encode · aminalaee · Sep 30, 2021 · Sep 27, 2021 · Sep 27, 2021 · Sep 29, 2021
diff --git a/docs/relationships.md b/docs/relationships.md
@@ -128,3 +128,44 @@ A database driver exception will be raised.
 
 This will set referencing objects `ForeignKey` column to `NULL`.
 The `ForeignKey` defined here should also have `allow_null=True`.
+
+
+## OneToOne
+
+Creating a  `OneToOne` relationship between models, this is basically
+the same as `ForeignKey` but it uses `unique=True` on the ForeignKey column:
+
+```python
+class Profile(orm.Model):
+    registry = models
+    fields = {
+        "id": orm.Integer(primary_key=True),
+        "website": orm.String(max_length=100),
+    }
+
+
+class Person(orm.Model):
+    registry = models
+    fields = {
+        "id": orm.Integer(primary_key=True),
+        "email": orm.String(max_length=100),
+        "profile": orm.OneToOne(Profile),
+    }
+```
+
+You can create a `Profile` and `Person` instance:
+
+```python
+profile = await Profile.objects.create(website="https://encode.io")
+await Person.objects.create(email="info@encode.io", profile=profile)
+```
+
+Now creating another `Person` using the same `profile` will fail
+and will raise an exception:
+
+```python
+await Person.objects.create(email="info@encode.io", profile=profile)
+```
+
+`OneToOne` accepts the same `on_delete` parameters as `ForeignKey` which is
+described [here](#foreignkey-constraints).
diff --git a/orm/__init__.py b/orm/__init__.py
@@ -12,6 +12,7 @@
     Float,
     ForeignKey,
     Integer,
+    OneToOne,
     String,
     Text,
     Time,
@@ -32,13 +33,14 @@
     "Decimal",
     "Enum",
     "Float",
+    "ForeignKey",
     "Integer",
     "JSON",
+    "OneToOne",
     "String",
     "Text",
     "Time",
     "UUID",
-    "ForeignKey",
     "Model",
     "ModelRegistry",
 ]
diff --git a/orm/fields.py b/orm/fields.py
@@ -163,8 +163,7 @@ def get_column(self, name: str) -> sqlalchemy.Column:
         column_type = to_field.get_column_type()
         constraints = [
             sqlalchemy.schema.ForeignKey(
-                f"{target.tablename}.{target.pkname}",
-                ondelete=self.on_delete,
+                f"{target.tablename}.{target.pkname}", ondelete=self.on_delete
             )
         ]
         return sqlalchemy.Column(
@@ -181,6 +180,27 @@ def expand_relationship(self, value):
         return target(pk=value)
 
 
+class OneToOne(ForeignKey):
+    def get_column(self, name: str) -> sqlalchemy.Column:
+        target = self.target
+        to_field = target.fields[target.pkname]
+
+        column_type = to_field.get_column_type()
+        constraints = [
+            sqlalchemy.schema.ForeignKey(
+                f"{target.tablename}.{target.pkname}", ondelete=self.on_delete
+            ),
+        ]
+
+        return sqlalchemy.Column(
+            name,
+            column_type,
+            *constraints,
+            nullable=self.allow_null,
+            unique=True,
+        )
+
+
 class Enum(ModelField):
     def __init__(self, enum, **kwargs):
         super().__init__(**kwargs)

diff --git a/tests/test_foreignkey.py b/tests/test_foreignkey.py
@@ -1,3 +1,5 @@
+import sqlite3
+
 import asyncpg
 import databases
 import pymysql
@@ -56,6 +58,23 @@ class Member(orm.Model):
     }
 
 
+class Profile(orm.Model):
+    registry = models
+    fields = {
+        "id": orm.Integer(primary_key=True),
+        "website": orm.String(max_length=100),
+    }
+
+
+class Person(orm.Model):
+    registry = models
+    fields = {
+        "id": orm.Integer(primary_key=True),
+        "email": orm.String(max_length=100),
+        "profile": orm.OneToOne(Profile),
+    }
+
+
 @pytest.fixture(autouse=True, scope="module")
 def create_test_database():
     models.create_all()
@@ -229,3 +248,24 @@ async def test_on_delete_set_null():
 
     member = await Member.objects.first()
     assert member.team.pk is None
+
+
+async def test_one_to_one_crud():
+    profile = await Profile.objects.create(website="https://encode.io")
+    await Person.objects.create(email="info@encode.io", profile=profile)
+
+    person = await Person.objects.get(email="info@encode.io")
+    assert person.profile.pk == profile.pk
+    assert not hasattr(person.profile, "website")
+
+    await person.profile.load()
+    assert person.profile.website == "https://encode.io"
+
+    exceptions = (
+        asyncpg.exceptions.UniqueViolationError,
+        pymysql.err.IntegrityError,
+        sqlite3.IntegrityError,
+    )
+
+    with pytest.raises(exceptions):
+        await Person.objects.create(email="contact@encode.io", profile=profile)