Create longest_value_in_field.md

django/longest_value_in_field.md

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+# Finding the longest value of a particular field 
+
+```python
+from django.db.models import Max, F
+from django.db.models.functions import Length
+from myapp.models import MyModel
+
+# Annotate the queryset with the length of each string in `my_field`
+annotated_queryset = MyModel.objects.annotate(my_field_length=Length('my_field'))
+
+# Aggregate the annotated queryset to find the maximum length
+max_length = annotated_queryset.aggregate(max_length=Max('my_field_length'))['max_length']
+
+print(f"The longest string in `my_field` is {max_length} characters long.")
+```