I have a model class in my django project:
*user_id
*amount
*net_balance
*created_on
I have a list of user_ids(let's say 3). I need to get the last row for each user_id and then do some operation and create a new row for each user id. How do this efficiently. I can certainly do 6 transactions (if there are 3 items in list of userids).
If you want the most recent entry then
YourModel.objects.filter(user=user_id).latest('created_on')
If I understand your question correctly then you need to get all the user_ids (presumably you have a separate User model?) and then loop through them - for each user getting the most recent entry and then create the new row.
You need 1 select (at least) for all the records you interested and 1 insert query for each record returned.
The select query can be generated by ORM abilities (aggregation) or you can use raw SQL if you fill comfortable. If you use PostgreSQL, you can use distinct ability (I recommended) as:
Model.objects.order_by('user_id', '-created_on').distinct('user_id')
or you can use aggregation abilities as:
Model.objects.filter(user_id__in=[1,2,3]).values('user_id', 'created_on').annotate(last_row=Max('created_on')).filter(created_on=F('last_row'))
The correct answer depends on your Django version and database. But there are lots of good features in Django to achieve this kind of stuffs.
Related
I have 5 different models....similar structure but seperated due to lots of record so easier management and scalability.
I'm not sure if this is possible with django, but i have a div on the website which is called latest....i want to grab the latest 3 records and show it in this div....however having 5 different models its difficult....each have a timestamp field. Is it possible to query something like show latest 3 records in total, but check 5 of these models and display?
Usually if it was in one model I could have just easily said show latest x....but separated models makes its complicated. So i don't want to grab latest 3 records from each model....rather 3 in total but just consider from 5 different models and show latest 3 (either through filtering of id of timestamp field)
Please kindly let me know if there is a solution.
There is two different solution.
At first you can merge models and make a general model with common columns and each of 5 models connect to general model with ForeignKey. So you can make query on general model and access specific columns with related objects.
Second way is usingdjango contenttypes framework
I try to run a query for paged results, like this
Model.objects.all()[start: start+page_size].
I also want to know whether there are more pages to load, that is to say, I want to know whether start+page_size < Model.objects.all().count().
My question is, if I call all() twice here, whether Django executes the same query twice (one for slice operation[], one for count()).
Another question is if I slice on a Model.objects.all() like this Model.objects.all()[2:9] whether Django fetch all data from DB and slice by python, or Django only fetch with SQL limit limit 2 to 9
Yes, it makes two queries, but these are not the "same query" at all. One is SELECT * FROM mymodel LIMIT <page_size> OFFSET <start> and the other is SELECT COUNT(*) FROM mymodel.
If you want to avoid two queries, a simple fix is to ask for one more record than you actually need:
Model.objects.all()[start: start+page_size+1]
then you can iterate up to page_size, and show the Next button if the extra record is there.
Can Django support Oracle nested tables or varrays or collections in some manner? Asking just for completeness as our project is reworking the data model, attempting to move away from EAV organization, but I don't like creating a bucket load of dependent supporting tables for each main entity.
e.g.
(not the proper Oracle syntax, but gets the idea across)
Events
eventid
report_id
result_tuple (result_type_id, result_value)
anomaly_tuple(anomaly_type_id, anomaly_value)
contributing_factors_tuple(cf_type_id, cf_value)
etc,
where the can be multiple rows of the tuples for one eventid
each of these tuples can, of course exist as separate tables, but this seems to be more concise. If it 's something Django can't do, or I can't modify the model classes to do easily, then perhaps just having django create the extra tables is the way to go.
--edit--
I note that django-hstore is doing something very similar to what I want to do, but using postgresql's hstore capability. Maybe I can branch off of that for an Oracle nested table implementation. I dunno...I'm pretty new to python and django, so my reach may exceed my grasp in this case.
Querying a nested table gives you a cursor to traverse the tuples, one member of which is yet another cursor, so you can get the rows from the nested table.
I have a Django model that defines a TimeSlot. Each TimeSlot can hold a certain number of users (TimeSlot.spots). Each TimeSlot also has a certain number of users already held in it (a many to many field, TimeSlot.participants.
When I pass to the template that displays the available TimeSlots to the user, I annotate with TimeSlot.objects.annotate(Count('participants')),which gives the number of users currently held by the TimeSlot as participants__count.
However, what I really want is the number of spots remaining, the capacity (TimeSlot.spots) minus the number currently held (participants__count). How can I annotate another field with this new number, so I can pass it to the template?
It's still not possible with annotation (though it is planned to implement in Django). But you can do it with an .extra() query. See my answer to another question for details.
Upd.:
Essentially, you need somethig like this query:
items = MyModel.objects.extra(
select = {'variance': 'Model.someField - SUM(relatedModel__someField)'},
)
Not possible with only an annotation. I'd create a method on the model which does the annotation, and then subtract that from your TimeSlot.spots value. This will use more database queries, but thats your only option. Or I guess you could drop down to raw SQL...
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.