django1/tests/regressiontests/aggregation_regress/tests.py

import datetime
import pickle
from decimal import Decimal
from operator import attrgetter

from django.core.exceptions import FieldError
from django.test import TestCase, Approximate, skipUnlessDBFeature
from django.db.models import Count, Max, Avg, Sum, StdDev, Variance, F

from models import Author, Book, Publisher, Clues, Entries, HardbackBook


class AggregationTests(TestCase):
    def assertObjectAttrs(self, obj, **kwargs):
        for attr, value in kwargs.iteritems():
            self.assertEqual(getattr(obj, attr), value)

    def test_aggregates_in_where_clause(self):
        """
        Regression test for #12822: DatabaseError: aggregates not allowed in
        WHERE clause

        Tests that the subselect works and returns results equivalent to a
        query with the IDs listed.

        Before the corresponding fix for this bug, this test passed in 1.1 and
        failed in 1.2-beta (trunk).
        """
        qs = Book.objects.values('contact').annotate(Max('id'))
        qs = qs.order_by('contact').values_list('id__max', flat=True)
        # don't do anything with the queryset (qs) before including it as a
        # subquery
        books = Book.objects.order_by('id')
        qs1 = books.filter(id__in=qs)
        qs2 = books.filter(id__in=list(qs))
        self.assertEqual(list(qs1), list(qs2))

    def test_aggregates_in_where_clause_pre_eval(self):
        """
        Regression test for #12822: DatabaseError: aggregates not allowed in
        WHERE clause

        Same as the above test, but evaluates the queryset for the subquery
        before it's used as a subquery.

        Before the corresponding fix for this bug, this test failed in both
        1.1 and 1.2-beta (trunk).
        """
        qs = Book.objects.values('contact').annotate(Max('id'))
        qs = qs.order_by('contact').values_list('id__max', flat=True)
        # force the queryset (qs) for the subquery to be evaluated in its
        # current state
        list(qs)
        books = Book.objects.order_by('id')
        qs1 = books.filter(id__in=qs)
        qs2 = books.filter(id__in=list(qs))
        self.assertEqual(list(qs1), list(qs2))

    @skipUnlessDBFeature('supports_subqueries_in_group_by')
    def test_annotate_with_extra(self):
        """
        Regression test for #11916: Extra params + aggregation creates
        incorrect SQL.
        """
        #oracle doesn't support subqueries in group by clause
        shortest_book_sql = """
        SELECT name
        FROM aggregation_regress_book b
        WHERE b.publisher_id = aggregation_regress_publisher.id
        ORDER BY b.pages
        LIMIT 1
        """
        # tests that this query does not raise a DatabaseError due to the full
        # subselect being (erroneously) added to the GROUP BY parameters
        qs = Publisher.objects.extra(select={
            'name_of_shortest_book': shortest_book_sql,
        }).annotate(total_books=Count('book'))
        # force execution of the query
        list(qs)

    def test_aggregate(self):
        # Ordering requests are ignored
        self.assertEqual(
            Author.objects.order_by("name").aggregate(Avg("age")),
            {"age__avg": Approximate(37.444, places=1)}
        )

        # Implicit ordering is also ignored
        self.assertEqual(
            Book.objects.aggregate(Sum("pages")),
            {"pages__sum": 3703},
        )

        # Baseline results
        self.assertEqual(
            Book.objects.aggregate(Sum('pages'), Avg('pages')),
            {'pages__sum': 3703, 'pages__avg': Approximate(617.166, places=2)}
        )

        # Empty values query doesn't affect grouping or results
        self.assertEqual(
            Book.objects.values().aggregate(Sum('pages'), Avg('pages')),
            {'pages__sum': 3703, 'pages__avg': Approximate(617.166, places=2)}
        )

        # Aggregate overrides extra selected column
        self.assertEqual(
            Book.objects.extra(select={'price_per_page' : 'price / pages'}).aggregate(Sum('pages')),
            {'pages__sum': 3703}
        )

    def test_annotation(self):
        # Annotations get combined with extra select clauses
        obj = Book.objects.annotate(mean_auth_age=Avg("authors__age")).extra(select={"manufacture_cost": "price * .5"}).get(pk=2)
        self.assertObjectAttrs(obj,
            contact_id=3,
            id=2,
            isbn=u'067232959',
            mean_auth_age=45.0,
            name='Sams Teach Yourself Django in 24 Hours',
            pages=528,
            price=Decimal("23.09"),
            pubdate=datetime.date(2008, 3, 3),
            publisher_id=2,
            rating=3.0
        )
        # Different DB backends return different types for the extra select computation
        self.assertTrue(obj.manufacture_cost == 11.545 or obj.manufacture_cost == Decimal('11.545'))

        # Order of the annotate/extra in the query doesn't matter
        obj = Book.objects.extra(select={'manufacture_cost' : 'price * .5'}).annotate(mean_auth_age=Avg('authors__age')).get(pk=2)
        self.assertObjectAttrs(obj,
            contact_id=3,
            id=2,
            isbn=u'067232959',
            mean_auth_age=45.0,
            name=u'Sams Teach Yourself Django in 24 Hours',
            pages=528,
            price=Decimal("23.09"),
            pubdate=datetime.date(2008, 3, 3),
            publisher_id=2,
            rating=3.0
        )
        # Different DB backends return different types for the extra select computation
        self.assertTrue(obj.manufacture_cost == 11.545 or obj.manufacture_cost == Decimal('11.545'))

        # Values queries can be combined with annotate and extra
        obj = Book.objects.annotate(mean_auth_age=Avg('authors__age')).extra(select={'manufacture_cost' : 'price * .5'}).values().get(pk=2)
        manufacture_cost = obj['manufacture_cost']
        self.assertTrue(manufacture_cost == 11.545 or manufacture_cost == Decimal('11.545'))
        del obj['manufacture_cost']
        self.assertEqual(obj, {
            "contact_id": 3,
            "id": 2,
            "isbn": u"067232959",
            "mean_auth_age": 45.0,
            "name": u"Sams Teach Yourself Django in 24 Hours",
            "pages": 528,
            "price": Decimal("23.09"),
            "pubdate": datetime.date(2008, 3, 3),
            "publisher_id": 2,
            "rating": 3.0,
        })

        # The order of the (empty) values, annotate and extra clauses doesn't
        # matter
        obj = Book.objects.values().annotate(mean_auth_age=Avg('authors__age')).extra(select={'manufacture_cost' : 'price * .5'}).get(pk=2)
        manufacture_cost = obj['manufacture_cost']
        self.assertTrue(manufacture_cost == 11.545 or manufacture_cost == Decimal('11.545'))
        del obj['manufacture_cost']
        self.assertEqual(obj, {
            'contact_id': 3,
            'id': 2,
            'isbn': u'067232959',
            'mean_auth_age': 45.0,
            'name': u'Sams Teach Yourself Django in 24 Hours',
            'pages': 528,
            'price': Decimal("23.09"),
            'pubdate': datetime.date(2008, 3, 3),
            'publisher_id': 2,
            'rating': 3.0
        })

        # If the annotation precedes the values clause, it won't be included
        # unless it is explicitly named
        obj = Book.objects.annotate(mean_auth_age=Avg('authors__age')).extra(select={'price_per_page' : 'price / pages'}).values('name').get(pk=1)
        self.assertEqual(obj, {
            "name": u'The Definitive Guide to Django: Web Development Done Right',
        })

        obj = Book.objects.annotate(mean_auth_age=Avg('authors__age')).extra(select={'price_per_page' : 'price / pages'}).values('name','mean_auth_age').get(pk=1)
        self.assertEqual(obj, {
            'mean_auth_age': 34.5,
            'name': u'The Definitive Guide to Django: Web Development Done Right',
        })

        # If an annotation isn't included in the values, it can still be used
        # in a filter
        qs = Book.objects.annotate(n_authors=Count('authors')).values('name').filter(n_authors__gt=2)
        self.assertQuerysetEqual(
            qs, [
                {"name": u'Python Web Development with Django'}
            ],
            lambda b: b,
        )

        # The annotations are added to values output if values() precedes
        # annotate()
        obj = Book.objects.values('name').annotate(mean_auth_age=Avg('authors__age')).extra(select={'price_per_page' : 'price / pages'}).get(pk=1)
        self.assertEqual(obj, {
            'mean_auth_age': 34.5,
            'name': u'The Definitive Guide to Django: Web Development Done Right',
        })

        # Check that all of the objects are getting counted (allow_nulls) and
        # that values respects the amount of objects
        self.assertEqual(
            len(Author.objects.annotate(Avg('friends__age')).values()),
            9
        )

        # Check that consecutive calls to annotate accumulate in the query
        qs = Book.objects.values('price').annotate(oldest=Max('authors__age')).order_by('oldest', 'price').annotate(Max('publisher__num_awards'))
        self.assertQuerysetEqual(
            qs, [
                {'price': Decimal("30"), 'oldest': 35, 'publisher__num_awards__max': 3},
                {'price': Decimal("29.69"), 'oldest': 37, 'publisher__num_awards__max': 7},
                {'price': Decimal("23.09"), 'oldest': 45, 'publisher__num_awards__max': 1},
                {'price': Decimal("75"), 'oldest': 57, 'publisher__num_awards__max': 9},
                {'price': Decimal("82.8"), 'oldest': 57, 'publisher__num_awards__max': 7}
            ],
            lambda b: b,
        )

    def test_aggrate_annotation(self):
        # Aggregates can be composed over annotations.
        # The return type is derived from the composed aggregate
        vals = Book.objects.all().annotate(num_authors=Count('authors__id')).aggregate(Max('pages'), Max('price'), Sum('num_authors'), Avg('num_authors'))
        self.assertEqual(vals, {
            'num_authors__sum': 10,
            'num_authors__avg': Approximate(1.666, places=2),
            'pages__max': 1132,
            'price__max': Decimal("82.80")
        })

    def test_field_error(self):
        # Bad field requests in aggregates are caught and reported
        self.assertRaises(
            FieldError,
            lambda: Book.objects.all().aggregate(num_authors=Count('foo'))
        )

        self.assertRaises(
            FieldError,
            lambda: Book.objects.all().annotate(num_authors=Count('foo'))
        )

        self.assertRaises(
            FieldError,
            lambda: Book.objects.all().annotate(num_authors=Count('authors__id')).aggregate(Max('foo'))
        )

    def test_more(self):
        # Old-style count aggregations can be mixed with new-style
        self.assertEqual(
            Book.objects.annotate(num_authors=Count('authors')).count(),
            6
        )

        # Non-ordinal, non-computed Aggregates over annotations correctly
        # inherit the annotation's internal type if the annotation is ordinal
        # or computed
        vals = Book.objects.annotate(num_authors=Count('authors')).aggregate(Max('num_authors'))
        self.assertEqual(
            vals,
            {'num_authors__max': 3}
        )

        vals = Publisher.objects.annotate(avg_price=Avg('book__price')).aggregate(Max('avg_price'))
        self.assertEqual(
            vals,
            {'avg_price__max': 75.0}
        )

        # Aliases are quoted to protected aliases that might be reserved names
        vals = Book.objects.aggregate(number=Max('pages'), select=Max('pages'))
        self.assertEqual(
            vals,
            {'number': 1132, 'select': 1132}
        )

        # Regression for #10064: select_related() plays nice with aggregates
        obj = Book.objects.select_related('publisher').annotate(num_authors=Count('authors')).values()[0]
        self.assertEqual(obj, {
            'contact_id': 8,
            'id': 5,
            'isbn': u'013790395',
            'name': u'Artificial Intelligence: A Modern Approach',
            'num_authors': 2,
            'pages': 1132,
            'price': Decimal("82.8"),
            'pubdate': datetime.date(1995, 1, 15),
            'publisher_id': 3,
            'rating': 4.0,
        })

        # Regression for #10010: exclude on an aggregate field is correctly
        # negated
        self.assertEqual(
            len(Book.objects.annotate(num_authors=Count('authors'))),
            6
        )
        self.assertEqual(
            len(Book.objects.annotate(num_authors=Count('authors')).filter(num_authors__gt=2)),
            1
        )
        self.assertEqual(
            len(Book.objects.annotate(num_authors=Count('authors')).exclude(num_authors__gt=2)),
            5
        )

        self.assertEqual(
            len(Book.objects.annotate(num_authors=Count('authors')).filter(num_authors__lt=3).exclude(num_authors__lt=2)),
            2
        )
        self.assertEqual(
            len(Book.objects.annotate(num_authors=Count('authors')).exclude(num_authors__lt=2).filter(num_authors__lt=3)),
            2
        )

    def test_aggregate_fexpr(self):
        # Aggregates can be used with F() expressions
        # ... where the F() is pushed into the HAVING clause
        qs = Publisher.objects.annotate(num_books=Count('book')).filter(num_books__lt=F('num_awards')/2).order_by('name').values('name','num_books','num_awards')
        self.assertQuerysetEqual(
            qs, [
                {'num_books': 1, 'name': u'Morgan Kaufmann', 'num_awards': 9},
                {'num_books': 2, 'name': u'Prentice Hall', 'num_awards': 7}
            ],
            lambda p: p,
        )

        qs = Publisher.objects.annotate(num_books=Count('book')).exclude(num_books__lt=F('num_awards')/2).order_by('name').values('name','num_books','num_awards')
        self.assertQuerysetEqual(
            qs, [
                {'num_books': 2, 'name': u'Apress', 'num_awards': 3},
                {'num_books': 0, 'name': u"Jonno's House of Books", 'num_awards': 0},
                {'num_books': 1, 'name': u'Sams', 'num_awards': 1}
            ],
            lambda p: p,
        )

        # ... and where the F() references an aggregate
        qs = Publisher.objects.annotate(num_books=Count('book')).filter(num_awards__gt=2*F('num_books')).order_by('name').values('name','num_books','num_awards')
        self.assertQuerysetEqual(
            qs, [
                {'num_books': 1, 'name': u'Morgan Kaufmann', 'num_awards': 9},
                {'num_books': 2, 'name': u'Prentice Hall', 'num_awards': 7}
            ],
            lambda p: p,
        )

        qs = Publisher.objects.annotate(num_books=Count('book')).exclude(num_books__lt=F('num_awards')/2).order_by('name').values('name','num_books','num_awards')
        self.assertQuerysetEqual(
            qs, [
                {'num_books': 2, 'name': u'Apress', 'num_awards': 3},
                {'num_books': 0, 'name': u"Jonno's House of Books", 'num_awards': 0},
                {'num_books': 1, 'name': u'Sams', 'num_awards': 1}
            ],
            lambda p: p,
        )

    def test_db_col_table(self):
        # Tests on fields with non-default table and column names.
        qs = Clues.objects.values('EntryID__Entry').annotate(Appearances=Count('EntryID'), Distinct_Clues=Count('Clue', distinct=True))
        self.assertQuerysetEqual(qs, [])

        qs = Entries.objects.annotate(clue_count=Count('clues__ID'))
        self.assertQuerysetEqual(qs, [])

    def test_empty(self):
        # Regression for #10089: Check handling of empty result sets with
        # aggregates
        self.assertEqual(
            Book.objects.filter(id__in=[]).count(),
            0
        )

        vals = Book.objects.filter(id__in=[]).aggregate(num_authors=Count('authors'), avg_authors=Avg('authors'), max_authors=Max('authors'), max_price=Max('price'), max_rating=Max('rating'))
        self.assertEqual(
            vals,
            {'max_authors': None, 'max_rating': None, 'num_authors': 0, 'avg_authors': None, 'max_price': None}
        )

        qs = Publisher.objects.filter(pk=5).annotate(num_authors=Count('book__authors'), avg_authors=Avg('book__authors'), max_authors=Max('book__authors'), max_price=Max('book__price'), max_rating=Max('book__rating')).values()
        self.assertQuerysetEqual(
            qs, [
                {'max_authors': None, 'name': u"Jonno's House of Books", 'num_awards': 0, 'max_price': None, 'num_authors': 0, 'max_rating': None, 'id': 5, 'avg_authors': None}
            ],
            lambda p: p
        )

    def test_more_more(self):
        # Regression for #10113 - Fields mentioned in order_by() must be
        # included in the GROUP BY. This only becomes a problem when the
        # order_by introduces a new join.
        self.assertQuerysetEqual(
            Book.objects.annotate(num_authors=Count('authors')).order_by('publisher__name', 'name'), [
                "Practical Django Projects",
                "The Definitive Guide to Django: Web Development Done Right",
                "Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp",
                "Artificial Intelligence: A Modern Approach",
                "Python Web Development with Django",
                "Sams Teach Yourself Django in 24 Hours",
            ],
            lambda b: b.name
        )

        # Regression for #10127 - Empty select_related() works with annotate
        qs = Book.objects.filter(rating__lt=4.5).select_related().annotate(Avg('authors__age'))
        self.assertQuerysetEqual(
            qs, [
                (u'Artificial Intelligence: A Modern Approach', 51.5, u'Prentice Hall', u'Peter Norvig'),
                (u'Practical Django Projects', 29.0, u'Apress', u'James Bennett'),
                (u'Python Web Development with Django', Approximate(30.333, places=2), u'Prentice Hall', u'Jeffrey Forcier'),
                (u'Sams Teach Yourself Django in 24 Hours', 45.0, u'Sams', u'Brad Dayley')
            ],
            lambda b: (b.name, b.authors__age__avg, b.publisher.name, b.contact.name)
        )

        # Regression for #10132 - If the values() clause only mentioned extra
        # (select=) columns, those columns are used for grouping
        qs = Book.objects.extra(select={'pub':'publisher_id'}).values('pub').annotate(Count('id')).order_by('pub')
        self.assertQuerysetEqual(
            qs, [
                {'pub': 1, 'id__count': 2},
                {'pub': 2, 'id__count': 1},
                {'pub': 3, 'id__count': 2},
                {'pub': 4, 'id__count': 1}
            ],
            lambda b: b
        )

        qs = Book.objects.extra(select={'pub':'publisher_id', 'foo':'pages'}).values('pub').annotate(Count('id')).order_by('pub')
        self.assertQuerysetEqual(
            qs, [
                {'pub': 1, 'id__count': 2},
                {'pub': 2, 'id__count': 1},
                {'pub': 3, 'id__count': 2},
                {'pub': 4, 'id__count': 1}
            ],
            lambda b: b
        )

        # Regression for #10182 - Queries with aggregate calls are correctly
        # realiased when used in a subquery
        ids = Book.objects.filter(pages__gt=100).annotate(n_authors=Count('authors')).filter(n_authors__gt=2).order_by('n_authors')
        self.assertQuerysetEqual(
            Book.objects.filter(id__in=ids), [
                "Python Web Development with Django",
            ],
            lambda b: b.name
        )

    def test_duplicate_alias(self):
        # Regression for #11256 - duplicating a default alias raises ValueError.
        self.assertRaises(ValueError, Book.objects.all().annotate, Avg('authors__age'), authors__age__avg=Avg('authors__age'))

    def test_field_name_conflict(self):
        # Regression for #11256 - providing an aggregate name that conflicts with a field name on the model raises ValueError
        self.assertRaises(ValueError, Author.objects.annotate, age=Avg('friends__age'))

    def test_m2m_name_conflict(self):
        # Regression for #11256 - providing an aggregate name that conflicts with an m2m name on the model raises ValueError
        self.assertRaises(ValueError, Author.objects.annotate, friends=Count('friends'))

    def test_reverse_relation_name_conflict(self):
        # Regression for #11256 - providing an aggregate name that conflicts with a reverse-related name on the model raises ValueError
        self.assertRaises(ValueError, Author.objects.annotate, book_contact_set=Avg('friends__age'))

    def test_pickle(self):
        # Regression for #10197 -- Queries with aggregates can be pickled.
        # First check that pickling is possible at all. No crash = success
        qs = Book.objects.annotate(num_authors=Count('authors'))
        pickle.dumps(qs)

        # Then check that the round trip works.
        query = qs.query.get_compiler(qs.db).as_sql()[0]
        qs2 = pickle.loads(pickle.dumps(qs))
        self.assertEqual(
            qs2.query.get_compiler(qs2.db).as_sql()[0],
            query,
        )

    def test_more_more_more(self):
        # Regression for #10199 - Aggregate calls clone the original query so
        # the original query can still be used
        books = Book.objects.all()
        books.aggregate(Avg("authors__age"))
        self.assertQuerysetEqual(
            books.all(), [
                u'Artificial Intelligence: A Modern Approach',
                u'Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp',
                u'Practical Django Projects',
                u'Python Web Development with Django',
                u'Sams Teach Yourself Django in 24 Hours',
                u'The Definitive Guide to Django: Web Development Done Right'
            ],
            lambda b: b.name
        )

        # Regression for #10248 - Annotations work with DateQuerySets
        qs = Book.objects.annotate(num_authors=Count('authors')).filter(num_authors=2).dates('pubdate', 'day')
        self.assertQuerysetEqual(
            qs, [
                datetime.datetime(1995, 1, 15, 0, 0),
                datetime.datetime(2007, 12, 6, 0, 0)
            ],
            lambda b: b
        )

        # Regression for #10290 - extra selects with parameters can be used for
        # grouping.
        qs = Book.objects.annotate(mean_auth_age=Avg('authors__age')).extra(select={'sheets' : '(pages + %s) / %s'}, select_params=[1, 2]).order_by('sheets').values('sheets')
        self.assertQuerysetEqual(
            qs, [
                150,
                175,
                224,
                264,
                473,
                566
            ],
            lambda b: int(b["sheets"])
        )

        # Regression for 10425 - annotations don't get in the way of a count()
        # clause
        self.assertEqual(
            Book.objects.values('publisher').annotate(Count('publisher')).count(),
            4
        )
        self.assertEqual(
            Book.objects.annotate(Count('publisher')).values('publisher').count(),
            6
        )

        publishers = Publisher.objects.filter(id__in=[1, 2])
        self.assertQuerysetEqual(
            publishers, [
                "Apress",
                "Sams"
            ],
            lambda p: p.name
        )

        publishers = publishers.annotate(n_books=Count("book"))
        self.assertEqual(
            publishers[0].n_books,
            2
        )

        self.assertQuerysetEqual(
            publishers, [
                "Apress",
                "Sams",
            ],
            lambda p: p.name
        )

        books = Book.objects.filter(publisher__in=publishers)
        self.assertQuerysetEqual(
            books, [
                "Practical Django Projects",
                "Sams Teach Yourself Django in 24 Hours",
                "The Definitive Guide to Django: Web Development Done Right",
            ],
            lambda b: b.name
        )
        self.assertQuerysetEqual(
            publishers, [
                "Apress",
                "Sams",
            ],
            lambda p: p.name
        )

        # Regression for 10666 - inherited fields work with annotations and
        # aggregations
        self.assertEqual(
            HardbackBook.objects.aggregate(n_pages=Sum('book_ptr__pages')),
            {'n_pages': 2078}
        )

        self.assertEqual(
            HardbackBook.objects.aggregate(n_pages=Sum('pages')),
            {'n_pages': 2078},
        )

        qs = HardbackBook.objects.annotate(n_authors=Count('book_ptr__authors')).values('name', 'n_authors')
        self.assertQuerysetEqual(
            qs, [
                {'n_authors': 2, 'name': u'Artificial Intelligence: A Modern Approach'},
                {'n_authors': 1, 'name': u'Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp'}
            ],
            lambda h: h
        )

        qs = HardbackBook.objects.annotate(n_authors=Count('authors')).values('name', 'n_authors')
        self.assertQuerysetEqual(
            qs, [
                {'n_authors': 2, 'name': u'Artificial Intelligence: A Modern Approach'},
                {'n_authors': 1, 'name': u'Paradigms of Artificial Intelligence Programming: Case Studies in Common Lisp'}
            ],
            lambda h: h,
        )

        # Regression for #10766 - Shouldn't be able to reference an aggregate
        # fields in an an aggregate() call.
        self.assertRaises(
            FieldError,
            lambda: Book.objects.annotate(mean_age=Avg('authors__age')).annotate(Avg('mean_age'))
        )

    def test_empty_filter_count(self):
        self.assertEqual(
            Author.objects.filter(id__in=[]).annotate(Count("friends")).count(),
            0
        )

    def test_empty_filter_aggregate(self):
        self.assertEqual(
            Author.objects.filter(id__in=[]).annotate(Count("friends")).aggregate(Count("pk")),
            {"pk__count": None}
        )

    def test_annotate_and_join(self):
        self.assertEqual(
            Author.objects.annotate(c=Count("friends__name")).exclude(friends__name="Joe").count(),
            Author.objects.count()
        )

    def test_f_expression_annotation(self):
        # Books with less than 200 pages per author.
        qs = Book.objects.values("name").annotate(
            n_authors=Count("authors")
        ).filter(
            pages__lt=F("n_authors") * 200
        ).values_list("pk")
        self.assertQuerysetEqual(
            Book.objects.filter(pk__in=qs), [
                "Python Web Development with Django"
            ],
            attrgetter("name")
        )

    @skipUnlessDBFeature('supports_stddev')
    def test_stddev(self):
        self.assertEqual(
            Book.objects.aggregate(StdDev('pages')),
            {'pages__stddev': Approximate(311.46, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(StdDev('rating')),
            {'rating__stddev': Approximate(0.60, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(StdDev('price')),
            {'price__stddev': Approximate(24.16, 2)}
        )

        self.assertEqual(
            Book.objects.aggregate(StdDev('pages', sample=True)),
            {'pages__stddev': Approximate(341.19, 2)}
        )

        self.assertEqual(
            Book.objects.aggregate(StdDev('rating', sample=True)),
            {'rating__stddev': Approximate(0.66, 2)}
        )

        self.assertEqual(
            Book.objects.aggregate(StdDev('price', sample=True)),
            {'price__stddev': Approximate(26.46, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(Variance('pages')),
            {'pages__variance': Approximate(97010.80, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(Variance('rating')),
            {'rating__variance': Approximate(0.36, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(Variance('price')),
            {'price__variance': Approximate(583.77, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(Variance('pages', sample=True)),
            {'pages__variance': Approximate(116412.96, 1)}
        )

        self.assertEqual(
            Book.objects.aggregate(Variance('rating', sample=True)),
            {'rating__variance': Approximate(0.44, 2)}
        )

        self.assertEqual(
            Book.objects.aggregate(Variance('price', sample=True)),
            {'price__variance': Approximate(700.53, 2)}
        )