fastapi pydantic nested models

[grillazz]

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My view models below:

class EnvContainersStatusResponse(BaseModel):
    description: str = Field(...)

    class Config:
        orm_mode = True
        allow_population_by_field_name = True


class EnvContainersResponse(BaseModel):
    container_name: str = Field(alias='_name')
    container_status: EnvContainersStatusResponse

    class Config:
        orm_mode = True
        allow_population_by_field_name = True


class EnvResponse(EnvBase):
    env_id: str = Field(alias='environment_name')
    container_collection: List[EnvContainersResponse] = Field(alias='compute_node_containers')

    class Config:
        orm_mode = True
        allow_population_by_field_name = True

my response is like below

{
  "environment_name": "somenewenv",
  "compute_node_containers": [
    {
      "_name": "compute001",
      "container_status": {
        "description": "online"
      }
    }
  ]
}

question: how i can flatten my response model to have below:

want eliminate another dict and have only key value like "container_status": "online"

{
  "environment_name": "somenewenv",
  "compute_node_containers": [
    {
      "_name": "compute001",
      "container_status": "online"
      
    }
  ]
}

[ycd]

As far as i know, no. Are there any reasons to have a class for container_status instead of just using container_status: str from your EnvContainersResponse model?

[grillazz]

@ycd to take container description i need to follow relation from container to container status as this is how models are designed in legacy codebase

class Container(Base):
    __tablename__ = "container"
...
container_status = relationship("ContainerStatus", lazy='joined')

[paloderosa]

I think this is rather related to pydantic. A discussion has already taken place about flattening (at, pydantic/pydantic#717, for example). Two possible solutions I can think of (and would have to implement in order to understand the details, sorry for the lack of time):

1. The description attribute in your ORM model is available as container_status.description. You could give that as an alias for the container_status field in your EnvContainersResponse model, but then you would have to subclass the GetterDict class to be able to get nested object attributes and register that new class in the EnvContainersResponse model's Config.
2. Add to your EnvContainersResponse a validator on container_status field with pre=True that explicitly extracts the subattribute from the ORM object or add a non-public field _container_status that gets the nested object and compute the container_status field from that.

[grillazz]

gracias @paloderosa i also fund few suggestions about overwrite presentation for pydatinc. I will try both of solutions you suggested and back with some code examples.

[paloderosa]

Welcome! (and no issues with having my username changed hahahaha).

[paloderosa]

@grillazz I actually came up today to the need to implement this functionality in a many-to-many relationship. Here is my implementation of the first proposal. I will assume that you are using SQLAlchemy for ORM, that you are traversing the entities relationships through relationship model attributes and that you have initialized the declarative Base class somewhere.

import functools
from typing import Any, List

from pydantic import BaseModel, Field
from pydantic.utils import GetterDict
from sqlalchemy import Column, String
from sqlalchemy.orm import relationship


# implementation of a recursive getattr
# taken from https://stackoverflow.com/a/31174427
def rgetattr(obj, attr, *args):
    def _getattr(obj, attr):
        return getattr(obj, attr, *args)
    return functools.reduce(_getattr, [obj] + attr.split('.'))


class NestedGetterDict(GetterDict):
    def __getitem__(self, key: str) -> Any:
        try:
            return rgetattr(self._obj, key)
        except AttributeError as e:
            raise KeyError(key) from e

    def get(self, key: Any, default: Any = None) -> Any:
        return rgetattr(self._obj, key, default)


class Container(Base):
    __tablename__ = 'container'
    ...
    container_status = relationship("ContainerStatus", lazy='joined')

class ContainerStatus(Base):
    __tablename__ = 'container_status'
    ...
    description = Column(String, nullable=False)


class EnvContainersResponse(BaseModel):
    container_name: str = Field(alias='_name')
    container_status: str = Field(alias='container_status.description')

    class Config:
        orm_mode = True
        allow_population_by_field_name = True
        getter_dict = NestedGetterDict


class EnvResponse(EnvBase):
    env_id: str = Field(alias='environment_name')
    container_collection: List[EnvContainersResponse] = Field(alias='compute_node_containers')

    class Config:
        orm_mode = True
        allow_population_by_field_name = True

This should result in what you are looking for.

[grillazz]

sorry for messing your name @paloderosa

[aaronstephenson]

@grillazz I have a similar need to flatten some nested models, did you get this working?

@paloderosa Thanks for the example you posted! But when I try to implement it, the flattened field name has dot notation (container_status.description). How would I get the field name to be container_status while still having the field value be the nested description value?

[shrutikaponde]

@aaronstephenson, I solved this by adding response_model_by_alias.

@router.get("/",
            response_model=List[schemas.Command],
            response_model_by_alias=False)

[scd75]

hello, building on solution proposed by @paloderosa :

I created this method: get_custom_mapper, that take a dict of key, value such as
{'your_pydantic_custom_name': 'your_model_attribute.nested_attribute'}

def get_custom_mapper(mapping: dict):

    def rgetattr(obj, attr, *args):
        def _getattr(obj, attr):
            return getattr(obj, attr, *args)
        return functools.reduce(_getattr, [obj] + attr.split('.'))

    class CustomGetterDict(GetterDict):
    
        def __getitem__(self, key: str) -> Any:
            if key in mapping:
                try:
                    return rgetattr(self._obj, mapping[key])
                except AttributeError as e:
                    raise KeyError(key) from e
            try:
                return getattr(self._obj, key)
            except AttributeError as e:
                raise KeyError(key) from e

        def get(self, key: Any, default: Any = None) -> Any:
            if key in mapping:
                return rgetattr(self._obj, mapping[key], default)
            return getattr(self._obj, key, default)

    return CustomGetterDict

then you just have to call this in the config class of your pydantic model:

    class Config:
        getter_dict = get_custom_mapper(mapping={
            'your_pydantic_custom_name': 'your_model_attribute.nested_attribute' 
        })

[ben519]

I came across this issue as well and found this discussion and @paloderosa's solution very helpful. I ended up documenting the problem and a few other potential solutions here.