Django objects queryset

Question

I have a question regarding django queries. I have a query like so:

x = Foo.objects.order_by('date').values('amount')

and this returns something like:

[{'amount': Decimal('100.2200')},{'amount': Decimal('100.2200')}....]

What I want to do is get the standard deviation of these decimals but I cannot seem to figure out how to get them from the dictionary to just the decimals in a list. I tried a for statement but that doesn't work and list comprehension returns:

[Decimal('100.2200'), Decimal('100.2300')...]

With this code:

x = Foo.objects.order_by('date').values('amount')
x = ([amount.values()[0] for amount in x])
print x

Am I doing this wrong? Should I be making sql queries to the server if I want to get the info back and manipulate it?

Basically what I want to do is get the standard dev of all the amounts in a certain table based on specific filters...

Thank you so much for your help

karthikr · Accepted Answer · 2016-07-13 00:01:26Z

2

First, you can get the list of decimals directly by using values_list

qs = Foo.objects.order_by('date').values_list('amount', flat=True)

Secondly, if it is easier to calculate SD on floats, rather than on a list of decimals, you can do

qs = Foo.objects.order_by('date').values_list('amount', flat=True)
float_qs = map(float, qs)

answered Jul 13, 2016 at 0:01

karthikr

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5 Comments

rafalmp Over a year ago

You can also get SD directly from DB (SQLite requires an extension module for that, other DBs support it out of the box): Foo.objects.all().aggregate(StdDev('amount')) - docs

Harp Angell Over a year ago

@karthikr your first idea still prints out [Decimal('100.2200'), Decimal('100.2300')...]

Harp Angell Over a year ago

@rafalmp does it matter if I'm using postgres? I'm running my program on C9 and cant update the python to run 3.

Harp Angell Over a year ago

Thanks @karthikr second one is what I need I believe.

rafalmp Over a year ago

@HarpAngell Postgres supports StdDev since at least version 8.1 so I think there will be no problem.

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