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/
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´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'])
I'm trying get an attribute from a given object in Djano. I'm getting the value properly but I'm curious as if there is a better way to grab this data.
I'm getting the name attribute by using:
owner_name = Owner.objects.filter(id=id).values('name')
And it properly returns the name attribute I'm looking for, but it is in the form of:
<QuerySet [{'name': u'John Doe'}]>
How can I get it to just return "John Doe" instead of <QuerySet [{'name': u'John Doe'}]>?
Edit: I've found out that I can get the info I need by doing:
owner_name[0].get('name')
and it will return John Doe. Is there a better way to do this still just to get 1 attribute?
You want a values_list query with the flat=True parameter. From the docs:
A common need is to get a specific field value of a certain model instance. To achieve that, use values_list() followed by a get() call:
Entry.objects.values_list('headline', flat=True).get(pk=1)
Or in your case:
Owner.objects.values_list('name', flat=True).get(id=id)
Note that get will raise an exception if there is not exactly one matching result. If that's a possibility, say because you were filtering on something other than id or because there might be no matching object, you could catch the exceptions or you could work with the list of names returned and check its length.
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))
This line works and returns the value that I'm looking for:
logs = Log.objects.filter(filterURI=aFilter.uri).values()[0]['yk']
However, when I try to add another filter and do the same I get errors:
logs = Log.objects.filter(filterURI=aFilter.uri).filter(k=k-1).values()[0]['yk']
My understanding is that a object.filter returns a queryset but so does a 'filter of a filter'. So I should be able to do the 'values' call in the same way regardless of whether I have one filter or 1000.
What am I doing wrong here.
Thanks in advance.
I don't think the error is in the fact that you have two filters - it's in the actual second filter. k=k-1 will only work if you have both a model field and a local variable called k - the first is on the left of the expression, the second on the right.
If you want to refer to the model field on the right of the expression, use F:
.filter(k=(F('k')-1)