ManyToMany relation with extra field

Hello,

I would like to know if it’s possible to have a ManyToMany relationship with an extra filed.
I have a drug model that may contains multiple molecules models.
I would like to display this relation with an additional field: quantity.
I need to display the quantity at the same level as the molecule's information.

This is my endpoint :

@router.get('/', response_model=List[DrugResponse])
async def drugs():
    """Returns a list of drugs. """
    
    return await DrugResponse.from_queryset(Drug.all())

This is my models :

class Drug(Model):
    id = fields.IntField(pk=True)
    label = fields.CharField(max_length=128, null=False)

    class Meta:
        table = "drugs"


class DrugMolecule(Model):
    drug = fields.ForeignKeyField("models.Drug", related_name='molecules', null=False)
    molecule = fields.ForeignKeyField("models.Molecule", related_name=False, null=False)
    quantity = fields.FloatField(null=True)

    class Meta:
        table = 'drug_molecule'


class Molecule(Model):
    id = fields.IntField(pk=True)
    name = fields.CharField(max_length=128, null=False)

    class Meta:
        table = 'molecules'

    class PydanticMeta:
        exclude = ["administration_context"]

I would like that my endpoint return an object structured like this :

[
  {
    "id": 1,
    "label": "ADVIL 200mg, comprimé enrobé",
    "molecules": [
      {
        "id": 1, // molecule_id
        "name": "Ibuprofen",
        "quantity": 100 // extra field quantity
      },
      {
        "id": 2, // molecule_id
        "name": "Hydroxychloroquine",
        "quantity": 100 // extra field quantity
      }
    ]
  }
]

But for now, my endpoint return an object like this :

[
  {
    "id": 1,
    "label": "ADVIL 200mg, comprimé enrobé",
    "molecules": [
      {
        "id": 1,
        "molecule": {
          "id": 1,
          "name": "Ibuprofen"
        },
        "quantity": 100 // extra field quantity
      },
      {
        "id": 2,
        "molecule": {
          "id": 2,
          "name": "Hydroxychloroquine"
        },
        "quantity": 100 // extra field quantity
      }
    ]
  }
] 

Could you please explain me how I can get the well structured return please ?

Start with the Drug, DrugMolecule, and Molecule models and the drugs endpoint shown in the issue; inspect how DrugResponse serializes the reverse relation and nested molecule. Compare that behavior with the requested flattened molecules JSON, including quantity. Done means the endpoint returns the requested shape without losing the extra field.