from_builtins, but for objects? (e.g instantiate from SQLAlchemy models) #416

from sqlalchemy import ForeignKey
from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
from typing import List


# SQLAlchemy 2.0 Models
class Base(DeclarativeBase):
    pass


class GrommetTable(Base):
    __tablename__ = 'grommet'

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str]
    widget_id: Mapped[int] = mapped_column(ForeignKey("widget.id"))

    widget: Mapped["WidgetTable"] = relationship(back_populates="grommets")


class WidgetTable(Base):
    __tablename__ = 'widget'

    id: Mapped[int] = mapped_column(primary_key=True)
    name: Mapped[str]

    grommets: Mapped[List["GrommetTable"]] = relationship(back_populates="widget")

from msgspec import Struct


class Grommet(Struct):
    id: int
    name: str
    widget_id: int


class Widget(Struct):
    id: int
    name: str
    grommets: List[Grommet]

from sqlalchemy import create_engine
from sqlalchemy.orm import Session

from models import Grommet, Widget
from tables import GrommetTable, WidgetTable

engine = create_engine("sqlite+pysqlite:///:memory:", echo=True)

# Create table structure
Base.metadata.create_all(engine)

# Create demo data
g1 = GrommetTable(id=1, name="Grommet 1")
g2 = GrommetTable(id=2, name="Grommet 2")
g3 = GrommetTable(id=3, name="Grommet 3")
g4 = GrommetTable(id=4, name="Grommet 4")
g5 = GrommetTable(id=5, name="Grommet 5")
g6 = GrommetTable(id=6, name="Grommet 6")

w1 = WidgetTable(id=1, name="Widget 1", grommets=[g1, g2, g3])
w2 = WidgetTable(id=2, name="Widget 2", grommets=[g4, g5, g6])

with Session(engine) as session:
    session.add_all([w1, w2])
    session.commit()

from sqlalchemy import select
from sqlalchemy.orm import selectinload

from models import Widget
from tables import WidgetTable

with Session(engine) as session:
    query = select(WidgetTable).options(selectinload(WidgetTable.grommets))
    result = session.execute(query)

    for widget in result.scalars():
        widget = from_obj(widget, Widget)   # NEW FEATURE
        print(widget)

Widget(id=1, name='Widget 1', grommets=[Grommet(id=1, name='Grommet 1', widget_id=1), Grommet(id=2, name='Grommet 2', widget_id=1), Grommet(id=3, name='Grommet 3', widget_id=1)])
Widget(id=2, name='Widget 2', grommets=[Grommet(id=4, name='Grommet 4', widget_id=2), Grommet(id=5, name='Grommet 5', widget_id=2), Grommet(id=6, name='Grommet 6', widget_id=2)])

from msgspec import from_builtins
from msgspec.inspect import ListType, type_info

def from_obj(obj, type, _data_only=False):
    data = {}
    for field in type_info(type).fields:
        if isinstance(field.type, ListType):
            value = [from_obj(o, field.type.item_type.cls, _data_only=True) for o in getattr(obj, field.name)]
        else:
            value = getattr(obj, field.name)

        data[field.name] = value

    if _data_only:
        return data
    else:
        return from_builtins(data, type)