fix: validate nested models with default_factory

PR pydantic#1504 introduced a regression by bypassing `populate_validators()`,
which would skip the validation of children in nested models
with `default_factory`

closes pydantic#1710

changes/1710-PrettyWood.md

@@ -0,0 +1 @@
+fix validation of nested models with `default_factory`

pydantic/fields.py

@@ -341,8 +341,22 @@ def prepare(self) -> None:
         e.g. calling it it multiple times may modify the field and configure it incorrectly.
         """
 
-        # To prevent side effects by calling the `default_factory` for nothing, we only call it
-        # when we want to validate the default value i.e. when `validate_all` is set to True.
+        self._set_default_and_type()
+        self._type_analysis()
+        if self.required is Undefined:
+            self.required = True
+            self.field_info.default = Required
+        if self.default is Undefined and self.default_factory is None:
+            self.default = None
+        self.populate_validators()
+
+    def _set_default_and_type(self) -> None:
+        """
+        Set the default value, infer the type if needed and check if `None` value is valid.
+
+        Note: to prevent side effects by calling the `default_factory` for nothing, we only call it
+        when we want to validate the default value i.e. when `validate_all` is set to True.
+        """
         if self.default_factory is not None:
             if self.type_ is None:
                 raise errors_.ConfigError(
@@ -368,14 +382,6 @@ def prepare(self) -> None:
         if self.required is False and default_value is None:
             self.allow_none = True
 
-        self._type_analysis()
-        if self.required is Undefined:
-            self.required = True
-            self.field_info.default = Required
-        if self.default is Undefined and self.default_factory is None:
-            self.default = None
-        self.populate_validators()
-
     def _type_analysis(self) -> None:  # noqa: C901 (ignore complexity)
         # typing interface is horrible, we have to do some ugly checks
         if lenient_issubclass(self.type_, JsonWrapper):

tests/test_main.py

@@ -1174,6 +1174,18 @@ class MyModel(BaseModel):
     assert m2.id == 2
 
 
+def test_default_factory_validate_children():
+    class Child(BaseModel):
+        x: int
+
+    class Parent(BaseModel):
+        children: List[Child] = Field(default_factory=list)
+
+    Parent(children=[{'x': 1}, {'x': 2}])
+    with pytest.raises(ValidationError):
+        Parent(children=[{'x': 1}, {'y': 2}])
+
+
 @pytest.mark.skipif(sys.version_info < (3, 7), reason='field constraints are set but not enforced with python 3.6')
 def test_none_min_max_items():
     # None default