How to access field from ManyToMany relation on top of Foreign key relation in a django serializer?

Question

I have the following structure:

class Model_1():
    model_2_ref = models.ForeignKey(to="Model_2", null=True, on_delete=models.SET_NULL)
    model_1_field_1 = models.CharField(max_length=120)
    ...

class Model_2():
    model_2_field_1 = models.CharField(max_length=120)
    model_2_field_2 = models.CharField(max_length=120)
    ....

class Model_3():
    model_2_ref = models.ManyToManyField(to="Model_2", related_name="model_2_rel_name")
    model_3_field_1 = models.CharField(max_length=120)
    user_id = models.Integer()
    ....

Each record of Model_1 is related to only one record of Model_2. Also each record of the Model_2 is related to only one record of Model_3 for the current user. The serializer will be used by a viewset that filters the matches from model_3 by user_id, transforming this into a One-to-one relation of sorts.

I have a serializer for Model_1, where I would like to include it's related fields from Model_2 and Model_3.

In theory, this is what I would like to achieve:

class Model_1_Serializer():
    model_1_field_1 = serializers.CharField()
    model_2_field_1 = serializers.CharField(source='model_2_ref.model_2_field_1')
    model_2_field_2 = serializers.CharField(source='model_2_ref.model_2_field_1')
    model_3_field_1 = serializers.CharField(source='model_2_ref.model_3_ref.model_3_field_1')

For model_3_field_1 this syntax does not work, but for the fields of model 2 it does. How can I access the related fields of Model_3 from the serializer of Model_1?

Answer 1

Maybe something like (untested)?

models.py

class Model2(...):
    
    # this method returns the first model_3 instance or None:
    @property
    def model_3_single_ref(self): return self.model_3_ref.first() 


class Model3(...):
    
    model_3_field_1 = models.CharField(...)
    model_2_ref = models.ForeignKey(..., related_name = 'model_3_ref')

serializers.py

class Model_1_Serializer(...):

    # list of model_3_field_1s
    model_3_field_1s = serializers.SlugRelatedField(
        many = True,
        read_only = True,
        slug_field = 'model_3_field_1',
        source = 'model_2_ref.model_3_ref'
    )
    
    # model_3_field_1 as just a string:
    model_3_field_1 = serializers.SlugRelatedField(
        read_only = True,
        slug_field = 'model_3_field_1',
        source = 'model_2_ref.model_3_single_ref'            
    )