How to get all instances in serializer method field - python

How to get all instances in serializer method field
I have a serializer method field and I am passing list data in the form of context to serializer like below.
name_list = [ "abc", "def",....]
obj_list = abc.objects.all()
Serializer = abcSerializer (obj_list, context=name_list, many=True)
class abcSerializer (serializers.ModelSerializer):
xyz = serializers.SerializerMethodField ("getXYZ", read_only=True)
class Meta:
model = abc
def getXYZ (self, data):
# here I want all instanceses, but I got only one instance in data.
I want to attach name_list data one by one to instace data with same index?
How I can get all instanceses in my serializer method field?

Why do you need all instances? If you want to manipulate something in all instances, better do it before passing it as argument in Serializer. If you want to get indivisual instance, you should get the value in data parameter. But your indentations are wrong. Try like this:
class abcSerializer (serializers.ModelSerializer):
xyz = serializers.SerializerMethodField("getXYZ")
class Meta:
model = abc
def getXYZ(self, data):
print(data) # it will print a instance of abc
return value_based_on_data
Update
Then I think you should try like this:
First update serializer class:
class abcSerializer (serializers.ModelSerializer): # use PascalCase for naming classes
xyz = serializers.ReadOnlyField()
class Meta:
model = abc
fields = '__all__' # use PascalCase for naming classes
Then use the following code to get values of xyz:
obj_list = []
for i, item in enumerate(abc.objects.all()):
item.xyz = name_list[i]
obj_list.append(item)
abcSerializer(obj_list, many=True).data

After dig deep into debugging I realized that I should share my findings with the community.
Look at the below line:
Serializer = abcSerializer (obj_list, context=name_list, many=True)
Here many=True make's abcSerializer to list serializer, according to rest framework documentation in list serializer we can access all objects of queryset in update method like below
class BookListSerializer(serializers.ListSerializer):
def update(self, instance, validated_data):
# Maps for id->instance and id->data item.
book_mapping = {book.id: book for book in instance}
data_mapping = {item['id']: item for item in validated_data}
# Perform creations and updates.
I found that we can also access all objects of queryset in any method even in serializerMethodField using below syntax
def getXYZ (self, data):
objects = self.instance

Related

Copying only non-relation attributes django/python

I am copying a model object to another, but I want that it doesn’t copy the relations
For example, assume you have a model like this:
class Dish(models.Model):
name = models.CharField(max_length=100)
description = models.CharField(max_length=500)
category = models.ForeignKey(Category, on_delete=models.CASCADE, default=1)
def __str__(self):
return self.name
Then I do:
my_dish = Dish.objects.get(pk=dish.id)
serializer = Dish_Serializer(my_dish)
my_new_object = serializer.data
I want my_new_object to include only those attributes that are not relations, in this case, name and description.
How do I do that without accessing name and description directly?
I assume in your serializer you don't want to explicitly define which field to serialize. Otherwise you could do the following:
class Dish_Serializer(serializers.ModelSerializer):
class Meta:
model = Dish
fields = ['id','name', 'description']
You probably can define these fields dynamically:
fields = [f.name for f in Dish._meta.concrete_fields]
or
fields = [f.name for f in Dish._meta.fields if not isinstance(f,ForeignKey)]
Ultimately, you want my_new_object in dictionary format and as per condition pk will give you only one object of dish.
So, you can do this instead :
my_new_object = Dish.objects.filter(pk=dish.id).values("name", "description")[0]
It will give you exact what you want, just declare the fields you need in values as an attribute fields.
You can remove a field from your serializer using .fields.pop(field_name) method like the below example According that I took from Dynamically modifying fields [drf-docs]:
class DynamicFieldsModelSerializer(serializers.ModelSerializer):
"""
A ModelSerializer that takes an additional `fields` argument that
controls which fields should be displayed.
"""
def __init__(self, *args, **kwargs):
# Don't pass the 'fields' arg up to the superclass
fields = kwargs.pop('fields', None)
# Instantiate the superclass normally
super().__init__(*args, **kwargs)
if fields is not None:
# Drop any fields that are not specified in the `fields` >argument.
allowed = set(fields)
existing = set(self.fields)
for field_name in existing - allowed:
self.fields.pop(field_name)
Also, you can do this in your view like the below code snippet:
my_dish = Dish.objects.get(pk=dish.id)
serializer = Dish_Serializer(my_dish)
desired_fields = {'id', 'name', 'description'}
all_fields = set(serializer.fields)
for field in all_fields:
if field not in desired_fields:
serializer.fields.pop(field)
my_new_object = serializer.data

DRF SerializerMethodField how to pass parameters

Is there a way to pass paremeters to a Django Rest Framework's SerializerMethodField?
Assume I have the models:
class Owner(models.Model):
name = models.CharField(max_length=10)
class Item(models.Model):
name = models.CharField(max_length=10)
owner = models.ForeignKey('Owner', related_name='items')
itemType = models.CharField(max_length=5) # either "type1" or "type2"
What I need is to return an Owner JSON object with the fields: name, type1items, type2items.
My current solution is this:
class ItemSerializer(serializers.ModelSerializer):
class Meta:
model = models.Item
fields = ('name', 'itemType')
class OwnerSerializer(serializers.ModelSerializer):
type1items = serializers.SerializerMethodField(method_name='getType1Items')
type2items = serializers.SerializerMethodField(method_name='getType2Items')
class Meta:
model = models.Owner
fields = ('name', 'type1items', 'type2items')
def getType1Items(self, ownerObj):
queryset = models.Item.objects.filter(owner__id=ownerObj.id).filter(itemType="type1")
return ItemSerializer(queryset, many=True).data
def getType2Items(self, ownerObj):
queryset = models.Item.objects.filter(owner__id=ownerObj.id).filter(itemType="type2")
return ItemSerializer(queryset, many=True).data
This works. But it would be much cleaner if I could pass a parameter to the method instead of using two methods with almost the exact code. Ideally it would look like this:
...
class OwnerSerializer(serializers.ModelSerializer):
type1items = serializers.SerializerMethodField(method_name='getItems', "type1")
type2items = serializers.SerializerMethodField(method_name='getItems', "type2")
class Meta:
model = models.Owner
fields = ('name', 'type1items', 'type2items')
def getItems(self, ownerObj, itemType):
queryset = models.Item.objects.filter(owner__id=ownerObj.id).filter(itemType=itemType)
return ItemSerializer(queryset, many=True).data
In the docs SerializerMethodField accepts only one parameter which is method_name.
Is there any way to achieve this behaviour using SerializerMethodField? (The example code here is overly simplified so there might be mistakes.)
There is no way to do this with the base field.
You need to write a custom serializer field to support it. Here is an example one, which you'll probably want to modify depending on how you use it.
This version uses the kwargs from the field to pass as args to the function. I'd recommend doing this rather than using *args since you'll get more sensible errors, and flexibility in how you write your function/field definitions.
class MethodField(SerializerMethodField):
def __init__(self, method_name=None, **kwargs):
# use kwargs for our function instead, not the base class
super().__init__(method_name)
self.func_kwargs = kwargs
def to_representation(self, value):
method = getattr(self.parent, self.method_name)
return method(value, **self.func_kwargs)
Using the field in a serializer:
class Simple(Serializer):
field = MethodField("get_val", name="sam")
def get_val(self, obj, name=""):
return "my name is " + name
>>> print(Simple(instance=object()).data)
{'field': 'my name is sam'}
You could just refactor what you have:
class OwnerSerializer(serializers.ModelSerializer):
type1items = serializers.SerializerMethodField(method_name='getType1Items')
type2items = serializers.SerializerMethodField(method_name='getType2Items')
class Meta:
model = models.Owner
fields = ('name', 'type1items', 'type2items')
def getType1Items(self, ownerObj):
return getItems(ownerObj,"type1")
def getType2Items(self, ownerObj):
return getItems(ownerObj,"type2")
def getItems(self, ownerObj, itemType):
queryset = models.Item.objects.filter(owner__id=ownerObj.id).filter(itemType=itemType)
return ItemSerializer(queryset, many=True).data

Django rest framework nested serializer create method

I have created a nested serializer, when I try to post data in it it keeps on displaying either the foreign key value cannot be null or dictionary expected. I have gone through various similar questions and tried the responses but it is not working for me. Here are the models
##CLasses
class Classes(models.Model):
class_name = models.CharField(max_length=255)
class_code = models.CharField(max_length=255)
created_date = models.DateTimeField(auto_now_add=True)
def __str__(self):
return self.class_name
class Meta:
ordering = ['class_code']
##Streams
class Stream(models.Model):
stream_name = models.CharField(max_length=255)
classes = models.ForeignKey(Classes,related_name="classes",on_delete=models.CASCADE)
created_date = models.DateTimeField(auto_now_add=True)
def __str__(self):
return self.stream_name
class Meta:
ordering = ['stream_name']
Here is the view
class StreamViewset(viewsets.ModelViewSet):
queryset = Stream.objects.all()
serializer_class = StreamSerializer
Here is the serializer class
class StreamSerializer(serializers.ModelSerializer):
# classesDetails = serializers.SerializerMethodField()
classes = ClassSerializer()
class Meta:
model = Stream
fields = '__all__'
def create(self,validated_data):
classes = Classes.objects.get(id=validated_data["classes"])
return Stream.objects.create(**validated_data, classes=classes)
# def perfom_create(self,serializer):
# serializer.save(classes=self.request.classes)
#depth = 1
# def get_classesDetails(self, obj):
# clas = Classes.objects.get(id=obj.classes)
# classesDetails = ClassSerializer(clas).data
# return classesDetails
I have tried several ways of enabling the create method but like this displays an error {"classes":{"non_field_errors":["Invalid data. Expected a dictionary, but got int."]}}. Any contribution would be deeply appreciated
This is a very common situation when developing APIs with DRF.
The problem
Before DRF reaches the create() method, it validates the input, which I assume has a form similar to
{
"classes": 3,
"stream_name": "example"
}
This means that, since it was specified that
classes = ClassSerializer()
DRF is trying to build the classes dictionary from the integer. Of course, this will fail, and you can see that from the error dictionary
{"classes":{"non_field_errors":["Invalid data. Expected a dictionary, but got int."]}}
Solution 1 (requires a new writable field {field_name}_id)
A possible solution is to set read_only=True in your ClassSerializer, and use an alternative name for the field when writing, it's common to use {field_name}_id. That way, the validation won't be done. See this answer for more details.
class StreamSerializer(serializers.ModelSerializer):
classes = ClassSerializer(read_only=True)
class Meta:
model = Stream
fields = (
'pk',
'stream_name',
'classes',
'created_date',
'classes_id',
)
extra_kwargs = {
'classes_id': {'source': 'classes', 'write_only': True},
}
This is a clean solution but requires changing the user API. In case that's not an option, proceed to the next solution.
Solution 2 (requires overriding to_internal_value)
Here we override the to_internal_value method. This is where the nested ClassSerializer is throwing the error. To avoid this, we set that field to read_only and manage the validation and parsing in the method.
Note that since we're not declaring a classes field in the writable representation, the default action of super().to_internal_value is to ignore the value from the dictionary.
from rest_framework.exceptions import ValidationError
class StreamSerializer(serializers.ModelSerializer):
classes = ClassSerializer(read_only=True)
def to_internal_value(self, data):
classes_pk = data.get('classes')
internal_data = super().to_internal_value(data)
try:
classes = Classes.objects.get(pk=classes_pk)
except Classes.DoesNotExist:
raise ValidationError(
{'classes': ['Invalid classes primary key']},
code='invalid',
)
internal_data['classes'] = classes
return internal_data
class Meta:
model = Stream
fields = (
'pk',
'stream_name',
'classes',
'created_date',
)
With this solution you can use the same field name for both reading and writing, but the code is a bit messy.
Additional notes
You're using the related_name argument incorrectly, see this question. It's the other way around,
classes = models.ForeignKey(
Classes,
related_name='streams',
on_delete=models.CASCADE,
)
In this case it should be streams.
Kevin Languasco describes the behaviour of the create method quite well and his solutions are valid ones. I would add a variation to solution 1:
class StreamSerializer(serializers.ModelSerializer):
classes = ClassSerializer(read_only=True)
classes_id = serializers.IntegerField(write_only=True)
def create(self,validated_data):
return Stream.objects.create(**validated_data, classes=classes)
class Meta:
model = Stream
fields = (
'pk',
'stream_name',
'classes',
'classes_id',
'created_date',
)
The serializer will work without overriding the create method, but you can still do so if you want to as in your example.
Pass the value classes_id in the body of your POST method, not classes. When deserializing the data, the validation will skip classes and will check classes_id instead.
When serializing the data (when you perform a GET request, for example), classes will be used with your nested dictionary and classes_id will be omitted.
You can also solve this issue in such a way,
Serializer class
# Classes serializer
class ClassesSerializer(ModelSerializer):
class Meta:
model = Classes
fields = '__all__'
# Stream serializer
class StreamSerializer(ModelSerializer):
classes = ClassesSerializer(read_only=True)
class Meta:
model = Stream
fields = '__all__'
View
# Create Stream view
#api_view(['POST'])
def create_stream(request):
classes_id = request.data['classes'] # or however you are sending the id
serializer = StreamSerializer(data=request.data)
if serializer.is_valid():
classes_instance = get_object_or_404(Classes, id=classes_id)
serializer.save(classes=classes_instance)
else:
return Response(serializer.errors)
return Response(serializer.data)

How to make a field editable=False in DRF

I've a serializer. I want to restrict updating a field. How would I do that?
class ABCSerializer(serializers.ModelSerializer):
class Meta:
"""Meta."""
model = ModelA
fields = ('colA', 'colB', 'colC',)
colA is a required field while creating the object. However, it should not be allowed to update. How can I do that??
Sounds like you need different serializers for PUT and POST methods. In the serializer for the PUT method you can set the colA field to readonly
class ABCViewSet(ModelViewSet):
serializer_class = ABCSerializer
def get_serializer_class(self):
serializer_class = self.serializer_class
if self.request.method == 'PUT':
serializer_class = SerializerWithReadOnlyColA
return serializer_class
You can use Django REST Frameworks field-level validation by validating that field has not changed on update like so:
from rest_framework.exceptions import ValidationError
class ABCSerializer(serializers.ModelSerializer):
colA = serializers.CharField(max_length=100)
def validate_colA(self, value):
if self.instance and self.instance.colA != value:
raise ValidationError("You may not edit colA")
return value
class Meta:
"""Meta."""
model = ModelA
fields = ('colA', 'colB', 'colC',)
This will check whether or not this is an update (via checking if an instance is populated on the serializer) and if so it will then check to see if you have made a change to the field and if you have it will throw a ValidationError. The benefit of this approach is that you can keep your view code the same as before and continue to keep your validation behaviour in your serializer.
You can override the serializer's update method to only update fields that you want.
class ABCSerializer(serializers.ModelSerializer):
def update(self, instance, validated_data):
instance.colB = validated_data.get('colB', instance.colB)
instance.colC = validated_data.get('colC', instance.colC)
# do nothing to instance.colA
instance.save()
return instance
class Meta:
model = ModelA
fields = ('colA', 'colB', 'colC',)
Or if you have many fields, and just want to omit updating colA, you could write your update method like this:
def update(self, instance, validated_data):
validated_data.pop('colA') # validated_data no longer has colA
return super().update(instance, validated_data)
You can read more about overriding update here: https://www.django-rest-framework.org/api-guide/serializers/#saving-instances
I think it's too late to answer but this may be useful for others:)
you can solve your problem this way:
class ABCSerializer(serializers.ModelSerializer):
class Meta:
model = ModelA
fields = ('colA', 'colB', 'colC',)
def get_fields(self):
fields = super().get_fields()
if self.instance:
fields["colA"].read_only = True
return fields
When you want to create, the self.instance is None, it will pass the if clause, and in case of updating the if clause will make the field read only and non-editable.
You can do this with the read_only_fieldsoption
class ABCSerializer(serializers.ModelSerializer):
class Meta:
"""Meta."""
model = ModelA
fields = ('colB', 'colC',)
read_only_fields = ('colA',)

django rest framework and model inheritance

I have an "abstract" model class MyField:
class MyField(models.Model):
name = models.CharField(db_index = True, max_length=100)
user = models.ForeignKey("AppUser", null=False)
I have a few other subclasses of MyField each defining a value of a specific type.
for example:
class MyBooleanField(MyField):
value = models.BooleanField(db_index = True, default=False)
In MyField I have a method get_value() that returns the value based on the specific subclass.
In django rest I want to fetch all the fields of a user
class AppUserSerializer(serializers.ModelSerializer):
appuserfield_set = MyFieldSerializer(many=True)
class Meta:
model = AppUser
fields = ('appuser_id', 'appuserfield_set')
On the client side I want the user to be able to add new fields and set values to them and then on the server I want to be able to create the correct field based on the value.
What is the correct way to achieve this behavior?
After some digging, here is what I ended up doing. Aside from the code below I had to implement get_or_create and create the relevant subclass of MyField based on the passed value.
class ValueField(serializers.WritableField):
#called when serializing a field to a string. (for example when calling seralizer.data)
def to_native(self, obj):
return obj;
"""
Called when deserializing a field from a string
(for example when calling is_valid which calles restore_object)
"""
def from_native(self, data):
return data
class MyFieldSerializer(serializers.ModelSerializer):
value = ValueField(source='get_value', required=False)
def restore_object(self, attrs, instance=None):
"""
Called by is_valid (before calling save)
Create or update a new instance, given a dictionary
of deserialized field values.
Note that if we don't define this method, then deserializing
data will simply return a dictionary of items.
"""
if instance:
# Update existing instance
instance.user = attrs.get('user', instance.user)
instance.name = attrs.get('name', instance.name)
else:
# Create new instance
instance = MyField.get_or_create(end_user=attrs['user'],
name=attrs['name'],
value=attrs['get_value'])[0]
instance.value = attrs['get_value']
return instance
def save_object(self, obj, **kwargs):
#called when saving the instance to the DB
instance = MyField.get_or_create(end_user=obj.user,
name=obj.name,
value=obj.value)[0]
class Meta:
model = MyField
fields = ('id', 'user', 'name', 'value')

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