fix: processing complex params in multipart-form (#210)

Signed-off-by: Michele Dolfi <dol@zurich.ibm.com>

docling_serve/helper_functions.py

@@ -1,9 +1,30 @@
 import inspect
+import json
 import re
-from typing import Union
+from typing import Union, get_args, get_origin
 
 from fastapi import Depends, Form
-from pydantic import BaseModel
+from pydantic import BaseModel, TypeAdapter
+
+
+def is_pydantic_model(type_):
+    try:
+        if inspect.isclass(type_) and issubclass(type_, BaseModel):
+            return True
+
+        origin = get_origin(type_)
+        if origin is Union:
+            args = get_args(type_)
+            return any(
+                inspect.isclass(arg) and issubclass(arg, BaseModel)
+                for arg in args
+                if arg is not type(None)
+            )
+
+    except Exception:
+        pass
+
+    return False
 
 
 # Adapted from
@@ -12,25 +33,62 @@ def FormDepends(cls: type[BaseModel]):
     new_parameters = []
 
     for field_name, model_field in cls.model_fields.items():
+        annotation = model_field.annotation
+        description = model_field.description
+        default = (
+            Form(..., description=description)
+            if model_field.is_required()
+            else Form(
+                model_field.default,
+                examples=model_field.examples,
+                description=description,
+            )
+        )
+
+        # Flatten nested Pydantic models by accepting them as JSON strings
+        if is_pydantic_model(annotation):
+            annotation = str
+            default = Form(
+                None
+                if model_field.default is None
+                else json.dumps(model_field.default.model_dump(mode="json")),
+                description=description,
+                examples=None
+                if not model_field.examples
+                else [
+                    json.dumps(ex.model_dump(mode="json"))
+                    for ex in model_field.examples
+                ],
+            )
+
         new_parameters.append(
             inspect.Parameter(
                 name=field_name,
                 kind=inspect.Parameter.POSITIONAL_ONLY,
-                default=(
-                    Form(...)
-                    if model_field.is_required()
-                    else Form(model_field.default)
-                ),
-                annotation=model_field.annotation,
+                default=default,
+                annotation=annotation,
             )
         )
 
     async def as_form_func(**data):
+        for field_name, model_field in cls.model_fields.items():
+            value = data.get(field_name)
+            annotation = model_field.annotation
+
+            # Parse nested models from JSON string
+            if value is not None and is_pydantic_model(annotation):
+                try:
+                    validator = TypeAdapter(annotation)
+                    data[field_name] = validator.validate_json(value)
+                except Exception as e:
+                    raise ValueError(f"Invalid JSON for field '{field_name}': {e}")
+
         return cls(**data)
 
     sig = inspect.signature(as_form_func)
     sig = sig.replace(parameters=new_parameters)
     as_form_func.__signature__ = sig  # type: ignore
+
     return Depends(as_form_func)