Allow Discriminator for discriminator in Field

sqlmodel/__init__.py

@@ -1,5 +1,9 @@
 __version__ = "0.0.31"
 
+# Re-export from Pydantic
+from pydantic import Discriminator as Discriminator
+from pydantic import Tag as Tag
+
 # Re-export from SQLAlchemy
 from sqlalchemy.engine import create_engine as create_engine
 from sqlalchemy.engine import create_mock_engine as create_mock_engine

sqlmodel/main.py

@@ -22,7 +22,7 @@ from typing import (
     overload,
 )
 
-from pydantic import BaseModel, EmailStr
+from pydantic import BaseModel, Discriminator, EmailStr
 from pydantic.fields import FieldInfo as PydanticFieldInfo
 from sqlalchemy import (
     Boolean,
@@ -228,7 +228,7 @@ def Field(
     max_length: Optional[int] = None,
     allow_mutation: bool = True,
     regex: Optional[str] = None,
-    discriminator: Optional[str] = None,
+    discriminator: Union[str, Discriminator, None] = None,
     repr: bool = True,
     primary_key: Union[bool, UndefinedType] = Undefined,
     foreign_key: Any = Undefined,
@@ -271,7 +271,7 @@ def Field(
     max_length: Optional[int] = None,
     allow_mutation: bool = True,
     regex: Optional[str] = None,
-    discriminator: Optional[str] = None,
+    discriminator: Union[str, Discriminator, None] = None,
     repr: bool = True,
     primary_key: Union[bool, UndefinedType] = Undefined,
     foreign_key: str,
@@ -323,7 +323,7 @@ def Field(
     max_length: Optional[int] = None,
     allow_mutation: bool = True,
     regex: Optional[str] = None,
-    discriminator: Optional[str] = None,
+    discriminator: Union[str, Discriminator, None] = None,
     repr: bool = True,
     sa_column: Union[Column[Any], UndefinedType] = Undefined,
     schema_extra: Optional[dict[str, Any]] = None,
@@ -356,7 +356,7 @@ def Field(
     max_length: Optional[int] = None,
     allow_mutation: bool = True,
     regex: Optional[str] = None,
-    discriminator: Optional[str] = None,
+    discriminator: Union[str, Discriminator, None] = None,
     repr: bool = True,
     primary_key: Union[bool, UndefinedType] = Undefined,
     foreign_key: Any = Undefined,

tests/test_pydantic/test_field.py

@@ -1,9 +1,9 @@
 from decimal import Decimal
-from typing import Literal, Optional, Union
+from typing import Annotated, Any, Literal, Optional, Union
 
 import pytest
 from pydantic import ValidationError
-from sqlmodel import Field, SQLModel
+from sqlmodel import Discriminator, Field, SQLModel, Tag
 
 
 def test_decimal():
@@ -47,6 +47,39 @@ def test_discriminator():
         Model(pet={"pet_type": "dog"}, n=1)  # type: ignore[arg-type]
 
 
+def test_discriminator_callable():
+    # Example adapted from
+    # [Pydantic docs](https://docs.pydantic.dev/latest/concepts/unions/#discriminated-unions-with-callable-discriminator):
+
+    class Pie(SQLModel):
+        pass
+
+    class ApplePie(Pie):
+        fruit: Literal["apple"] = "apple"
+
+    class PumpkinPie(Pie):
+        filling: Literal["pumpkin"] = "pumpkin"
+
+    def get_discriminator_value(v: Any) -> str:
+        if isinstance(v, dict):
+            return v.get("fruit", v.get("filling"))
+        return getattr(v, "fruit", getattr(v, "filling", None))
+
+    class ThanksgivingDinner(SQLModel):
+        dessert: Union[
+            Annotated[ApplePie, Tag("apple")],
+            Annotated[PumpkinPie, Tag("pumpkin")],
+        ] = Field(
+            discriminator=Discriminator(get_discriminator_value),
+        )
+
+    apple_pie = ThanksgivingDinner.model_validate({"dessert": {"fruit": "apple"}})
+    assert isinstance(apple_pie.dessert, ApplePie)
+
+    pumpkin_pie = ThanksgivingDinner.model_validate({"dessert": {"filling": "pumpkin"}})
+    assert isinstance(pumpkin_pie.dessert, PumpkinPie)
+
+
 def test_repr():
     class Model(SQLModel):
         id: Optional[int] = Field(primary_key=True)