imp_points

imp points:


1). AGGREGATORS and ANNOTATIONS
https://docs.djangoproject.com/en/1.10/topics/db/aggregation/


aggregators and annotations are similar but aggregator works on  group of objects and yields the result as a dictionary but annotate performs operations on a per objects basis and returns a queryset so as to perform multiple further operations to it.
Annotate uses joins instead of sub queries therefore annotate on multiple fields would result wrong result(we can perform multiple op. like filter etc but on a single one)
eg:- aggregate
Book.objects.aggregate(average_price=Avg('price'))
result:-
{'average_price': 34.35}

multiple aggregate:
Book.objects.aggregate(Avg('price'), Max('price'), Min('price'))
result:-
{'price__avg': 34.35, 'price__max': Decimal('81.20'), 'price__min': Decimal('12.99')}

annotate:
q = Book.objects.annotate(Count('authors')) // here q is a queryset
q[0].authors__count

multiple annotate:-
q = Book.objects.annotate(Count('authors'), Count('store')) // this will give wrong results

A distint parameter may help prevent the problem for 'Count' aggregator.
eg:-q = Book.objects.annotate(Count('authors', distinct=True), Count('store', distinct=True))


We can apply filters on both aggregators n annotations... :p but only before.. eg:-
>>> Book.objects.filter(name__startswith="Django").annotate(num_authors=Count('authors'))
>>> Book.objects.filter(name__startswith="Django").aggregate(Avg('price'))

When used with an annotate() clause, a filter has the effect of constraining the objects FOR which an annotation is calculated. For example, you can generate an annotated list of all books that have a title starting with “Django” using the above query.

When used with an aggregate() clause, a filter has the effect of constraining the objects OVER which the aggregate is calculated. For example, you can generate the average price of all books with a title that starts with “Django” using the above query.

Annotated values can also be filtered. The alias for the annotation can be used in filter() and exclude() clauses in the same way as any other model field.

For example, to generate a list of books that have more than one author, you can issue the query:
>>> Book.objects.annotate(num_authors=Count('authors')).filter(num_authors__gt=1)


Order of annotate() and filter() clauses
Given:

Publisher A has two books with ratings 4 and 5.
Publisher B has two books with ratings 1 and 4.
Publisher C has one book with rating 1.


>>> a, b = Publisher.objects.annotate(num_books=Count('book', distinct=True)).filter(book__rating__gt=3.0)
>>> a, a.num_books
(<Publisher: A>, 2)
>>> b, b.num_books
(<Publisher: B>, 2)

>>> a, b = Publisher.objects.filter(book__rating__gt=3.0).annotate(num_books=Count('book'))
>>> a, a.num_books
(<Publisher: A>, 2)
>>> b, b.num_books
(<Publisher: B>, 1)


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rest more imp pts on the webpage...link provided with title


2). Trying to inherit from more than one class that inherits from View