fix: forward ref with nested models and optional fields

PR pydantic#1712 introduced a regression for forward refs in `ModelField.prepare`
as it would not return early for forward refs anymore.
Optional fields would hence have `required` set to `True`.

closes pydantic#1736

changes/1736-PrettyWood.md

@@ -0,0 +1 @@
+Fix behaviour with forward refs and optional fields in nested models

pydantic/fields.py

@@ -342,6 +342,11 @@ def prepare(self) -> None:
         """
 
         self._set_default_and_type()
+        if self.type_.__class__ == ForwardRef:
+            # self.type_ is currently a ForwardRef and there's nothing we can do now,
+            # user will need to call model.update_forward_refs()
+            return
+
         self._type_analysis()
         if self.required is Undefined:
             self.required = True
@@ -374,11 +379,6 @@ def _set_default_and_type(self) -> None:
         if self.type_ is None:
             raise errors_.ConfigError(f'unable to infer type for attribute "{self.name}"')
 
-        if self.type_.__class__ == ForwardRef:
-            # self.type_ is currently a ForwardRef and there's nothing we can do now,
-            # user will need to call model.update_forward_refs()
-            return
-
         if self.required is False and default_value is None:
             self.allow_none = True
 

tests/test_forward_ref.py

@@ -439,3 +439,35 @@ class Foo(BaseModel):
     raise AssertionError('error not raised')
     """
     )
+
+
+def test_forward_ref_optional(create_module):
+    module = create_module(
+        """
+from __future__ import annotations
+from pydantic import BaseModel, Field
+from typing import List, Optional
+
+
+class Spec(BaseModel):
+    spec_fields: List[str] = Field(..., alias="fields")
+    filter: Optional[str]
+    sort: Optional[str]
+
+
+class PSpec(Spec):
+    g: Optional[GSpec]
+
+
+class GSpec(Spec):
+    p: Optional[PSpec]
+
+PSpec.update_forward_refs()
+
+class Filter(BaseModel):
+    g: Optional[GSpec]
+    p: Optional[PSpec]
+    """
+    )
+    Filter = module.Filter
+    assert isinstance(Filter(p={'sort': 'some_field:asc', 'fields': []}), Filter)