I'm hitting my head on something that should be simple.
I have a query like this:
MyModel.objects.filter(...).aggregate(max_value=Max('value'))
and this does, indeed, return a (dictionary) with key 'max_value' and value that maximum value.
But what I want is the object that has that maximum value. Or, at least, some subset of the fields of that object, but the max value is just the selector
You can work with the .latest(…) [Django-doc] to obtain the object with the maximum of certain field(s). This will internally order the queryset and then obtain the last object, so:
MyModel.objects.filter(…).latest('value')
Related
Model.objects.all().values() returns a list of object values.
Model.objects.all().first() - This just appears to print an object reference with id field attribute.
Is there a way to do something like: Model.objects.all().first().values()?
The following does the trick, but is there something for this purpose?
Model.objects.filter(id=Model.objects.all().last().id).values()
You can chain the .first() after the .values(), since .values(…) [Django-doc] does not return a list, but a QuerySet that is not evaluated unless you consume it.
You thus can query with:
Model.objects.values().first()
This will return a single dictionary, and the .first() will thus force evaluation.
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 want to get the first value from a sequence for a key within a dictionary object.
This is the dictionary object:
{0: (mdb.models['EXP-100'].parts['hatpart-100'].vertices[1],
(62.5242172081597, 101.192447407436, 325.0))}
The Key is 0, and I'd like to get the first value in the sequence of values
(vertex object and coordinates), specifically the vertex object.
When you're trying to get a specific item from a dictionary containing a list you can do it by indicating the key and then index that you're looking for. So:
your dictionary ={'key':[1,2,3]}
result = your_dictionary['key'][0]
where key is the first item you want and if it contains a list, then the "0" is asking for the first item of the list.
The above will return the value "1".
You can iterate over this to get all values and you can add as many [] as you need to get to the specific value you need.
That being said, your data doesn't look quite right. The dictionary should contain "key":"object" sets. Those objects can be further dictionaries themselves or lists, but what you've provided seems to be off. You may have to manipulate the data to fit what you need it to do before your can perform any operations on it. This could help with that first: "Deparsing" a list using pyparsing
I have a model(Delivery) with 2 fields called name and to_date. I just need to a object with the specific name and it's maximum to_date.
Delivery.objects.filter(name__exact = 'name1').aggregate(Max('valid_to'))
The above query will return the maximum date. Is it possible to fetch the complete object?
To get a single object ordered by valid_to:
obj = Delivery.objects.filter(name='name1', to_date=my_date).order_by('-valid_to')[0]
Try this:
maximum_to_date = Delivery.objects.filter(name__exact='name1').aggregate(maximum to_date=Max('valid_to'))
result = Delivery.objects.filter(valid_to=maximum_to_date)
Note that you need filter() in the second line, because two or more Deliveries might have the same valid_to value. In such case you can either accept them all, or e.g. take the one with the smallest ID, depending on what you need.
I have a model which looks like this:
class MyModel(models.Model)
value = models.DecimalField()
date = models.DatetimeField()
I'm doing this request:
MyModel.objects.aggregate(Min("value"))
and I'm getting the expected result:
{"mymodel__min": the_actual_minimum_value}
However, I can't figure out a way to get at the same time the minimum value AND the associated date (the date at which the minimum value occured).
Does the Django ORM allow this, or do I have to use raw SQL ?
What you want to do is annotate the query, so that you get back your usual results but also have some data added to the result. So:
MyModel.objects.annotate(Min("value"))
Will return the normal result with mymodel__min as an additional value
In reply to your comment, I think this is what you are looking for? This will return the dates with their corresponding Min values.
MyModel.objects.values('date').annotate(Min("value"))
Edit: In further reply to your comment in that you want the lowest valued entry but also want the additional date field within your result, you could do something like so:
MyModel.objects.values('date').annotate(min_value=Min('value')).order_by('min_value')[0]
This will get the resulting dict you are asking for by ordering the results and then simply taking the first index which will always be the lowest value.
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