Graphene-SQLAlchemy automatically converts columns of type SmallInteger or Integer to ID! fields if they are primary keys, but does not convert such columns to ID fields if they are foreign keys.

Take for example this schema:

class Department(Base):
    __tablename__ = 'department'
    id = Column(Integer, primary_key=True)
    name = Column(String)

class User(Base):
    __tablename__ = 'users'
    id = Column(Integer, primary_key=True)
    name = Column(String)
    department_id = Column(Integer, ForeignKey('department.id'))
    department = relationship(Department)

class DepartmentType(SQLAlchemyObjectType):
    class Meta:
        model = Department

class UserType(SQLAlchemyObjectType):
    class Meta:
        model = User

class Query(ObjectType):

    departments = List(DepartmentType)
    users = List(UserType)

    def resolve_departments(self, info):
        return DepartmentType.get_query(info)

    def resolve_users(self, info):
        return UserType.get_query(info)

You can run the following query:

query {
  users {
    id
    name
    departmentId
    department {
      id
    }
  }
}

As a result, you get something like:

{
  "data": {
    "users": [
      {
        "id": "1",
        "firstName": "Fred",
        "departmentId": 1,
        "department": {
          "id": "1"
        }
      },
      {
        "id": "2",
        "firstName": "Barnie",
        "departmentId": 2,
        "department": {
          "id": "2"
        }
      }
    ]
  }
}

As you see, department.id is a string (because IDs are returned as strings), while departmentId is a number. This turned out to be a huge problem and source of error in practice. Working with this inconsistent, fault-prone interface has bitten me many times. When storing ids in objects on the frontend, or using ids as filters, I never know whether I should use numbers or strings. Currently I have conversions from number to string and vice versa everywhere in my frontend code, and if I don't do it correctly, things stop working in hard to debug ways because you often don't recognize such type mismatches. On the server side, do I take ids used as filter parameters as IDs or Ints? If I do the former, I must then convert them to integer when using them as filter arguments for SQLAlchemy. So, really, this is no fun to work with and doesn't work in practice, because you always have this mental burden of thinking about whether your ids should be represented as strings or numbers and whether you need to convert them when passing them around.

I suggest the conversions should be consistent. Either convert all keys, including foreign keys, to IDs, or do not make a special case conversion for primary keys. Actually I'd prefer the latter, since then I never need to think about the type and since storing numbers on the frontend uses less memory.

Now of course I know that there is the relay specification which assumes there is an id field with a type of ID. So when using the relay interface, things are different. In this case, I suggest converting to IDs everywhere (including foreign keys) - but here we need conversion of the values to global ids anyway, they are not just the row ids converted to strings.