Django Serializers, how to show data in nested bridge model serializer

Question

I'm using Django REST Framework.

I have 3 different models, 2 are standalone models while the 3rd one is a bridge model that connects the first 2 models

# First standalone model
class Build(TimeStampedModel):
    name = models.CharField(max_length=64, null=False, blank=False)
    type = models.CharField(max_length=64, null=False, blank=False)
    play_as = models.CharField(max_length=64, null=False, blank=False)
    link = models.CharField(max_length=255, null=False, blank=False)

    class Meta:
        ordering = ['-created_at']

    def __str__(self):
        return f'{self.play_as} - {self.name}'


# Second standalone model
class Run(TimeStampedModel):
    name = models.CharField(max_length=64, null=False, blank=False)
    difficulty = models.CharField(max_length=64, null=False, blank=False)
    origin = models.CharField(max_length=64, null=False, blank=False)
    alignment = models.CharField(max_length=64, null=False, blank=False)

    class Meta:
        ordering = ['-created_at']

    def __str__(self):
        return f'{self.name} - {self.difficulty}'
    

# Bridge model
class RunBuild(TimeStampedModel):
    run = models.ForeignKey(
        Run, related_name='runBuild_run', on_delete=models.CASCADE, null=False, blank=False
    )
    build = models.ForeignKey(
        Build, related_name='runBuild_builds', on_delete=models.CASCADE, null=False, blank=False
    )

    class Meta:
        ordering = ['-run']

    def __str__(self):
        return f'{self.run.name} - {self.build.name}'

The Build serializer is fine, the problem stands in RunBuild and Run serializer.

# Model Serializer for Build model   
class BuildSerializer(serializers.ModelSerializer):
    created_at = serializers.SerializerMethodField()

    class Meta:
        model = Build
        exclude = ['updated_at']

    def get_created_at(self, instance):
        return instance.created_at.strftime('%B %d, %Y')


# Model Serializer for RunBuild model    
class RunBuildSerializer(serializers.ModelSerializer):
    # This doesn't work
    runBuild_builds = BuildSerializer(many=True, read_only=True)
    created_at = serializers.SerializerMethodField()

    class Meta:
        model = RunBuild
        fields = ['run', 'build', 'created_at', 'runBuild_builds']

    def get_created_at(self, instance):
        return instance.created_at.strftime('%B %d, %Y')
    

# Model Serializer for Run model
class RunSerializer(serializers.ModelSerializer):
    # This works
    runBuild_run = RunBuildSerializer(many=True, read_only=True)
    created_at = serializers.SerializerMethodField()

    class Meta:
        model = Run
        fields = [
            'name',
            'difficulty',
            'origin',
            'alignment',
            'created_at',
            'runBuild_run'
        ]

    def get_created_at(self, instance):
        # We will format the date
        return instance.created_at.strftime('%B %d, %Y')

A Run is composed by few fields and many Builds under it, that's why I needed a bridge model. I'm able to show many RunBuild in the Run serializer, like this

{
    "name": "Versatile Bard",
    "difficulty": "Tactician",
    "origin": "The Dark Urge",
    "alignment": "Chaotic Neutral",
    "created_at": "March 14, 2024",
    "runBuild_run": [
        {
            "run": 1,
            "build": 1,
            "created_at": "March 14, 2024"
        },
        {
            "run": 1,
            "build": 2,
            "created_at": "March 14, 2024"
        },
        ....
    ]
}

But if I do the same thing for RunBuild serializer, it won't show a thing (and I want it to show each Build on it)

{
    "run": 1,
    "build": 1,
    "created_at": "March 14, 2024"
    "runBuild_builds": [
        {
            "name": "Cleric / Bard - CC / Versatile Spellcaster",
            "type": "Main Character",
            "play_as": "Main Character",
            "link": "......."
        },
        ....
    ]
}

The runBuild_builds field is not present in the answer, I added by hand now to show you how it should be.

The ultimate result should be, in Run serializer, having a result like this

{
    "name": "Versatile Bard",
    "difficulty": "Tactician",
    "origin": "The Dark Urge",
    "alignment": "Chaotic Neutral",
    "created_at": "March 14, 2024",
    "runBuild_run": [
        {
            "run": 1,
            "build": 1,
            "created_at": "March 14, 2024",
            "runBuild_builds": [
                {
                    "name": "Cleric / Bard - CC / Versatile Spellcaster",
                    "type": "Main Character",
                    "play_as": "Main Character",
                    "link": "......."
                },
                ....
            ]
        },
        ....
    ]
}

Any idea on how to modify my code?

Answer 1

Update your RunBuildSerializer like this,

class RunBuildSerializer(serializers.ModelSerializer):
# This doesn't work
build = BuildSerializer(many=True, read_only=True)
created_at = serializers.SerializerMethodField()

class Meta:
    model = RunBuild
    fields = ['run', 'build', 'created_at']

def get_created_at(self, instance):
    return instance.created_at.strftime('%B %d, %Y')

Remove the runBuild_builds fields from this serializer. Because this is related_name field it needs to use when we need to access RunBuild class from Build class.