Create m2m_queries_how_many_objects_have_no_connections.md

django/m2m_queries_how_many_objects_have_no_connections.md

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+# Finding out how many objects have N connections to the other model in a M2M relationship 
+
+Assume we have a model `Item` with a ManyToMany field to `Category` through `ItemCategory`. 
+
+```python
+class Category(models.Model):
+...
+
+class Item(models.Model):
+    categories = models.ManyToManyField("Category", through="ItemCategory"...)
+
+class ItemCategory(models.Model):
+    item = models.ForeignKey("Item"...)
+    category = models.Category("Category"...)
+```
+
+We want to know how many Items do not have any Categories. 
+
+```python
+from django.db.models import Count
+from items.models import Item
+
+# Annotate each item with the count of related categories
+items_with_category_count = Item.objects.annotate(categories_count=Count('categories'))
+
+# Filter items that have zero categories
+items_with_zero_categories = items_with_category_count.filter(categories_count=0)
+
+# Get the count of items with zero categories
+count_of_items_with_zero_categories = items_with_zero_categories.count()
+
+print(count_of_items_with_zero_categories)
+```