Question: I have a multilang FastAPI connected to MongoDB. My document in MongoDB is duplicated in the two languages available and structured this way (simplified example):  { "_id": xxxxxxx, "en": { "title": "Drinking Water Composition", "description": "Drinking water composition expressed in... with pesticides.", "category": "Water", "tags": ["water","pesticides"] }, "fr": { "title": "Composition de l'eau de boisson", "description": "Composition de l'eau de boisson exprimée en... présence de pesticides....", "category": "Eau", "tags": ["eau","pesticides"] }, }  I therefore implemented two models DatasetFR and DatasetEN, each one makes references with specific external Models (Enum) for category and tags in each lang. class DatasetFR(BaseModel): title:str description: str category: CategoryFR tags: Optional[List[TagsFR]] # same for DatasetEN chnaging the lang tag to EN  In the routes definition I forced the language parameter to declare the corresponding Model and get the corresponding validation.  @router.post("?lang=fr", response_description="Add a dataset") async def create_dataset(request:Request, dataset: DatasetFR = Body(...), lang:str="fr"): ... return JSONResponse(status_code=status.HTTP_201_CREATED, content=created_dataset) @router.post("?lang=en", response_description="Add a dataset") async def create_dataset(request:Request, dataset: DatasetEN = Body(...), lang:str="en"): ... return JSONResponse(status_code=status.HTTP_201_CREATED, content=created_dataset)  But this seems to be in contradiction with the DRY principle. So, I wonder here if someone knows an elegant solution to: - given the lang parameter, dynamically call the corresponding model. Or, if we can create a Parent Model Dataset that takes the lang argument and retrieve the child model Dataset<LANG>. This would incredibly ease building my API routes and the call of my models and mathematically divide by two the writing.

Answer: Option 1 A solution would be the following: Define lang as Query paramter and add a regular expression that the parameter should match. In your case, that would be ^(fr|en)$, meaning that only fr or en would be valid inputs. Thus, if no match was found, the request would stop there and the client would receive a "string does not match regex..." error. Next, define the body parameter as a generic type of dict and declare it as Body field; thus, instructing FastAPI to expect a JSON body. Following, create a dictionary of your models that you can use to look up for a model using the lang attribute. Once you find the corresponding model, try to parse the JSON body using models[lang].parse_obj(body) (equivalent to using models[lang](**body)). If no ValidationError is raised, you know the resulting model instance is valid. Otherwise, return an HTTP_422_UNPROCESSABLE_ENTITY error, including the errors, which you can handle as desired. If you would also like FR and EN being valid lang values, adjust the regex to ignore case using ^(?i)(fr|en)$ instead, and make sure to convert lang to lower case when looking up for a model (i.e., models[lang.lower()].parse_obj(body)). import pydantic from fastapi import FastAPI, Response, status, Body, Query from fastapi.responses import JSONResponse from fastapi.encoders import jsonable_encoder models = {"fr": DatasetFR, "en": DatasetEN} @router.post("/", response_description="Add a dataset") async def create_dataset(body: dict = Body(...), lang: str = Query(..., regex="^(fr|en)$")): try: model = models[lang].parse_obj(body) except pydantic.ValidationError as e: return Response(content=e.json(), status_code=status.HTTP_422_UNPROCESSABLE_ENTITY, media_type="application/json") return JSONResponse(content=jsonable_encoder(dict(model)), status_code=status.HTTP_201_CREATED)  Update Since the two models have identical attributes (i.e., title and description), you could define a parent model (e.g., Dataset) with those two attributes, and have DatasetFR and DatasetEN models inherit those. class Dataset(BaseModel): title:str description: str class DatasetFR(Dataset): category: CategoryFR tags: Optional[List[TagsFR]] class DatasetEN(Dataset): category: CategoryEN tags: Optional[List[TagsEN]]  Additionally, it might be a better approach to move the logic from inside the route to a dependency function and have it return the model, if it passes the validation; otherwise, raise an HTTPException, as also demonstrated by @tiangolo. You can use jsonable_encoder, which is internally used by FastAPI, to encode the validation errors() (the same function can also be used when returning the JSONResponse). from fastapi.exceptions import HTTPException from fastapi import Depends models = {"fr": DatasetFR, "en": DatasetEN} async def checker(body: dict = Body(...), lang: str = Query(..., regex="^(fr|en)$")): try: model = models[lang].parse_obj(body) except pydantic.ValidationError as e: raise HTTPException(detail=jsonable_encoder(e.errors()), status_code=status.HTTP_422_UNPROCESSABLE_ENTITY) return model @router.post("/", response_description="Add a dataset") async def create_dataset(model: Dataset = Depends(checker)): return JSONResponse(content=jsonable_encoder(dict(model)), status_code=status.HTTP_201_CREATED)  Option 2 A further approach would be to have a single Pydantic model (let's say Dataset) and customize the validators for category and tags fields. You can also define lang as part of Dataset, thus, no need to have it as query parameter. You can use a set, as described here, to keep the values of each Enum class, so that you can efficiently check if a value exists in the Enum; and have dictionaries to quickly look up for a set using the lang attribute. In the case of tags, to verify that every element in the list is valid, use set.issubset, as described here. If an attribute is not valid, you can raise ValueError, as shown in the documentation, "which will be caught and used to populate ValidationError" (see "Note" section here). Again, if you need the lang codes written in uppercase being valid inputs, adjust the regex pattern, as described earlier. P.S. You don't even need to use Enum with this approach. Instead, populate each set below with the permitted values. For instance, categories_FR = {"Eau"} categories_EN = {"Water"} tags_FR = {"eau", "pesticides"} tags_EN = {"water", "pesticides"}. Additionally, if you would like not to use regex, but rather have a custom validation error for lang attribute as well, you could add it in the same validator decorator and perform validation similar (and previous) to the other two fields. from pydantic import validator categories_FR = set(item.value for item in CategoryFR) categories_EN = set(item.value for item in CategoryEN) tags_FR = set(item.value for item in TagsFR) tags_EN = set(item.value for item in TagsEN) cats = {"fr": categories_FR, "en": categories_EN} tags = {"fr": tags_FR, "en": tags_EN} def raise_error(values): raise ValueError(f'value is not a valid enumeration member; permitted: {values}') class Dataset(BaseModel): lang: str = Body(..., regex="^(fr|en)$") title: str description: str category: str tags: List[str] @validator("category", "tags") def validate_atts(cls, v, values, field): lang = values.get('lang') if lang: if field.name == "category": if v not in cats[lang]: raise_error(cats[lang]) elif field.name == "tags": if not set(v).issubset(tags[lang]): raise_error(tags[lang]) return v @router.post("/", response_description="Add a dataset") async def create_dataset(model: Dataset): return JSONResponse(content=jsonable_encoder(dict(model)), status_code=status.HTTP_201_CREATED)  Update Note that in Pydantic V2, @validator has been deprecated and was replaced by @field_validator. Please have a look at this answer for more details and examples. Option 3 Another approach would be to use Discriminated Unions, as described in this answer. As per the documentation:  When Union is used with multiple submodels, you sometimes know exactly which submodel needs to be checked and validated and want to enforce this. To do that you can set the same field - let's call it my_discriminator - in each of the submodels with a discriminated value, which is one (or many) Literal value(s). For your Union, you can set the discriminator in its value: Field(discriminator='my_discriminator'). Setting a discriminated union has many benefits:  validation is faster since it is only attempted against one model only one explicit error is raised in case of failure the generated JSON schema implements the associated OpenAPI specification