django prefetch_related vs select_related

class ModelA(models.Model):
    pass

class ModelB(models.Model):
    a = ForeignKey(ModelA)

ModelB.objects.select_related('a').all() # Forward ForeignKey relationship
ModelA.objects.prefetch_related('modelb_set').all() # Reverse ForeignKey relationship

* django select_related and prefetch_related
* Prefetch_related and select_related functions in django
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select_related() “follows” foreign-key relationships,
selecting additional related-object data when it executes its query.

prefetch_related() does a separate lookup for each relationship, 
and does the “joining” in Python.
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Example

class A(models.Model):
pass

class B(models.Model):
a = ForeignKey(ModelA)
# Forward ForeignKey relationship
A.objects.select_related('a').all()
# Reverse ForeignKey relationship
A.objects.prefetch_related('modelb_set').all()