I´m trying to update the data of an existing model with a csv. I read the file and assign the values with no problem.
If I try `MyModel.update() everything runs with no error but the data is not saved.
with open('Productosold.csv') as csvfile:
reader = csv.DictReader(csvfile)
for row in reader:
p = ProductosBase.objects.filter(codigo_barra = row['codigo_barra'])
p.region=row['region']
p.producto_ing=row['producto_ing']
p.packaging_ing=row['packaging_ing']
p.precio_compra=row['precio_compra']
p.uom=row['uom']
p.units_inner=row['units_inner']
p.inner_master=row['inner_master']
p.tier=row['tier']
p.precio_lista_internacional=row['precio_lista_internacional']
p.update()
I usualy upload new data using the MyModel.save() method and have no problem.
Now, if I use that I get "Queryset has no attribute save".
p.save()
If I print some of the p.values I can see they are populated correctly from the csv file.
What I´m doing wrong?
Thanks in advance!
.filter always returns a queryset, not a single instance. When you set all those values, you are just setting (previously non-existing) attributes onto that queryset object; you are not setting fields in a model instance. You should use .get to get an instance and save that.
p = ProductosBase.objects.get(codigo_barra = row['codigo_barra'])
p.region=row['region']
...
p.save()
However, since all the columns in your CSV map precisely to fields on the model, you could in fact use filter and update to do the whole thing in one go:
for row in reader:
ProductosBase.objects.filter(codigo_barra=row['codigo_barra']).update(**row)
and no need for any of the rest of the code.
You need filter() whenever you expect more than just one object that matches your criteria. If no item was found matching your criteria, filter() returns am empty queryset without throwing an error.
Also you can use get() but when you expect one (and only one) item that matches your criteria. Get throws an error if the item does not exist or if multiple items exist that match your criteria. You should therefore always use if in a try.. except .. block or with a shortcut function like get_object_or_404 in order to handle the exceptions properly. I'd recommend using get_object_or_404 in this case.
p = get_object_or_404(ProductosBase, codigo_barra=row['codigo_barra'])
Related
I am trying load some data into datatables. I am trying to specify columns in the model.objects query by using .only() --- at first glance at the resulting QuerySet, it does in fact look like the mySQL query is only asking for those columns.
However, When I try to pass the QuerySet into Paginator, and/or a Serializer, the result has ALL columns in it.
I cannot use .values_list() because that does not return the nested objects that I need to have serialized as part of my specific column ask. I am not sure what is happening to my .only()
db_result_object = model.objects.prefetch_related().filter(qs).order_by(asc+sort_by).only(*columns_to_return)
paginated_results = Paginator(db_result_object,results_per_page)
serialized_results = serializer(paginated_results.object_list,many=True)
paginated_results.object_list = serialized_results.data
return paginated_results
This one has tripped me up too. In Django, calling only() doesn't return data equivalent to a SQL statement like this:
SELECT col_to_return_1, ... col_to_return_n
FROM appname_model
The reason it doesn't do it like this is because Django returns data to you not when you construct the QuerySet, but when you first access data from that QuerySet (see lazy QuerySets).
In the case of only() (a specific example of what is called a deferred field) you still get all of the fields like you normally would, but the difference is that it isn't completely loaded in from the database immediately. When you access the data, it will only load the fields included in the only statement. Some useful docs here.
My recommendation would be to write your Serializer so that it is only taking care of the one specific filed, likely using a SerializerMethodField with another serializer to serialize your related fields.
I get a queryset object every time i want some data from models.
So when i say,
"items = Items.object.get(value=value)"
I get --
"<QuerySet [<Item-name>]>"
I have to iterate through the queryset object to get the data and I do that with
"items[0]"
Is there any way I can avoid this?
Edit: I meant "items = Items.object.filter(value=value)"
first of all items = Items.objects.get(value=value) does not return a queryset,
rather it returns an object of <Items: Items object (1)>
To get the first(or just one result) or last date from the object, do this Items.objects.first() or Items.objects.last()
To get the desired data without using its index position, then you can filter it like this Items.objects.filter(value=value)
You are mistaken. items = Items.object.get(value=value) will not give you a queryset, but an object. items = Items.object.filter(value=value)
would give you a queryset.
Filter method will always give you a queryset, because; in order to minimize the need of database hits, django considers you might add additional filters through your code. So if you not execute that queryset, e.g. by using list(your_queryset) django never hits the database.
# when you are using 'get' in your query, you don't need to iterate, directly get an access to the field values
try:
items = Items.object.get(value=value)
except Items.DoesNotExist:
items = None
if items:
print(items.value)
I have a Django app where users post photos, and other leave comments under the photos.
When a comment is left, I need to notify:
Everyone else who wrote in this thread
The owner of the photo, in case they're not included in (1)
For (1), I do:
#I slice by 25 because I arbitrarily deem anyone beyond that irrelevant.
all_commenter_ids = PhotoComment.objects.filter(which_photo=which_photo).order_by('-id').values_list('submitted_by', flat=True)[:25]
Next, for (2), I try:
all_relevant_ids = all_commenter_ids.append(which_photo.owner_id)
all_relevant_ids = list(set(all_relevant_ids))
I end up with an error:
'ValuesListQuerySet' object has no attribute 'append'
I find this strange, because I'm extracting a values_list.
Isn't that a list object, and in that case, shouldn't the attribute append work in this scenario? Please explain what's wrong, and suggest alternatives.
The values_list method returns a ValuesListQuerySet. This means it has the advantages of a queryset. For example it is lazy, so you only fetch the first 25 elements from the database when you slice it.
To convert it to a list, use list().
all_commenter_ids = PhotoComment.objects.filter(which_photo=which_photo).order_by('-id').values_list('submitted_by', flat=True)[:25]
all_commenter_ids = list(all_commenter_ids)
You might be able to start the queryset from your User model instead of using values_list. You haven't shown your models, so the following code is a guess:
from django.db.models import Q
commenters = User.objects.filter(Q(id=which_photo.owner_id)|Q(photocomment=which_photo))
I have an import of objects where I want to check against the database if it has already been imported earlier, if it has I will update it, if not I will create a new one. But what is the best way of doing this.
Right now I have this:
old_books = Book.objects.filter(foreign_source="import")
for book in new_books:
try:
old_book = old_books.get(id=book.id):
#update book
except:
#create book
But that creates a database call for each book in new_books. So I am looking for a way where it will only make one call to the database, and then just fetch objects from that queryset.
Ps: not looking for a get_or_create kind of thing as the update and create functions are more complex than that :)
--- EDIT---
I guess I haven't been good enough in my explanation, as the answers does not reflect what the problem is. So to make it more clear (I hope):
I want to pick out a single object from a queryset, based on an id of that object. I want the full object so I can update it and save it with it's changed values. So lets say I have a queryset with 3 objects, A and B and C. Then I want a way to ask if the queryset has object B and if it has then get it, without an extra database call.
Assuming new_books is another queryset of Book you can try filter on id of it as
old_books = Book.objects.filter(foreign_source="import").filter(id__in=[b.id for b in new_books])
With this old_books has books that are already created.
You can use the values_list('id', flat=True) to get all ids in a single DB call (is much faster than querysets). Then you can use sets to find the intersections.
new_book_ids = new_books.values_list('id', flat=True)
old_book_ids = Book.objects.filter(foreign_source="import") \
.values_list('id', flat=True)
to_update_ids = set(new_book_ids) & set(old_book_ids)
to_create_ids = set(new_book_ids) - to_update_ids
-- EDIT (to include the updated part) --
I guess the problem you are facing is in bulk updating rather than bulk fetch.
If the updates are simple, then something like this might work:
old_book_ids = Book.objects.filter(foreign_source="import") \
.values_list('id', flat=True)
to_update = []
to_create = []
for book in new_books:
if book.id in old_book_ids:
# list of books to update
# to_update.append(book.id)
else:
# create a book object
# Book(**details)
# Update books
Book.objects.filter(id__in=to_update).update(field='new_value')
Book.objects.bulk_create(to_create)
But if the updates are complex (update fields are dependent upon related fields), then you can check insert... on duplicated key update option in MySQL and its custom manager for Django.
Please leave a comment if the above is completely off the track.
You'll have to do more than one query. You need two groups of objects, you can't fetch them both and split them up at the same time arbitrarily like that. There's no bulk_get_or_create method.
However, the example code you've given will do a query for every object which really isn't very efficient (or djangoic for that matter). Instead, use the __in clause to create smart subqueries, and then you can limit database hits to only two queries:
old_to_update = Book.objects.filter(foreign_source="import", pk__in=new_books)
old_to_create = Book.objects.filter(foreign_source="import").exclude(pk__in=new_books)
Django is smart enough to know how to use that new_books queryset in that context (it can also be a regular list of ids)
update
Queryset objects are just a sort of list of objects. So all you need to do now is loop over the objects:
for book in old_to_update:
#update book
for book in old_to_create:
#create book
At this point, when it's fetching the books from the QuerySet, not from the databse, which is a lot more efficient than using .get() for each and every one of them - and you get the same result. each iteration you get to work with an object, the same as if you got it from a direct .get() call.
The best solution I have found is using the python next() function.
First evaluate the queryset into a set and then pick the book you need with next:
old_books = set(Book.objects.filter(foreign_source="import"))
old_book = next((book for book in existing_books if book.id == new_book.id), None )
That way the database is not queried everytime you need to get a specific book from the queryset. And then you can just do:
if old_book:
#update book
old_book.save()
else:
#create new book
In Django 1.7 there is an update_or_create() method that might solve this problem in a better way: https://docs.djangoproject.com/en/dev/ref/models/querysets/#django.db.models.query.QuerySet.update_or_create
I'm trying to output a list of images that belong to each record in my app as below:
pri_photo = vehicle.images.all()[:1]
sec_photos = vehicle.images.all()[1:]
This first part is OK. The part I'm having issues with is when I try
pri_photo.original_image.url
sec_photos.original_image.url
The above two lines of code give me a 'QuerySet' object has no attribute 'original_image'. What could be the issue?
I also want the photos in sec_photos to be output as image1, image2,... upto the last one
[:1] just limits the number of records returned by the queryset, still it is a queryset result, not a single object.
to get a single object, you shoud use
[0]
or
[0:1].get()
but it's not safe, as it will raise an error if none objects match the query. to do it properly, use filter() and if result are present then get(), or
try:
#get()
except modelname.DoesNotExist:
# do shomething else
also, if you are looking for the latest object, maybe you can just use http://www.djangoproject.com/documentation/models/get_latest/