I am having trouble in understanding how to iterate over QuerySet , Tuple and Dictionarty in django.
I am confused which djnago functions returns what like objects.all or objects.get
Suppose i have
a = model.objects.all()
b = model.object.get(pk=1)
c = Blog.objects.values('name', 'entry__headline')
d = Entry.objects.values_list('id', 'headline')
e = Person.objects.raw('SELECT * FROM myapp_person')
What is the retured in each scenario and the biggest problem is how can i iterate over.
All these confuse me very much . I studies the docs but they tell one thing and don't tell how to use in template. I know its related to python but then python don't have template to deal with
QuerySet: A Django class that processes SQL responses and returns a python construct representing the results. Although it functions like a list in many ways, it's actually what's called an "iterable". It simply mocks the behavior of a list to allow you to use things like for-loops on it.
Tuple: An immutable list. That means that once it's set, it can't be altered. In virtually every other way it behaves just like a list.
Dictionary: Also known as a hash in other languages. It can be considered a "keyed list". A "list" in the strictest of senses is a group of items stored serially in memory. In the old days of programming, you'd have to "pop" items off and "push" items onto a list, and they could only be retrieved in a FIFO, or first-in-first-out fashion. Dictionaries provide a way to "lookup" items in a list. It is composed of key-value pairs, so you can reference a key and get the attached value.
Now in terms of Django templates:
QuerySets: You iterate over these using the standard methods. Once you get a result from something like MyModel.objects.all(), you can use a {% for value in queryset %} tag.
Tuples: As far as iteration goes, these behave exactly as standard lists. You can also just simply use a {% for value in tuple %} tag. The only potential hangup is that sometimes you'll end up with tuples of tuples or a list of tuples. These are just like multi-level lists. You have to iterate over the outer list or tuple and then iterate over the inner ones.
Dictionaries: These are probably the most complicated, only because they require a method call to get an iterable object.
mydict.iteritems() # returns an iterable consisting of `(key, value)` tuples.
mydict.iterkeys() # returns an iterable consisting of the keys. You can then use mydict[key] to get the values
mydict.itervalues() # returns an iterable consisting of the values.
The last method is probably the best of simple iteration in a Django template:
{% for value in mydict.itervalues %}
Are you referring to these docs? https://docs.djangoproject.com/en/dev/ref/templates/builtins/#for
I think that's what you were looking for.
Basically you iterate over them like:
{% for item in a %}
{{item.field}}
{{item.field2}}
{% endfor %}
{{b.field}}
{% for item in c %}
{{item.name}}
{{item.entry__headline}}
{% endfor %}
{% for item in d %}
{{item}}
{% endfor %}
# Also you can do this if you want to access just a particular index:
{{d.0}}
{{d.1}}
{% for item in e %}
{{item.field}}
{{item.field2}}
{% endfor %}
As for your data types:
a would be a QuerySet or list of model objects
b would be a model object
c would be a ValuesQuerySet or a list of dictionaries
d would also be a ValuesQuerySet but it's actually a list of tuples
e would be a RawQuerySet, which acts like a normal QuerySet
Sources:
https://docs.djangoproject.com/en/dev/topics/db/sql/#django.db.models.Manager.raw
https://docs.djangoproject.com/en/dev/ref/models/querysets/#values
https://docs.djangoproject.com/en/dev/ref/models/querysets/#values-list
Related
In a template in Django I am importing a list of dicts and in one of the keys (tested) I can have either a single value or a list of values depending on the case.
My context dict in the template looks something like this:
context_dicts.append({'url': p.url,
'type_': p.type,
'error': error,
'tested': p.tested})
In the html template I want to if test the tested key to do something if it is a single value and something else if it's a list. So when looping through the dicts, if I use {% if value|length > 1% } it will give me the string size of the value when it's just a value and the length of the list when it's a list. How can I test the if to tell me if it's a "list of one value" or more?
Welcome to SO!
Storing possibly different kind of data in a single variables looks cumbersome to me. It makes your logic less easy to understand and may be prone to errors.
I think the best is to always store a list, but possibly a list with a single element. In the template, you could then do:
{% if tested.count == 1 %}
do stuff with {{ tested.0 }} value
{% else %}
do stuff with {{ tested }} list
{% endif %}
How do I find the length of a list of objects in a jinja template when the list of objects has been created by querying a database?
I thought There are {{ items|length }} items in this category. would work, but items is a result of:
items = db_session.query(Item).filter_by(category_id=category.id)
and I get the error
TypeError: object of type 'Query' has no len()
Obviously, I could calculate the length separately and pass into render_template(), but I wondered if there was a better way?
Any help from the community would be much appreciated :)
items object is not a list yet, it is an unprocessed Query object as you see in the error. You can use Query.all() method to get a list of items:
items = db_session.query(Item).filter_by(category_id=category.id).all()
After that length filter can be applicable.
Use {{ items | count }} in jinja template
Try to add loopcontrol extension
app.jinja_env.add_extension('jinja2.ext.loopcontrols')
Then
{% for item in items %}
{{loop.length}}
{% break %}
{% endfor %}
How do I iterate through 2 objects synchronously in the template form?
I have 2 objects in my views; one object is products, the other prices. Both are lists.
I want to cycle through both products and prices at the same time in the templates form.
Usually in python I would just make a simple while loop as such:
n = 0
while n < len(products):
print products[n], prices[n]
n+=1
I haven't found anything similar in the templates form yet.
Any help would be appreciated.
You could write a custom template filter to get the nth item in the list, but this is what I would do:
Zip the 2 lists together in the view:
products_prices = zip(products, prices)
Then loop through that looped list in the template:
{% for product, prices in products_prices %}
{{ product }}, {{ price }}
{% endfor %}
You might want to use forloop.counter, docs at https://docs.djangoproject.com/en/dev/ref/templates/builtins/
The built in template tags don't have an easy way to do this that I can recall. The simplest approach will be to zip the two lists together in your view, then provide the zipped data structure in the context. You could also write a custom zip template tag, if you're feeling adventurous.
Is there a way to present a partial tree using Django-MPTT's {% recursetree %} without retrieving the entire tree from database? I need to show the first 20 nodes encountered by a Depth First Search.
Either of these (which do not retrieve the full tree) cause an exception:
# resulting querySet passed to {% recursetree %} in template
Thing.objects.all()[:20]
# directly sliced in template
{% recursetree all_nodes|slice:":20" %}
AssertionError while rendering: Cannot reorder a query once a slice has been taken.
This on the other hand does work, but retrieves the entire tree:
# resulting querySet passed to {% recursetree %} in template
list(Thing.objects.all())[:20]
How can I do this without retrieving the entire tree form the DB?
MPTT uses pre-order (which is a depth-first search already.) So all you need to do is add a limit to your queryset before passing it to recursetree.
MPTT calls order_by() if you pass a queryset to recursetree, but it can't do that if you pass a list. That behaviour is kind of confusing and has caused other people issues too.
I've created a ticket on MPTT to address this.
In the meantime, you can just do the slicing before the list() call:
list(Thing.objects.all()[:20])
That will do the limit in the database, then convert the queryset to a list, which you can pass to recursetree without it trying to reorder things.
I want to list the number of items for each list. How can I find this number in a single query, rather than a query for each list?
Here is a simplified version of my current template code:
{% for list in lists %}
<li>
{{ listname }}:
{% with list.num_items as item_count %}
{{ item_count }} item{{ item_count|pluralize }}
{% endwith %}
</li>
{% endfor %}
lists is passed as: List.objects.filter(user=user)
and num_items is a property of the List model:
def _get_num_items(self):
return self.item_set.filter(archived=False).count()
num_items = property(_get_num_items)
This queries SELECT COUNT(*) FROM "my_app_item" WHERE... n times, where n is the number of lists. Is it possible to make a single query here?
You should do this in your view and send the dictionary, instead.
Model.objects.values_list('item').annotate(Count('num_items'))
This will produce the SQL same as, (or equivalent to) the one you have posted.
In the following, I tried to take into account all your constraints: filtering on fields of List, on fields of Item, counting items , and grouping by list.
The solution I see is that you could use values() (here is the django doc about this : http://docs.djangoproject.com/en/dev/topics/db/aggregation/#values)
from django.db.models import Count
lists = list(List.objects.filter(user=user))
items=Item.objects.values(list).filter(archived=False,list__in=lists).annotate(count=Count("id"))
#you will get a list of dicts of the form [{'count':2,'list':5},...] where 5 is the id of the list
#now, you can match you list with you item counts in python
list_items_count_dict={}
for item in items:
list_items_count_dict[item['list']]=item['count']
for list in lists :
list.item_count = list_items_count_dict.get(list.id)
That will make only 2 queries, one for getting the lists, the other for computing the item counts. Afterwards, you will have two loops (that could probably be replaced by list comprehension one-liners ), but only for the lists you are interested in.
afterwards, in your template, you can use
{{list.item_count}}
There might be a more elegant option, but that is what I have found right now. I am also certain that you could reduce the number of query to one by using custom sql.
Disclaimer: I have not tested this code, but I have tested similar code on similar models. You could have problems because list is one of the keyword of the Python language.