python / intermediate
Snippet
Django ORM QuerySet Chaining and Aggregation
Django QuerySets support method chaining for building complex queries. select_related() performs SQL joins for ForeignKey relationships (single query), while prefetch_related() optimizes ManyToMany with separate queries. Aggregation functions like Count and Avg compute statistics. Q objects build complex OR/AND conditions. Custom Managers encapsulate reusable query logic, keeping models clean and testable.
snippet.py
python
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from django.db.models import Count, Avg, Q, F, Prefetchfrom django.db.models.functions import TruncMonthclass BookManager(models.Manager):def get_published_with_authors(self):return self.get_queryset().filter(is_published=True).select_related('author').prefetch_related(Prefetch('collaborators', queryset=Author.objects.only('id', 'name')))def monthly_publication_stats(self):return self.get_queryset().filter(is_published=True).annotate(month=TruncMonth('published_date')).values('month').annotate(book_count=Count('id'),avg_rating=Avg('rating')).order_by('-month')def search_books(self, query, min_rating=None):filters = Q(title__icontains=query) | Q(description__icontains=query)if min_rating:filters &= Q(rating__gte=min_rating)return self.filter(filters)# Usagebooks = Book.objects.search_books('python', min_rating=4.0)top_rated = books.order_by('-rating')[:5]
django
Breakdown
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select_related('author')
Fetches ForeignKey relations in a single SQL JOIN, avoiding N+1 queries
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Prefetch('collaborators', queryset=...)
Optimizes ManyToMany with separate query, limiting fields to only what's needed
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Q(title__icontains=query) | Q(description__icontains=query)
Q objects combine conditions with OR; & for AND, ~ for NOT
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TruncMonth('published_date')
Truncates datetime to month for grouping publications by month