This is not really django specific; more to do with databases.  You add indexes on columns when you want to speed up searches on that column.

Typically, only the primary key is indexed by the database. This means look ups using the primary key are optimized.

If you do a lot of lookups on a secondary column, consider adding an index to that column to speed things up.

Keep in mind, like most problems of scale, these only apply if you have a statistically large number of rows (10,000 is not large).

Additionally, every time you do an insert, indexes need to be updated. So be careful on which column you add indexes.

As always, you can only optimize what you can measure - so use the `EXPLAIN` statement and your database logs (especially any slow query logs) to find out where indexes can be useful.

The above answer is correct but in some cases where the search is being done on columns that have only varchar datatype like ```email```. There you need to add an index.

Following is the way of doing that:

    Index(name=&#39;covering_index&#39;, fields=[&#39;headline&#39;], include=[&#39;pub_date&#39;])

reference from https://docs.djangoproject.com/en/3.2/ref/models/indexes/

This question is concerning joining two databases in Google spreadsheet using =QUERY function

I have a table like so in range A1:C3

    a d g
    b e h
    c f i

I have another table

    c j m
    a k n
    b l o

I want the final table to look like this

    a d g k n
    b e h l o 
    c f i j m

I can do this by using a vlookup function pretty easily in cell D1 and paste it down and across, but my dataset is huge. I would need a whole page of vlookups and Google Spreadsheet tells I&#39;m at my limit in complexities.

I look at the Google&#39;s Query Language reference... there doesn&#39;t seem to be an type of &quot;join&quot; functions mentioned. You would think it would be an easy &quot;join on A&quot; type operation.

Can anybody solves this without a vlookup?



Google spreadsheet &quot;=QUERY&quot; join() equivalent function?

Suppose I have a for loop:

    for i in range(1,10):
        if i is 5:
            i = 7

I want to change `i` if it meets certain condition. I tried this but didn&#39;t work.
How do I go about it?


How to change index of a for loop?

I&#39;m reading a book about coding style in Django and one thing they discuss is `db_index=True`. Ever since I started using Django, I&#39;ve never used this function because I&#39;m not really sure what it does.

So my question is, when to consider adding indexes?

Add Indexes (db_index=True)

<p>I'm reading a book about coding style in Django and one thing they discuss is <code>db_index=True</code>. Ever since I started using Django, I've never used this function because I'm not really sure what it does.</p>
<p>So my question is, when to consider adding indexes?</p>


I always use FBVs (Function Based Views) when creating a django app because it&#39;s very easy to handle. But most developers said that it&#39;s better to use CBVs (Class Based Views) and use only FBVs if it is complicated views that would be a pain to implement with CBVs. 

Why? What are the advantages of using CBVs? 


Class Based Views VS Function Based Views

I am using this to login the user in:

    def login_backend(request):
        if request.method == &#39;POST&#39;:
            username = request.POST[&#39;username&#39;]
            password = request.POST[&#39;password&#39;]
            user = authenticate(username=username, password=password)
            if user is not None:
                login(request, user)
                request.session.set_expiry(300)
                return HttpResponseRedirect(&#39;/overview/&#39;)
            else:
                return HttpResponseRedirect(&#39;/login_backend/&#39;)
        else:
            return render_to_response(&#39;login_backend.html&#39;, context_instance=RequestContext(request))

I want session to expire after 5mins thus I added `request.session.set_expiry(300)` in the view above. But the session is never expiring. What am I doing wrong?


How to expire Django session in 5minutes?

I have two models like this:

    class User(models.Model):
        email = models.EmailField()
    
    class Report(models.Model):
        user = models.ForeignKey(User)

In reality each model has more fields which are of no consequence to this question.

I want to filter all users who have an email which starts with &#39;a&#39; and have no reports. There will be more `.filter()` and `.exclude()` criteria based on other fields.

I want to approach it like this:

    users = User.objects.filter(email__like = &#39;a%&#39;)
    
    users = users.filter(&lt;other filters&gt;)
    
    users = ???

I would like ??? to filter out users who do not have reports associated with them. How would I do this? If this is not possible as I have presented it, what is an alternate approach?

In a Django QuerySet, how to filter for &quot;not exists&quot; in a many-to-one relationship

I have read the Django Rest Framework Guides and done all the tutorials. Everything seemed to make sense and work just how it should. I got basic and session authentication working as described.

[django rest framework - api guide](http://django-rest-framework.org/api-guide)

However, I&#39;m struggling with the Token Authentication part of the documentation, it&#39;s a little lacking or does not go into as much depth as the tutorials.  

[django-rest-framework - token authentication][1]

It says I need to create tokens for users but does state where in models.py?

Can someone explain the Token Authentication part of the documentation a little better for a first-timer?


  [1]: http://%20http://django-rest-framework.org/api-guide/authentication/#tokenauthentication

Django Rest Framework Token Authentication

I started using Django release 1.5 and got a problem with my old code:

    &lt;a href=&quot;{% url auto.views.viewpost post.slug %}&quot;&gt;&lt;h3&gt;{{post.title}}&lt;/h3&gt;&lt;/a&gt;
Error:  &#39;url&#39; requires a non-empty first argument. The syntax changed in Django 1.5, see the docs.
Docs: 

&gt; One deprecated feature worth noting is the shift to “new-style” url
&gt; tag. Prior to Django 1.3, syntax like {% url myview %} was interpreted
&gt; incorrectly (Django considered &quot;myview&quot; to be a literal name of a
&gt; view, not a template variable named myview). Django 1.3 and above
&gt; introduced the {% load url from future %} syntax to bring in the
&gt; corrected behavior where myview was seen as a variable.
&gt; 
&gt; The upshot of this is that if you are not using {% load url from
&gt; future %} in your templates, you’ll need to change tags like {% url
&gt; myview %} to {% url &quot;myview&quot; %}. If you were using {% load url from
&gt; future %} you can simply remove that line under Django 1.5

Then I tried:
 `&lt;a href=“{% url ‘auto.views.view_post’ post.slug %}”&gt;&lt;h3&gt;{{post.title}}&lt;/h3&gt;&lt;/a&gt;`
but got error Reverse for ‘auto.views.view_post’ with arguments ‘(’&#39;,)&#39; and keyword arguments ‘{}’ not found. :(
What am I doing wrong?
Thx!

Django release 1.5: &#39;url&#39; requires a non-empty first argument. The syntax changed in Django 1.5

From the [Django.Contrib.Auth docs][1]:

&gt; **Extending Django’s default User**
&gt; If you’re entirely happy with Django’s User model and you just want to add some additional profile information, you can simply subclass `django.contrib.auth.models.AbstractUser` and add your custom profile fields. This class provides the full implementation of the default User as an abstract model.


Said and done. I created a new model like below:

    class MyUser(AbstractUser):
      some_extra_data = models.CharField(max_length=100, blank=True)

This shows up in admin almost like Django&#39;s standard `User`. However, the most important difference in admin is that the password-(re)set field is not present, but a normal CharField is displayed instead. Do I really have to override stuff in the admin-config to get this to work? If so, how can I do that in somewhat DRY way (i.e. without copying stuff from the Django source... eww...)?

  [1]: https://docs.djangoproject.com/en/dev/topics/auth/customizing/#extending-django-s-default-user


Using Django auth UserAdmin for a custom user model

Is it possible to temporarily disable constraints in MySQL?

I have two Django models, each with a foreign key to the other one. Deleting instances of a model returns an error because of the foreign key constraint:

    cursor.execute(&quot;DELETE FROM myapp_item WHERE n = %s&quot;, n)
    transaction.commit_unless_managed()  #a foreign key constraint fails here

    cursor.execute(&quot;DELETE FROM myapp_style WHERE n = %s&quot;, n)
    transaction.commit_unless_managed()

Is it possible to temporarily disable constraints and delete anyway?





How can I temporarily disable a foreign key constraint in MySQL?

I am trying to get the latest Django model object but cannot seem to succeed. 

Neither of these are working:

    obj = Model.objects.filter(testfield=12).latest()

&lt;!-- language: lang-python --&gt;

    obj = Model.objects.latest().filter(testfield=12)

Get the latest record with filter in Django

So I have my TagStatus model. I&#39;m trying to make a ModelForm for it. However, my form requires that the hidden input be populated with the {{ tag.name }}. I&#39;ve been looking through the docs and I don&#39;t know how to make the tag field a hidden input. Perhaps a ModelForm isn&#39;t the way to go?

**models.py:**

    class TagStatus(models.Model):
        user = models.ForeignKey(User, null=True, unique=True)
        status = models.CharField(max_length=2, choices=tag_statuses)
        tag = models.ForeignKey(Tag, null=True, blank=True)
    
        def __unicode__(self):
            return self.status
    
        def save(self, *args, **kwargs):
            super(TagStatus, self).save(*args, **kwargs)
    
    class TagStatusForm(modelForm):
        class Meta:
            model = TagStatus
            fields = (&#39;status&#39;,&#39;tag&#39;) 
            widgets = {
                 &#39;select&#39;: Select,
                 &#39;tag&#39;: ???
            }

**django views.py:**

    @login_required
    def tags(request):
        all_tags = Tag.objects.all()
        context = base_context(request)
        if request.method == &#39;POST&#39;:
            if &#39;status_check&#39; in request.POST:
                status_form = TagStatusForm(request.POST)
                #if request.is_ajax():
                if status_form.is_valid():
                    status_form.save()
                    response = simplejson.dumps({&quot;status&quot;: &quot;Successfully changed status&quot;})
                else:
                    response = simplejson.dumps({&quot;status&quot;: &quot;Error&quot;})
                    return HttpResponse (response, mimetype=&#39;application/json&#39;)
        status_form = TagStatusForm()
        context[&#39;status_form&#39;] = status_form
        context[&#39;all_tags&#39;] = all_tags
        return render_to_response(&#39;tags/tags.html&#39;, context, context_instance=RequestContext(request))

template:

    {% for tag in all_tags %}
    ....
    &lt;form class=&quot;nice&quot; id=&quot;status-form&quot; method=&quot;POST&quot; action=&quot;&quot;&gt;
         {% csrf_token %}
          &lt;input type=&quot;hidden&quot; name=&quot;status_check&quot; /&gt;
          &lt;input type=&#39;hidden&#39; name=&quot;tag&quot; value=&quot;{{ tag.name }}&quot; /&gt;
         {{ status_form.status }}
    &lt;/form&gt;
    ...
    {% endfor %}


How would I go about making a hidden input through a django ModelForm and then populate it through the template?

Django ModelForm to have a hidden input

I am trying to create a view where I save an object but I&#39;d like to _undo_ that save if some exception is raised. This is what I tried:

    class MyView(View):
    
        @transaction.atomic
        def post(self, request, *args, **kwargs):
            try:
                some_object = SomeModel(...)
                some_object.save()
    
                if something:
                    raise exception.NotAcceptable()
                    # When the workflow comes into this condition, I think the previous save should be undone
                    # What am I missing?
    
            except exception.NotAcceptable, e:
                # do something

What am I doing wrong? even when the exception is raised `some_object` is still in Database.

Django - Rollback save with transaction atomic

I am trying to make a very simple `Subquery` that uses `OuterRef` (not for practical purposes, but just to get it working), but I keep running into the same error.

`posts/models.py` code


    from django.db import models

    class Tag(models.Model):
        name = models.CharField(max_length=120)
        def __str__(self):
            return self.name

    class Post(models.Model):
        title = models.CharField(max_length=120)
        tags = models.ManyToManyField(Tag)
        def __str__(self):
            return self.title

`manage.py shell` code

    &gt;&gt;&gt; from django.db.models import OuterRef, Subquery
    &gt;&gt;&gt; from posts.models import Tag, Post
    &gt;&gt;&gt; tag1 = Tag.objects.create(name=&#39;tag1&#39;)
    &gt;&gt;&gt; post1 = Post.objects.create(title=&#39;post1&#39;)
    &gt;&gt;&gt; post1.tags.add(tag1)
    &gt;&gt;&gt; Tag.objects.filter(post=post1.pk)
    &lt;QuerySet [&lt;Tag: tag1&gt;]&gt;
    &gt;&gt;&gt; tags_list = Tag.objects.filter(post=OuterRef(&#39;pk&#39;))
    &gt;&gt;&gt; Post.objects.annotate(count=Subquery(tags_list.count()))

The last two lines should give me number of tags for each Post object. And here I keep getting the same error:

    ValueError: This queryset contains a reference to an outer query and may only be used in a subquery.

Content Type	Original Author	Original Content on Stackoverflow
Question	catherine	View Question on Stackoverflow
Solution 1 - Django	Burhan Khalid	View Answer on Stackoverflow
Solution 2 - Django	Mohit Rathod	View Answer on Stackoverflow

Add Indexes (db_index=True)

Django Problem Overview

Django Solutions

Solution 1 - Django

Solution 2 - Django

How to change index of a for loop?

Google spreadsheet "=QUERY" join() equivalent function?

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