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BackAnt uses marshmallow for request body validation. Schemas live in api/schemas/ and are used in the service layer to validate incoming data before processing.

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Schema structure

Each schema is a marshmallow Schema subclass. The naming convention is Post<Name>Schema for POST request schemas: 
# api/schemas/PostUsers_schema.py
from marshmallow import fields, Schema

class PostUsersSchema(Schema):
    name = fields.String(required=True)
    email = fields.Email(required=True)
    age = fields.Integer(load_default=None)

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Using a schema in a service

from marshmallow import ValidationError
from schemas.PostUsers_schema import PostUsersSchema
from helper.functions.validation_error import validation_error

class UsersService:
    def create_user(self, data):
        try:
            validated = PostUsersSchema().load(data or {})
        except ValidationError as e:
            raise validation_error(e)

        return self.users_repository.add_user(**validated)
validation_error() is a helper in api/helper/functions/validation_error.py that converts a marshmallow ValidationError into an APIException(400) automatically.

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Common field types

FieldDescription
fields.String()String value
fields.Integer()Integer value
fields.Float()Float value
fields.Boolean()Boolean value
fields.Email()Validates email format
fields.URL()Validates URL format
fields.List(fields.String())List of strings
fields.Dict()Arbitrary dict
fields.DateTime()ISO 8601 datetime string

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Field options

OptionDescription
required=TrueField must be present in input
load_default=valueDefault value when field is missing
allow_none=TrueAllow null values
validate=Attach a validator function

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Validation example

from marshmallow import fields, Schema, validate

class PostProductSchema(Schema):
    name = fields.String(required=True, validate=validate.Length(min=1, max=100))
    price = fields.Float(required=True, validate=validate.Range(min=0))
    stock = fields.Integer(load_default=0)

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Nested schemas

class AddressSchema(Schema):
    street = fields.String(required=True)
    city = fields.String(required=True)

class PostUserSchema(Schema):
    name = fields.String(required=True)
    address = fields.Nested(AddressSchema, required=True)

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Error format

When validation fails, validation_error() raises APIException(400). The response body will be: 
{"message": "Ocurrió un error"}
For more detailed validation error messages, inspect error.messages in the ValidationError and pass them to APIException: 
except ValidationError as e:
    raise APIException(status_code=400, message=str(e.messages))