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Diffstat (limited to 'docs/topics/db/aggregation.txt')
| -rw-r--r-- | docs/topics/db/aggregation.txt | 40 |
1 files changed, 24 insertions, 16 deletions
diff --git a/docs/topics/db/aggregation.txt b/docs/topics/db/aggregation.txt index 125cd0bdee..1024d6b0c2 100644 --- a/docs/topics/db/aggregation.txt +++ b/docs/topics/db/aggregation.txt @@ -18,27 +18,29 @@ used to track the inventory for a series of online bookstores: .. code-block:: python + from django.db import models + class Author(models.Model): - name = models.CharField(max_length=100) - age = models.IntegerField() + name = models.CharField(max_length=100) + age = models.IntegerField() class Publisher(models.Model): - name = models.CharField(max_length=300) - num_awards = models.IntegerField() + name = models.CharField(max_length=300) + num_awards = models.IntegerField() class Book(models.Model): - name = models.CharField(max_length=300) - pages = models.IntegerField() - price = models.DecimalField(max_digits=10, decimal_places=2) - rating = models.FloatField() - authors = models.ManyToManyField(Author) - publisher = models.ForeignKey(Publisher) - pubdate = models.DateField() + name = models.CharField(max_length=300) + pages = models.IntegerField() + price = models.DecimalField(max_digits=10, decimal_places=2) + rating = models.FloatField() + authors = models.ManyToManyField(Author) + publisher = models.ForeignKey(Publisher) + pubdate = models.DateField() class Store(models.Model): - name = models.CharField(max_length=300) - books = models.ManyToManyField(Book) - registered_users = models.PositiveIntegerField() + name = models.CharField(max_length=300) + books = models.ManyToManyField(Book) + registered_users = models.PositiveIntegerField() Cheat sheet =========== @@ -123,7 +125,7 @@ If you want to generate more than one aggregate, you just add another argument to the ``aggregate()`` clause. So, if we also wanted to know the maximum and minimum price of all books, we would issue the query:: - >>> from django.db.models import Avg, Max, Min, Count + >>> from django.db.models import Avg, Max, Min >>> Book.objects.aggregate(Avg('price'), Max('price'), Min('price')) {'price__avg': 34.35, 'price__max': Decimal('81.20'), 'price__min': Decimal('12.99')} @@ -148,6 +150,7 @@ the number of authors: .. code-block:: python # Build an annotated queryset + >>> from django.db.models import Count >>> q = Book.objects.annotate(Count('authors')) # Interrogate the first object in the queryset >>> q[0] @@ -192,6 +195,7 @@ and aggregate the related value. For example, to find the price range of books offered in each store, you could use the annotation:: + >>> from django.db.models import Max, Min >>> Store.objects.annotate(min_price=Min('books__price'), max_price=Max('books__price')) This tells Django to retrieve the ``Store`` model, join (through the @@ -222,7 +226,7 @@ For example, we can ask for all publishers, annotated with their respective total book stock counters (note how we use ``'book'`` to specify the ``Publisher`` -> ``Book`` reverse foreign key hop):: - >>> from django.db.models import Count, Min, Sum, Max, Avg + >>> from django.db.models import Count, Min, Sum, Avg >>> Publisher.objects.annotate(Count('book')) (Every ``Publisher`` in the resulting ``QuerySet`` will have an extra attribute @@ -269,6 +273,7 @@ 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 query:: + >>> from django.db.models import Count, Avg >>> Book.objects.filter(name__startswith="Django").annotate(num_authors=Count('authors')) When used with an ``aggregate()`` clause, a filter has the effect of @@ -407,6 +412,8 @@ particularly, when counting things. By way of example, suppose you have a model like this:: + from django.db import models + class Item(models.Model): name = models.CharField(max_length=10) data = models.IntegerField() @@ -457,5 +464,6 @@ For example, if you wanted to calculate the average number of authors per book you first annotate the set of books with the author count, then aggregate that author count, referencing the annotation field:: + >>> from django.db.models import Count, Avg >>> Book.objects.annotate(num_authors=Count('authors')).aggregate(Avg('num_authors')) {'num_authors__avg': 1.66} |