&quot;)
    	area = models.ForeignKey(Areas, verbose_name = &#39;Area&#39;)
    	
    	def __unicode__ (self):
    		return self.name
    
    	class Meta:
    		verbose_name_plural = &quot;Countries&quot;
    		verbose_name = &quot;Country&quot;
    		ordering = [&#39;name&#39;]
    
    
    
    class Offices(models.Model):
    	country = models.ForeignKey(Countries, verbose_name = &#39;Country&#39;)
    	name = models.CharField(max_length = 255, help_text = &quot;The name of this office, IE London&quot;)
    	main_office = models.BooleanField(default= False, help_text = &quot;Is this office a key location?&quot;, verbose_name = &quot;Key Location&quot;)
    	address_1 = models.CharField(max_length = 255, null = True, blank = True)
    	address_2 = models.CharField(max_length = 255, null = True, blank = True)
    	address_3 = models.CharField(max_length = 255, null = True, blank = True)
    	city = models.CharField(max_length = 255, null = True, blank = True)
    	postcode = models.CharField(max_length = 20)
    	tel = models.CharField(max_length = 30, null= True, blank = True, help_text = &quot;Optional telephone contact number&quot;)
    	mobile = models.CharField(max_length = 30, null= True, blank = True, help_text = &quot;Optional mobile contact number&quot;)
    	fax = models.CharField(max_length = 30, null= True, blank = True, help_text = &quot;Optional fax contact number&quot;)
    	data_1 = models.CharField(max_length = 255, null = True, blank = True, help_text = &quot;Optional additional data&quot;, verbose_name = &quot;Additional information&quot;)
    	data_2 = models.CharField(max_length = 255, null = True, blank = True, help_text = &quot;Optional additional data&quot;, verbose_name = &quot;Additional information&quot;)
    	
    	class Meta:
    		verbose_name_plural = &quot;Offices&quot;
    		verbose_name = &quot;Office&quot;
    		ordering = [&#39;name&#39;]
    	
    	def __unicode__(self):
    		return self.name
    	
    class OfficeMembers(models.Model):
    	name = models.CharField(max_length = 60, help_text = &quot;Please tell us this person name&quot;)
    	title = models.CharField(max_length = 100, help_text = &quot;The person&#39;s title, IE Managing Director&quot;)
    	email = models.EmailField(max_length = 255, null = True, blank = True, help_text = &quot;Optional email address for this person&quot;)
    	email2 = models.EmailField(max_length = 255, null = True, blank = True, help_text = &quot;Optional second email address for this person&quot;)
    	phone = models.CharField(max_length = 30, null = True, blank  = True, help_text = &quot;Optional contact number for this person&quot;)
    	mobile = models.CharField(max_length = 30, null = True, blank  = True, help_text = &quot;Optional mobile contact number for this person&quot;)
    	office = models.ForeignKey(Offices, null = True)
    	class Meta:
    		verbose_name_plural = &quot;Office Memebers&quot;
    		verbose_name = &quot;Office memebr&quot;
    		ordering = [&#39;name&#39;]
    
    	def __unicode__(self):
    		return self.name

i have the following view set up

    def index(request):
    	
    	cache_key = &quot;world_areas&quot;
    	cache_time = 60
    	
    	world_areas_cache = cache.get(cache_key)
    		
    	#if no cache is set, grab the objects, and set the cache
    	
    	logger.debug(world_areas)
    	if not world_areas_cache:
    		logger.info(&#39;No cache found grabbing objects&#39;)
    		world_areas = Areas.objects.select_related().all()
    		#cache.set(cache_key, world_areas, cache_time)
    		logger.debug(world_areas)
    	else:
    		logger.info(&quot;Getting from cache&quot;)
    		world_areas = world_areas_cache
    		
    	return render_to_response(&#39;network/index.html&#39;, {&#39;world_areas&#39;:world_areas}, context_instance=RequestContext(request))

trying to iterate over the world_areas object like so

    {% for area in world_areas %}
produces a template syntax error

&gt;  &#39;RelatedManager&#39; object is not iterable

Any one got any ideas why this is happeing? really cant seem to get round this! strangley this is working for me in shell :S am i missing something obvious???

Big thanks to anyone able to help!

&#39;RelatedManager&#39; object is not iterable Django

I have a model `Employees` and I would like to have a QuerySet of all rows, but with some specific fields from each row, and not all fields.

I know how to query all rows from the table/model:

    Employees.objects.all()

I would like to know how to select fields for each of the Queryset element. How can I do that?

How to obtain a QuerySet of all rows, with specific fields for each one of them?

I have a model of work orders, with a field for when the work order is required by. To get a list of work orders, with those that are required early, I do this:

`wo = Work_Order.objects.order_by(&#39;dateWORequired&#39;)`

This works nicely, but ONLY if there is actually a value in that field. If there is no required date, then the value is `None`. Then, the list of work orders has all the `None`&#39;s at the top, and then the remaining work orders in proper order. 

How can I get the `None`&#39;s at the bottom?

Content Type	Original Author	Original Content on Stackoverflow
Question	user541686	View Question on Stackoverflow
Solution 1 - Python	rolling stone	View Answer on Stackoverflow

Django SUM Query?

Python Problem Overview

Python Solutions

Solution 1 - Python

How to compare values of generic types?

getting only name of the class Class.getName()

Attributions