DRF Serializers .orderitem_set.all() - python-3.x

Question

I was following serializer tutorial and came across some code that i did not understand its function.
def get_order_items(self, obj):
items = obj.orderitem_set.all()
print(obj)
print(items)
return OrderItemSerializer(items, many=True).data
What does the order item function do.
The snippet was from the serializer class below:
class OrderSerializer(serializers.ModelSerializer):
order_items = serializers.SerializerMethodField()
"""
Calculate order_items field
"""
def get_order_items(self, obj):
items = obj.orderitem_set.all()
print(obj)
print(items)
return OrderItemSerializer(items, many=True).data
class Meta:
model = Order
fields = ('phone',
'first_name',
'delivery_date',
'delivery_time',
'address',
'comment',
'payment',
'order_items',)
def create(self, validated_data):
items_data = validated_data.pop('order_items')
order = Order.objects.create(**validated_data)
for item_data in items_data:
OrderItem.objects.create(order=order, **item_data)
return order

Answer 1

The order items functions serializes the OrderItem set, essentially converting in into a dictionary (or json) ready for you frontend to use because of the use of .data.
Grab objects -> Serialize -> Convert into json
So the result of that function will be a format that you can send as a response.

Answer 2

This is a MethodSerializer that will obtain the orderitem_set of the given Order object obj. Then these items run through the OrderItemSerializer and the serializer data is collected and returned with the rest of the data of the Order if the user triggers the corresponding view.
Using a subserializer that way is however not recommended. You can use the subserializer directly as a field as is explained in the documentation [Django-rest-doc]:
class OrderSerializer(serializers.ModelSerializer):
order_items = serializers.OrderItemSerializer(source='orderitem_set', many=True) # 🖘 use the OrderItemSerializer directly
class Meta:
model = Order
fields = ('phone', 'first_name', 'delivery_date', 'delivery_time', 'address', 'comment', 'payment', 'order_items')
def create(self, validated_data):
items_data = validated_data.pop('order_items')
order = Order.objects.create(**validated_data)
for item_data in items_data:
OrderItem.objects.create(order=order, **item_data)
return order