How can I get computed elements of a table in a django queryset?

Question

I\'m trying to use django\'s queryset API to emulate the following query:

SELECT EXTRACT(year FROM chosen_date) AS year, 
EXTRACT(month FROM chosen_date) AS          


        

Answer 1

Well here're some workarounds

1. In your particular case you could do it with one extra:

if use_date_due:
    sum_qs = sum_qs.extra(select={
                          'year': 'EXTRACT(year FROM coalesce(date_due, date))',
                          'month': 'EXTRACT(month FROM coalesce(date_due, date))',
                          'is_paid':'date_paid IS NOT NULL'
                        })

2. It's also possible to use plain python to get data you need:

for x in sum_qs:
    chosen_date = x.date_due if use_date_due and x.date_due else x.date
    print chosen_date.year, chosen_date.month

or

[(y.year, y.month) for y in (x.date_due if use_date_due and x.date_due else x.date for x in sum_qs)]

3. In the SQL world this type of calculating new fields is usually done by uing subquery or common table expression. I like cte more because of it's readability. It could be like:

with cte1 as (
    select
        *, coalesce(date_due, date) as chosen_date
    from polls_invoice
)
select
    *,
    extract(year from chosen_date) as year,
    extract(month from chosen_date) as month,
    case when date_paid is not null then 1 else 0 end as is_paid
from cte1

you can also chain as many cte as you want:

with cte1 as (
    select
        *, coalesce(date_due, date) as chosen_date
    from polls_invoice
), cte2 as (
    select
        extract(year from chosen_date) as year,
        extract(month from chosen_date) as month,
        case when date_paid is not null then 1 else 0 end as is_paid
    from cte2
)
select
    year, month, sum(is_paid) as paid_count
from cte2
group by year, month

so in django you can use raw query like:

Invoice.objects.raw('
     with cte1 as (
        select
            *, coalesce(date_due, date) as chosen_date
        from polls_invoice
    )
    select
        *,
        extract(year from chosen_date) as year,
        extract(month from chosen_date) as month,
        case when date_paid is not null then 1 else 0 end as is_paid
    from cte1')

and you will have Invoice objects with some additional properties.

4. Or you can simply substitute fields in your query with plain python

if use_date_due:
    chosen_date = 'coalesce(date_due, date)'
else: 
    chosen_date = 'date'

year = 'extract(year from {})'.format(chosen_date)
month = 'extract(month from {})'.format(chosen_date)
fields = {'year': year, 'month': month, 'is_paid':'date_paid is not null'}, 'chosen_date':chosen_date)
sum_qs = sum_qs.extra(select = fields)