add a 'Unique' method and basic test

Signed-off-by: Grant Ramsay <seapagan@gmail.com>

sqliter/model/__init__.py

@@ -1,9 +1,11 @@
 """This module provides the base model class for SQLiter database models.
 
 It exports the BaseDBModel class, which is used to define database
-models in SQLiter applications.
+models in SQLiter applications, and the Unique class, which is used to
+define unique constraints on model fields.
 """
 
 from .model import BaseDBModel
+from .unique import Unique
 
-__all__ = ["BaseDBModel"]
+__all__ = ["BaseDBModel", "Unique"]

sqliter/model/unique.py

@@ -0,0 +1,19 @@
+"""Define a custom field type for unique constraints in SQLiter."""
+
+from typing import Any
+
+from pydantic.fields import FieldInfo
+
+
+class Unique(FieldInfo):
+    """A custom field type for unique constraints in SQLiter."""
+
+    def __init__(self, default: Any = ..., **kwargs: Any) -> None:  # noqa: ANN401
+        """Initialize a Unique field.
+
+        Args:
+            default: The default value for the field.
+            **kwargs: Additional keyword arguments to pass to FieldInfo.
+        """
+        super().__init__(default=default, **kwargs)
+        self.unique = True

sqliter/sqliter.py

@@ -27,6 +27,7 @@
     TableDeletionError,
 )
 from sqliter.helpers import infer_sqlite_type
+from sqliter.model.unique import Unique
 from sqliter.query.query import QueryBuilder
 
 if TYPE_CHECKING:  # pragma: no cover
@@ -239,7 +240,12 @@ def create_table(
         for field_name, field_info in model_class.model_fields.items():
             if field_name != primary_key:
                 sqlite_type = infer_sqlite_type(field_info.annotation)
-                fields.append(f"{field_name} {sqlite_type}")
+                unique_constraint = (
+                    "UNIQUE" if isinstance(field_info, Unique) else ""
+                )
+                fields.append(
+                    f"{field_name} {sqlite_type} {unique_constraint}".strip()
+                )
 
         create_str = (
             "CREATE TABLE IF NOT EXISTS" if exists_ok else "CREATE TABLE"

tests/test_unique.py

@@ -0,0 +1,55 @@
+"""Test the Unique constraint."""
+
+from typing import Annotated
+
+import pytest
+
+from sqliter import SqliterDB
+from sqliter.exceptions import SqliterError
+from sqliter.model import BaseDBModel
+from sqliter.model.unique import Unique
+
+
+class TestUnique:
+    """Test suite for the Unique constraint."""
+
+    def test_unique_constraint(self) -> None:
+        """Test that the Unique constraint is properly applied."""
+
+        class User(BaseDBModel):
+            name: str
+            email: Annotated[str, Unique()]
+
+        db = SqliterDB(":memory:")
+        db.create_table(User)
+
+        # Insert a user successfully
+        user1 = User(name="Alice", email="alice@example.com")
+        db.insert(user1)
+
+        # Attempt to insert a user with the same email
+        user2 = User(name="Bob", email="alice@example.com")
+
+        with pytest.raises(SqliterError) as excinfo:
+            db.insert(user2)
+
+        assert "UNIQUE constraint failed: users.email" in str(excinfo.value)
+
+        # Verify that only one user was inserted
+        users = db.select(User).fetch_all()
+        assert len(users) == 1
+        assert users[0].name == "Alice"
+        assert users[0].email == "alice@example.com"
+
+        # Insert a user with a different email successfully
+        user3 = User(name="Charlie", email="charlie@example.com")
+        db.insert(user3)
+
+        # Verify that two users are now in the database
+        users = db.select(User).fetch_all()
+        assert len(users) == 2
+        assert {u.name for u in users} == {"Alice", "Charlie"}
+        assert {u.email for u in users} == {
+            "alice@example.com",
+            "charlie@example.com",
+        }