Fix nested model field with alias parsing

pydantic_settings/sources.py

@@ -824,13 +824,14 @@ class Cfg(BaseSettings):
             fields = _get_model_fields(annotation)
             # `case_sensitive is None` is here to be compatible with the old behavior.
             # Has to be removed in V3.
-            if (case_sensitive is None or case_sensitive) and fields.get(key):
-                return fields[key]
-            elif not case_sensitive:
-                for field_name, f in fields.items():
-                    if field_name.lower() == key.lower():
+            for field_name, f in fields.items():
+                if case_sensitive is None or case_sensitive:
+                    if (field_name == key) or (isinstance(f.validation_alias, str) and f.validation_alias == key):
                         return f
-
+                elif (field_name.lower() == key.lower()) or (
+                    isinstance(f.validation_alias, str) and f.validation_alias.lower() == key.lower()
+                ):
+                    return f
         return None
 
     def explode_env_vars(self, field_name: str, field: FieldInfo, env_vars: Mapping[str, str | None]) -> dict[str, Any]:

tests/test_settings.py

@@ -5004,3 +5004,45 @@ class Settings(BaseSettings):
         foo: list[str] = []
 
     Settings()
+
+
+def test_nested_model_field_with_alias(env):
+    class NestedSettings(BaseModel):
+        foo: List[str] = Field(alias='fooalias')
+
+    class Settings(BaseSettings):
+        model_config = SettingsConfigDict(env_nested_delimiter='__')
+
+        nested: NestedSettings
+
+    env.set('nested__fooalias', '["one", "two"]')
+
+    s = Settings()
+    assert s.model_dump() == {'nested': {'foo': ['one', 'two']}}
+
+
+def test_nested_model_field_with_alias_case_sensitive(monkeypatch):
+    class NestedSettings(BaseModel):
+        foo: List[str] = Field(alias='fooAlias')
+
+    class Settings(BaseSettings):
+        model_config = SettingsConfigDict(env_nested_delimiter='__', case_sensitive=True)
+
+        nested: NestedSettings
+
+    # Need to patch os.environ to get build to work on Windows, where os.environ is case insensitive
+    monkeypatch.setattr(os, 'environ', value={'nested__fooalias': '["one", "two"]'})
+    with pytest.raises(ValidationError) as exc_info:
+        Settings()
+    assert exc_info.value.errors(include_url=False) == [
+        {
+            'type': 'missing',
+            'loc': ('nested', 'fooAlias'),
+            'msg': 'Field required',
+            'input': {'fooalias': '["one", "two"]'},
+        }
+    ]
+
+    monkeypatch.setattr(os, 'environ', value={'nested__fooAlias': '["one", "two"]'})
+    s = Settings()
+    assert s.model_dump() == {'nested': {'foo': ['one', 'two']}}