[Fixed]-How to add data to ManyToMany field using Django Rest Framework?

First of all, the data format is not in the post request is wrong:

it supposed to be following:

{"allergies": [{"name": "Beef"},{"name": "Pork"}]}

Now let’s modify your def create() inside your serializer

 def create(self, validated_data):

    allergies_data =validated_data.pop('allergies', [])
    names = [allergy.get('name') for allergy in allergies_data if allergy]
    al = IngFamily.objects.filter(name__in=names) # using names(list) for filtering by __in

    user1 = UserProfile.objects.create(**validated_data)
    user1.allergies.add(*al) # notice there have * before "al" (as we are putting list inside add()

    # user1.save() # we don't need to call `save()` as we are updating ManyToMany field.
    return user1

You should but .save() inside loop and edit your query to be like:

def create(self, validated_data):

    allergies_data =validated_data.pop('allergies', [])
    user1=UserProfile.objects.create(**validated_data)
    for allergy in allergies_data:

      al=IngFamily.objects.get(name=allergy['name'])
      user1.allergies.add(al)
    #here you should add it
      user1.save()

Couple of things can be tried :

1. It might be possible your database dont have some of the allergies coming in with user data. Thats why your al is coming empty.
2. saving user data before you add allergies to it. This is important.
3. By using ipdb or pdb, try to understand in what data form ‘allergy’ and ‘allergies_data’ is coming out. If indeed it is what you have written, there is no reason the following code shouldnt work.

def create(self, validated_data):
    allergies_data =validated_data.pop('allergies')
    user1=UserProfile.objects.create(**validated_data)
    user1.save()
    for allergy in allergies_data:
        al=IngFamily.objects.get_or_create(name=allergy['name'])
        user1.allergies.add(al)
    return user1

Use something like:

class ProjectSerializer(serializers.ModelSerializer):

    tag= serializers.StringRelatedField(many=True)

    class Meta:
       model = Song
       fields = '__all__'

Here is a reference doc – https://www.django-rest-framework.org/api-guide/relations/