✨ Add support for forbidding extra form fields with Pydantic models (#12134)

Co-authored-by: Sofie Van Landeghem <svlandeg@users.noreply.github.com>

docs/en/docs/tutorial/request-form-models.md

@@ -1,6 +1,6 @@
 # Form Models
 
-You can use Pydantic models to declare form fields in FastAPI.
+You can use **Pydantic models** to declare **form fields** in FastAPI.
 
 /// info
 
@@ -22,7 +22,7 @@ This is supported since FastAPI version `0.113.0`. 🤓
 
 ## Pydantic Models for Forms
 
-You just need to declare a Pydantic model with the fields you want to receive as form fields, and then declare the parameter as `Form`:
+You just need to declare a **Pydantic model** with the fields you want to receive as **form fields**, and then declare the parameter as `Form`:
 
 //// tab | Python 3.9+
 
@@ -54,7 +54,7 @@ Prefer to use the `Annotated` version if possible.
 
 ////
 
-FastAPI will extract the data for each field from the form data in the request and give you the Pydantic model you defined.
+**FastAPI** will **extract** the data for **each field** from the **form data** in the request and give you the Pydantic model you defined.
 
 ## Check the Docs
 
@@ -63,3 +63,72 @@ You can verify it in the docs UI at `/docs`:
 <div class="screenshot">
 <img src="/img/tutorial/request-form-models/image01.png">
 </div>
+
+## Restrict Extra Form Fields
+
+In some special use cases (probably not very common), you might want to **restrict** the form fields to only those declared in the Pydantic model. And **forbid** any **extra** fields.
+
+/// note
+
+This is supported since FastAPI version `0.114.0`. 🤓
+
+///
+
+You can use Pydantic's model configuration to `forbid` any `extra` fields:
+
+//// tab | Python 3.9+
+
+```Python hl_lines="12"
+{!> ../../../docs_src/request_form_models/tutorial002_an_py39.py!}
+```
+
+////
+
+//// tab | Python 3.8+
+
+```Python hl_lines="11"
+{!> ../../../docs_src/request_form_models/tutorial002_an.py!}
+```
+
+////
+
+//// tab | Python 3.8+ non-Annotated
+
+/// tip
+
+Prefer to use the `Annotated` version if possible.
+
+///
+
+```Python hl_lines="10"
+{!> ../../../docs_src/request_form_models/tutorial002.py!}
+```
+
+////
+
+If a client tries to send some extra data, they will receive an **error** response.
+
+For example, if the client tries to send the form fields:
+
+* `username`: `Rick`
+* `password`: `Portal Gun`
+* `extra`: `Mr. Poopybutthole`
+
+They will receive an error response telling them that the field `extra` is not allowed:
+
+```json
+{
+    "detail": [
+        {
+            "type": "extra_forbidden",
+            "loc": ["body", "extra"],
+            "msg": "Extra inputs are not permitted",
+            "input": "Mr. Poopybutthole"
+        }
+    ]
+}
+```
+
+## Summary
+
+You can use Pydantic models to declare form fields in FastAPI. 😎