fix: wrap RootModel primitive defaults with default_factory

src/datamodel_code_generator/model/_types.py

@@ -0,0 +1,18 @@
+"""Internal types for model module."""
+
+from __future__ import annotations
+
+from dataclasses import dataclass
+from typing import Any
+
+
+@dataclass(repr=False)
+class WrappedDefault:
+    """Represents a default value wrapped with its type constructor."""
+
+    value: Any
+    type_name: str
+
+    def __repr__(self) -> str:
+        """Return type constructor representation, e.g., 'CountType(10)'."""
+        return f"{self.type_name}({self.value!r})"

src/datamodel_code_generator/model/base.py

@@ -10,7 +10,6 @@
 from abc import ABC, abstractmethod
 from collections import defaultdict
 from copy import deepcopy
-from dataclasses import dataclass
 from functools import cached_property, lru_cache
 from pathlib import Path
 from typing import TYPE_CHECKING, Any, ClassVar, Optional, TypeVar, Union
@@ -26,6 +25,7 @@
     IMPORT_UNION,
     Import,
 )
+from datamodel_code_generator.model._types import WrappedDefault
 from datamodel_code_generator.reference import Reference, _BaseModel
 from datamodel_code_generator.types import (
     ANY,
@@ -39,6 +39,8 @@
 )
 from datamodel_code_generator.util import PYDANTIC_V2, ConfigDict
 
+__all__ = ["WrappedDefault"]
+
 if TYPE_CHECKING:
     from collections.abc import Iterator
 
@@ -113,18 +115,6 @@ def merge_constraints(a: ConstraintsBaseT | None, b: ConstraintsBaseT | None) ->
         })
 
 
-@dataclass(repr=False)
-class WrappedDefault:
-    """Represents a default value wrapped with its type constructor."""
-
-    value: Any
-    type_name: str
-
-    def __repr__(self) -> str:
-        """Return type constructor representation, e.g., 'CountType(10)'."""
-        return f"{self.type_name}({self.value!r})"
-
-
 class DataModelFieldBase(_BaseModel):
     """Base class for model field representation and rendering."""
 

src/datamodel_code_generator/model/pydantic/base_model.py

@@ -17,6 +17,7 @@
     DataModel,
     DataModelFieldBase,
 )
+from datamodel_code_generator.model._types import WrappedDefault
 from datamodel_code_generator.model.base import UNDEFINED
 from datamodel_code_generator.model.pydantic.imports import (
     IMPORT_ANYURL,
@@ -122,6 +123,8 @@ def _get_strict_field_constraint_value(self, constraint: str, value: Any) -> Any
         return int(value)
 
     def _get_default_as_pydantic_model(self) -> str | None:
+        if isinstance(self.default, WrappedDefault):
+            return f"lambda :{self.default!r}"
         for data_type in self.data_type.data_types or (self.data_type,):
             # TODO: Check nested data_types
             if data_type.is_dict:
@@ -141,15 +144,18 @@ def _get_default_as_pydantic_model(self) -> str | None:
                         f"{self._PARSE_METHOD}(v) for v in {self.default!r}]"
                     )
             elif data_type.reference and isinstance(data_type.reference.source, BaseModelBase):
+                source = data_type.reference.source
+                is_root_model = hasattr(source, "BASE_CLASS") and source.BASE_CLASS == "pydantic.RootModel"
                 if self.data_type.is_union:
                     if not isinstance(self.default, (dict, list)):
+                        if not is_root_model:
+                            continue
+                    elif isinstance(self.default, dict) and any(dt.is_dict for dt in self.data_type.data_types):
                         continue
-                    if isinstance(self.default, dict) and any(dt.is_dict for dt in self.data_type.data_types):
-                        continue
-                return (
-                    f"lambda :{data_type.alias or data_type.reference.source.class_name}."
-                    f"{self._PARSE_METHOD}({self.default!r})"
-                )
+                class_name = data_type.alias or source.class_name
+                if is_root_model:
+                    return f"lambda :{class_name}({self.default!r})"
+                return f"lambda :{class_name}.{self._PARSE_METHOD}({self.default!r})"
         return None
 
     def _process_data_in_str(self, data: dict[str, Any]) -> None:

tests/data/expected/main/jsonschema/root_model_default_value.py

@@ -25,13 +25,7 @@ class NameType(RootModel[str]):
 
 class Model(BaseModel):
     admin_state: AdminStateLeaf | None = AdminStateLeaf.enable
-    count: Annotated[
-        CountType | None,
-        Field(default_factory=lambda: CountType.model_validate(CountType(10))),
-    ]
+    count: Annotated[CountType | None, Field(default_factory=lambda: CountType(10))]
     name: Annotated[
-        NameType | None,
-        Field(
-            default_factory=lambda: NameType.model_validate(NameType('default_name'))
-        ),
+        NameType | None, Field(default_factory=lambda: NameType('default_name'))
     ]

tests/data/expected/main/jsonschema/root_model_default_value_branches.py

@@ -15,8 +15,7 @@ class CountType(RootModel[int]):
 
 class Model(BaseModel):
     count_with_default: Annotated[
-        CountType | None,
-        Field(default_factory=lambda: CountType.model_validate(CountType(10))),
+        CountType | None, Field(default_factory=lambda: CountType(10))
     ]
     count_no_default: CountType | None = None
     count_list_default: Annotated[

tests/data/expected/main/jsonschema/root_model_default_value_no_annotated.py

@@ -24,9 +24,5 @@ class NameType(RootModel[constr(min_length=1, max_length=50)]):
 
 class Model(BaseModel):
     admin_state: AdminStateLeaf | None = AdminStateLeaf.enable
-    count: CountType | None = Field(
-        default_factory=lambda: CountType.model_validate(10)
-    )
-    name: NameType | None = Field(
-        default_factory=lambda: NameType.model_validate('default_name')
-    )
+    count: CountType | None = Field(default_factory=lambda: CountType(10))
+    name: NameType | None = Field(default_factory=lambda: NameType('default_name'))

tests/data/expected/main/jsonschema/root_model_default_value_non_root.py

@@ -19,8 +19,7 @@ class PersonType(BaseModel):
 
 class Model(BaseModel):
     root_model_field: Annotated[
-        CountType | None,
-        Field(default_factory=lambda: CountType.model_validate(CountType(10))),
+        CountType | None, Field(default_factory=lambda: CountType(10))
     ]
     non_root_model_field: Annotated[
         PersonType | None,

tests/data/expected/main/openapi/pydantic_v2_default_object/Nested.py

@@ -23,6 +23,4 @@ class Bar(BaseModel):
             for v in [{'text': 'abc', 'number': 123}, {'text': 'efg', 'number': 456}]
         ]
     )
-    nested_foo: Foo | None = Field(
-        default_factory=lambda: Foo.model_validate('default foo')
-    )
+    nested_foo: Foo | None = Field(default_factory=lambda: Foo('default foo'))

tests/data/expected/main/openapi/referenced_default.py

@@ -13,6 +13,4 @@ class ModelSettingB(RootModel[confloat(ge=0.0, le=10.0)]):
 
 class Model(BaseModel):
     settingA: confloat(ge=0.0, le=10.0) | None = 5
-    settingB: ModelSettingB | None = Field(
-        default_factory=lambda: ModelSettingB.model_validate(5)
-    )
+    settingB: ModelSettingB | None = Field(default_factory=lambda: ModelSettingB(5))

tests/data/expected/main/openapi/referenced_default_use_annotated.py

@@ -16,6 +16,5 @@ class ModelSettingB(RootModel[float]):
 class Model(BaseModel):
     settingA: Annotated[float | None, Field(ge=0.0, le=10.0)] = 5
     settingB: Annotated[
-        ModelSettingB | None,
-        Field(default_factory=lambda: ModelSettingB.model_validate(ModelSettingB(5))),
+        ModelSettingB | None, Field(default_factory=lambda: ModelSettingB(5))
     ]

tests/data/expected/main/openapi/root_model_default_primitive.py

@@ -0,0 +1,15 @@
+# generated by datamodel-codegen:
+#   filename:  root_model_default_primitive.yaml
+#   timestamp: 2019-07-26T00:00:00+00:00
+
+from __future__ import annotations
+
+from pydantic import BaseModel, Field, RootModel, conint
+
+
+class Timeout(RootModel[conint(le=14400, gt=0)]):
+    root: conint(le=14400, gt=0)
+
+
+class CrawlConfiguration(BaseModel):
+    timeout: Timeout | None = Field(default_factory=lambda: Timeout(3600))

tests/data/openapi/root_model_default_primitive.yaml

@@ -0,0 +1,18 @@
+openapi: 3.1.0
+info:
+  title: Test
+  version: 0.1.0
+paths: {}
+components:
+  schemas:
+    Timeout:
+      type: integer
+      maximum: 14400
+      exclusiveMinimum: 0
+    CrawlConfiguration:
+      properties:
+        timeout:
+          anyOf:
+            - $ref: "#/components/schemas/Timeout"
+            - type: "null"
+          default: 3600

tests/main/openapi/test_main_openapi.py

@@ -3242,6 +3242,18 @@ def test_main_openapi_referenced_default_use_annotated(output_file: Path) -> Non
     )
 
 
+def test_main_openapi_root_model_default_primitive(output_file: Path) -> None:
+    """Test RootModel with primitive default value in union type."""
+    run_main_and_assert(
+        input_path=OPEN_API_DATA_PATH / "root_model_default_primitive.yaml",
+        output_path=output_file,
+        input_file_type="openapi",
+        assert_func=assert_file_content,
+        expected_file="root_model_default_primitive.py",
+        extra_args=["--output-model-type", "pydantic_v2.BaseModel"],
+    )
+
+
 @pytest.mark.cli_doc(
     options=["--parent-scoped-naming"],
     input_schema="openapi/duplicate_models2.yaml",