feat: Support GQL interfaces for polymorphic SQLA models (#365)

* Support GQL interfaces for polymorphic SQLA models using SQLALchemyInterface and SQLAlchemyBase. fixes #313 


Co-authored-by: Erik Wrede <erikwrede@users.noreply.github.com>
Co-authored-by: Erik Wrede <erikwrede2@gmail.com>

Author: polgfred

docs/inheritance.rst

@@ -0,0 +1,107 @@
+Inheritance Examples
+====================
+
+Create interfaces from inheritance relationships
+------------------------------------------------
+
+SQLAlchemy has excellent support for class inheritance hierarchies.
+These hierarchies can be represented in your GraphQL schema by means
+of interfaces_.  Much like ObjectTypes, Interfaces in
+Graphene-SQLAlchemy are able to infer their fields and relationships
+from the attributes of their underlying SQLAlchemy model:
+
+.. _interfaces: https://docs.graphene-python.org/en/latest/types/interfaces/
+
+.. code:: python
+
+    from sqlalchemy import Column, Date, Integer, String
+    from sqlalchemy.ext.declarative import declarative_base
+
+    import graphene
+    from graphene import relay
+    from graphene_sqlalchemy import SQLAlchemyInterface, SQLAlchemyObjectType
+
+    Base = declarative_base()
+
+    class Person(Base):
+        id = Column(Integer(), primary_key=True)
+        type = Column(String())
+        name = Column(String())
+        birth_date = Column(Date())
+
+        __tablename__ = "person"
+        __mapper_args__ = {
+            "polymorphic_on": type,
+        }
+
+    class Employee(Person):
+        hire_date = Column(Date())
+
+        __mapper_args__ = {
+            "polymorphic_identity": "employee",
+        }
+        
+    class Customer(Person):
+        first_purchase_date = Column(Date())
+
+        __mapper_args__ = {
+            "polymorphic_identity": "customer",
+        }
+
+    class PersonType(SQLAlchemyInterface):
+        class Meta:
+            model = Person
+
+    class EmployeeType(SQLAlchemyObjectType):
+        class Meta:
+            model = Employee
+            interfaces = (relay.Node, PersonType)
+            
+    class CustomerType(SQLAlchemyObjectType):
+        class Meta:
+            model = Customer
+            interfaces = (relay.Node, PersonType)
+
+Keep in mind that `PersonType` is a `SQLAlchemyInterface`. Interfaces must 
+be linked to an abstract Model that does not specify a `polymorphic_identity`, 
+because we cannot return instances of interfaces from a GraphQL query. 
+If Person specified a `polymorphic_identity`, instances of Person could 
+be inserted into and returned by the database, potentially causing 
+Persons to be returned to the resolvers.
+
+When querying on the base type, you can refer directly to common fields,
+and fields on concrete implementations using the `... on` syntax:
+
+
+.. code::
+
+    people {
+        name
+        birthDate
+        ... on EmployeeType {
+            hireDate
+        }
+        ... on CustomerType {
+            firstPurchaseDate
+        }
+    }
+    
+    
+Please note that by default, the "polymorphic_on" column is *not*
+generated as a field on types that use polymorphic inheritance, as
+this is considered an implentation detail. The idiomatic way to
+retrieve the concrete GraphQL type of an object is to query for the
+`__typename` field. 
+To override this behavior, an `ORMField` needs to be created
+for the custom type field on the corresponding  `SQLAlchemyInterface`. This is *not recommended* 
+as it promotes abiguous schema design
+
+If your SQLAlchemy model only specifies a relationship to the
+base type, you will need to explicitly pass your concrete implementation
+class to the Schema constructor via the `types=` argument:
+
+.. code:: python
+
+    schema = graphene.Schema(..., types=[PersonType, EmployeeType, CustomerType])
+    
+See also: `Graphene Interfaces <https://docs.graphene-python.org/en/latest/types/interfaces/>`_

graphene_sqlalchemy/__init__.py

@@ -1,11 +1,12 @@
 from .fields import SQLAlchemyConnectionField
-from .types import SQLAlchemyObjectType
+from .types import SQLAlchemyInterface, SQLAlchemyObjectType
 from .utils import get_query, get_session
 
 __version__ = "3.0.0b3"
 
 __all__ = [
     "__version__",
+    "SQLAlchemyInterface",
     "SQLAlchemyObjectType",
     "SQLAlchemyConnectionField",
     "get_query",

graphene_sqlalchemy/registry.py

@@ -18,15 +18,10 @@ def __init__(self):
         self._registry_unions = {}
 
     def register(self, obj_type):
+        from .types import SQLAlchemyBase
 
-        from .types import SQLAlchemyObjectType
-
-        if not isinstance(obj_type, type) or not issubclass(
-            obj_type, SQLAlchemyObjectType
-        ):
-            raise TypeError(
-                "Expected SQLAlchemyObjectType, but got: {!r}".format(obj_type)
-            )
+        if not isinstance(obj_type, type) or not issubclass(obj_type, SQLAlchemyBase):
+            raise TypeError("Expected SQLAlchemyBase, but got: {!r}".format(obj_type))
         assert obj_type._meta.registry == self, "Registry for a Model have to match."
         # assert self.get_type_for_model(cls._meta.model) in [None, cls], (
         #     'SQLAlchemy model "{}" already associated with '
@@ -38,14 +33,10 @@ def get_type_for_model(self, model):
         return self._registry.get(model)
 
     def register_orm_field(self, obj_type, field_name, orm_field):
-        from .types import SQLAlchemyObjectType
+        from .types import SQLAlchemyBase
 
-        if not isinstance(obj_type, type) or not issubclass(
-            obj_type, SQLAlchemyObjectType
-        ):
-            raise TypeError(
-                "Expected SQLAlchemyObjectType, but got: {!r}".format(obj_type)
-            )
+        if not isinstance(obj_type, type) or not issubclass(obj_type, SQLAlchemyBase):
+            raise TypeError("Expected SQLAlchemyBase, but got: {!r}".format(obj_type))
         if not field_name or not isinstance(field_name, str):
             raise TypeError("Expected a field name, but got: {!r}".format(field_name))
         self._registry_orm_fields[obj_type][field_name] = orm_field

graphene_sqlalchemy/tests/models.py

@@ -288,3 +288,39 @@ class KeyedModel(Base):
     __tablename__ = "test330"
     id = Column(Integer(), primary_key=True)
     reporter_number = Column("% reporter_number", Numeric, key="reporter_number")
+
+
+############################################
+# For interfaces
+############################################
+
+
+class Person(Base):
+    id = Column(Integer(), primary_key=True)
+    type = Column(String())
+    name = Column(String())
+    birth_date = Column(Date())
+
+    __tablename__ = "person"
+    __mapper_args__ = {
+        "polymorphic_on": type,
+    }
+
+class NonAbstractPerson(Base):
+    id = Column(Integer(), primary_key=True)
+    type = Column(String())
+    name = Column(String())
+    birth_date = Column(Date())
+
+    __tablename__ = "non_abstract_person"
+    __mapper_args__ = {
+        "polymorphic_on": type,
+        "polymorphic_identity": "person",
+    }
+
+class Employee(Person):
+    hire_date = Column(Date())
+
+    __mapper_args__ = {
+        "polymorphic_identity": "employee",
+    }

graphene_sqlalchemy/tests/test_query.py

@@ -1,10 +1,21 @@
+from datetime import date
+
 import graphene
 from graphene.relay import Node
 
 from ..converter import convert_sqlalchemy_composite
 from ..fields import SQLAlchemyConnectionField
-from ..types import ORMField, SQLAlchemyObjectType
-from .models import Article, CompositeFullName, Editor, HairKind, Pet, Reporter
+from ..types import ORMField, SQLAlchemyInterface, SQLAlchemyObjectType
+from .models import (
+    Article,
+    CompositeFullName,
+    Editor,
+    Employee,
+    HairKind,
+    Person,
+    Pet,
+    Reporter,
+)
 from .utils import to_std_dicts
 
 
@@ -334,3 +345,55 @@ class Mutation(graphene.ObjectType):
     assert not result.errors
     result = to_std_dicts(result.data)
     assert result == expected
+
+
+def add_person_data(session):
+    bob = Employee(name="Bob", birth_date=date(1990, 1, 1), hire_date=date(2015, 1, 1))
+    session.add(bob)
+    joe = Employee(name="Joe", birth_date=date(1980, 1, 1), hire_date=date(2010, 1, 1))
+    session.add(joe)
+    jen = Employee(name="Jen", birth_date=date(1995, 1, 1), hire_date=date(2020, 1, 1))
+    session.add(jen)
+    session.commit()
+
+
+def test_interface_query_on_base_type(session):
+    add_person_data(session)
+
+    class PersonType(SQLAlchemyInterface):
+        class Meta:
+            model = Person
+
+    class EmployeeType(SQLAlchemyObjectType):
+        class Meta:
+            model = Employee
+            interfaces = (Node, PersonType)
+
+    class Query(graphene.ObjectType):
+        people = graphene.Field(graphene.List(PersonType))
+
+        def resolve_people(self, _info):
+            return session.query(Person).all()
+
+    schema = graphene.Schema(query=Query, types=[PersonType, EmployeeType])
+    result = schema.execute(
+        """
+        query {
+            people {
+                __typename
+                name
+                birthDate
+                ... on EmployeeType {
+                    hireDate
+                }
+            }
+        }
+        """
+    )
+
+    assert not result.errors
+    assert len(result.data["people"]) == 3
+    assert result.data["people"][0]["__typename"] == "EmployeeType"
+    assert result.data["people"][0]["name"] == "Bob"
+    assert result.data["people"][0]["birthDate"] == "1990-01-01"
+    assert result.data["people"][0]["hireDate"] == "2015-01-01"

graphene_sqlalchemy/tests/test_registry.py

@@ -28,7 +28,7 @@ def test_register_incorrect_object_type():
     class Spam:
         pass
 
-    re_err = "Expected SQLAlchemyObjectType, but got: .*Spam"
+    re_err = "Expected SQLAlchemyBase, but got: .*Spam"
     with pytest.raises(TypeError, match=re_err):
         reg.register(Spam)
 
@@ -51,7 +51,7 @@ def test_register_orm_field_incorrect_types():
     class Spam:
         pass
 
-    re_err = "Expected SQLAlchemyObjectType, but got: .*Spam"
+    re_err = "Expected SQLAlchemyBase, but got: .*Spam"
     with pytest.raises(TypeError, match=re_err):
         reg.register_orm_field(Spam, "name", Pet.name)