I have a Table "Subscriber" in which one column whose name is Status.
I want to replace value of "status" column.Where status = 1 and 3 change it with 4. In other words all '1 and 3' status convert into 4.
What should i do to solve this problem.
#e4c5's solution is perfectly good, though I try to reserve Q objects for complex queries, in this case, I might just go with the __in filter: Subscriber.objects.filter(status__in=[1, 3]).update(status=4)
Use a Q object to get the OR condition. The rest of it is a simple call to update
Subscriber.objects.filter(Q(status=1)|Q(status=3)).update(status=4)
Keyword argument queries – in filter(), etc. – are “AND”ed together.
If you need to execute more complex queries (for example, queries with
OR statements), you can use Q objects.
Read the documentation for update here: https://docs.djangoproject.com/en/1.9/ref/models/querysets/#update
Related
My code
PriceListItem.objects.get(id=tarif_id).price_eur
In my settings.py
CURRENCY='eur'
My Question:
I would like to pick the different info depending on the CURRENCY variable in settings.py
Example:
PriceListItem.objects.get(id=tarif_id).price_+settings.CURRENCY
Is it possible?
Sure. This has nothing to do with Django actually. You can reach the instance's attribute through pure Python:
getattr(PriceListItem.objects.get(id=tarif_id), 'price_'+settings.CURRENCY)
Note it might be a better idea to have a method on the model which accepts the currency as a parameter and returns the correct piece of data (through the line I wrote above, for example).
I think this should work
item = PriceListItem.objects.get(id=tarif_id)
value = getattr(item, price_+settings.CURRENCY)
In case you are only interested in that specific column, you can make the query more efficient with .values_list:
my_price = PriceListItem.objects.values_list_(
'price_{}'.format(settings.CURRENCY),
flat=True
).get(id=tarif_id)
This will only fetch that specific column from the database, which can be a (a bit) faster than first fetching the entire row into memory, and then discard all the rest later.
Here my_price is thus not a PriceListItem object, but the value that is stored for the specific price_cur column.
It will thus result in a query that looks like:
SELECT pricelistitem.price_cur
FROM pricelistitem
WHERE id=tarif_id
I'm trying to make a query using Django's Exclude() and passing to it a list, as in:
(...).exclude(id__in=list(top_vip_deals_filter))
The problem is that, apparently, there is a Limit -- depending on your database --on the size of the list being passed.
Is this correct?
If so, How to overcome this?
If not, is there some explanation to the fact that queries silently fail when the list size is big?
Thanks
If the top_vip_deals_filter comes from the database, you can set an extra where in the query:
(...).extra(where=['model.id not in select blah blah'])
(put your lowercase model name instead of model.)
You can do better if the data model allows you to. If you can do it in SQL, you probably can do it in django.
Is there a difference between filter and exclude in django? If I have
self.get_query_set().filter(modelField=x)
and I want to add another criteria, is there a meaningful difference between to following two lines of code?
self.get_query_set().filter(user__isnull=False, modelField=x)
self.get_query_set().filter(modelField=x).exclude(user__isnull=True)
is one considered better practice or are they the same in both function and performance?
Both are lazily evaluated, so I would expect them to perform equivalently. The SQL is likely different, but with no real distinction.
It depends what you want to achieve. With boolean values it is easy to switch between .exclude() and .filter() but what about e.g. if you want to get all articles except those from March? You can write the query as
Posts.objects.exclude(date__month=3)
With .filter() it would be (but I not sure whether this actually works):
Posts.objects.filter(date__month__in=[1,2,4,5,6,7,8,9,10,11,12])
or you would have to use a Q object.
As the function name already suggest, .exclude() is used to exclude datasets from the resultset. For boolean values you can easily invert this and use .filter() instead, but for other values this can be more tricky.
In general exclude is opposite of filter. In this case both examples works the same.
Here:
self.get_query_set().filter(user__isnull=False, modelField=x)
You select entries that field user is not null and modelField has value x
In this case:
self.get_query_set().filter(modelField=x).exclude(user__isnull=True)
First you select entries that modelField has value x(both user in null and user is not null), then you exclude entries that have field user null.
I think that in this case it would be better use first option, it looks more cleaner. But both work the same.
That seems simple enough, but all Django Queries seems to be 'SELECT *'
How do I build a query returning only a subset of fields ?
In Django 1.1 onwards, you can use defer('col1', 'col2') to exclude columns from the query, or only('col1', 'col2') to only get a specific set of columns. See the documentation.
values does something slightly different - it only gets the columns you specify, but it returns a list of dictionaries rather than a set of model instances.
Append a .values("column1", "column2", ...) to your query
The accepted answer advising defer and only which the docs discourage in most cases.
only use defer() when you cannot, at queryset load time, determine if you will need the extra fields or not. If you are frequently loading and using a particular subset of your data, the best choice you can make is to normalize your models and put the non-loaded data into a separate model (and database table). If the columns must stay in the one table for some reason, create a model with Meta.managed = False (see the managed attribute documentation) containing just the fields you normally need to load and use that where you might otherwise call defer(). This makes your code more explicit to the reader, is slightly faster and consumes a little less memory in the Python process.
This can't be hard, but... I just need to get the most recent three objects added to my database field.
So, query with reverse ID ordering, maximum three objects.
Been fiddling round with
Records.objects.order_by(-id)[:3]
Records.objects.all[:3]
and including an if clause to check whether there are actually three objects:
num_maps = Records.objects.count()
if (num_maps > 3): # etc...
and using reverse() and filter() for a while...
But just can't figure it out! Nothing I do gives the right result and using num_maps feels pretty inelegant. Not getting much joy from the documentation. Can anyone help?!
All you should need is:
Records.objects.all().order_by('-id')[:3]
You need the all() first before the order_by and the argument you pass into order_by should be a string. No need to check if there are actually 3 before running this query because the [:3] will not break if there are less than 3.