I'm listing queryset results and would like to add an option for choosing the order results are displayed.
I would like to pass the actual data from the database to other page for sorting.
I was able to achieve such thing by getting all objects ids and use django session to recreate a new queryset based on the order criteria.
I was thinking if there is any other way to achieve such goal?
10x
Assuming you are currently displaying the data as a table, you could give chance to some javascript client side table sorter such as tablesorter. There are lots of javascript table sorte.
I'm away from my development machine right now, but I think you could just pass the list of ids to a new Queryset, pk__in=list_of_object_ids, and then use the native order_by function.
For example:
objs = Object.objects.filter(pk__in=list_of_object_ids).order_by('value_to_order_by')
Anyway, that's what I would try first, though I'm sure there are better optimizations.
For example, instead of a list of object ids, you could pass a dictionary with a key:value pair that has the value you want to order by.
For example:
[{'obj_id':1,'obj_value':'foo'},{'obj_id':2,'obj_value':'foo'}]
Then use some lambda function to sort it, like here.
Related
I am trying to build a database for my website. There are currently three entries with different attributes in my database. I have not created these entries in order, but I have assigned a 'Chapter number' attribute which indicates the order 1,2,3.
I am now trying to inject this using 'context' and 'render' function in my views. I am using the method 'objects.all()' to add all objects to my context. I have a simple Html file where I am inserting the data from the database by looping over (a simple for loop) these added objects.
Now the output that is being generated (naturally) is that it is following the order in which I created the database. I am not sure how I can have the loop run in such a way that I get these chapters in correct order. Thank you for patiently reading my question. Any help will be appreciated.
You may use the order_by method which is included in Djangos QuerySet API:
https://docs.djangoproject.com/en/3.0/ref/models/querysets/
If you offer some more information of your specific data I might provide you with an example.
For orientation purposes, sorting queried objects by date would work as follows:
most_recent = Entry.objects.order_by('-timestamp')
You can sort by any field like so:
sorted_by_field = Entry.objects.order_by('custom_field')
I'm thinking of using a raw query to quickly get around limitations with either my brain or the Django ORM, but I don't want to redevelop the infrastructure required to support the existing ORM code such as filters. Right now I'm stuck with two dead ends:
Writing an inner raw query and reusing that like any other query set. Even though my raw query selects the correct columns, I can't filter on it:
AttributeError: 'RawQuerySet' object has no attribute 'filter'
This is corroborated by another answer, but I'm still hoping that that information is out of date.
Getting the SQL and parameters from the query set and wrapping that in a raw query. It seems the raw SQL should be retrievable using queryset.query.get_compiler(DEFAULT_DB_ALIAS).as_sql() - how would I get the parameters as well (obviously without actually running the query)?
One option for dealing with complex queries is to write a VIEW that encapsulates the query, and then stick a model in front of that. You will still be able to filter (and depending upon your view, you may even get push-down of parameters to improve query performance).
All you need to do to get a model that is backed by a view is have it as "unmanaged", and then have the view created by a migration operation.
It's better to try to write a QuerySet if you can, but at times it is not possible (because you are using something that cannot be expressed using the ORM, for instance, or you need to to something like a LATERAL JOIN).
I want to retrieving unique foreign key instances and ordering randomly
However, I got an error when I want to use order_by('?')
My query is like this:
qs=Course.objects.distinct('courseschedule__object_id').order_by('courseschedule__object_id')
this query is works fine, but right now I want to order randomly(to get random result every time),I try this
qs=qs.order_by('?')
I got this error:
django.db.utils.ProgrammingError: SELECT DISTINCT ON expressions must match initial ORDER BY expressions
Any idea how to fix it? My database is Postgres, I don't want do rawSQL.... I really appreciate you guys help!!!!
First of all, random order using the database is expensive and slow according to: querysetsDjango . Second, in my opinion instead of using the database to return a random order get your query from the database without distinct (because it also can be slow), shuffle the query using python methods for example: randomOrder and make a set() and then . This way you are not using the database to do your stuff but using pure python code.
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.
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.