"""Supplier ↔ material link service (v2.5) — the direct edge of the new flow.

Linking a material to a supplier derives materials.category_id from the
supplier's category when the material has none (first-link-wins). The same
rule is enforced by the DB trigger trg_supplier_materials_derive_category;
this service applies it in the application layer so the ORM state is
consistent inside the same transaction.
"""
from sqlalchemy import select
from sqlalchemy.ext.asyncio import AsyncSession

from app.models.catalog import Material
from app.models.procurement import Supplier, SupplierMaterial
from app.utils.errors import NotFoundError, ValidationError


async def link_supplier_material(
    db: AsyncSession, supplier_id, material_id, is_active: bool = True
) -> SupplierMaterial:
    """Upsert a supplier<->material link (idempotent) + derive category (first-link-wins).

    Caller is responsible for the surrounding transaction.
    """
    supplier = await db.get(Supplier, supplier_id)
    if supplier is None:
        raise NotFoundError("تأمین‌کننده یافت نشد", field="supplier_id")
    material = await db.get(Material, material_id)
    if material is None:
        raise NotFoundError("متریال یافت نشد", field="material_id")

    link = await db.scalar(
        select(SupplierMaterial).where(
            SupplierMaterial.supplier_id == supplier_id,
            SupplierMaterial.material_id == material_id,
        )
    )
    if link is not None:
        if not link.is_active and is_active:
            link.is_active = True
        return link

    link = SupplierMaterial(supplier_id=supplier_id, material_id=material_id, is_active=is_active)
    db.add(link)
    await db.flush()

    # First-link-wins: only when the material has no category yet.
    if material.category_id is None:
        material.category_id = supplier.category_id
    return link


async def unlink_supplier_material(db: AsyncSession, supplier_id, material_id) -> None:
    """Remove the link. The material's category is NEVER auto-cleared (documented rule)."""
    link = await db.scalar(
        select(SupplierMaterial).where(
            SupplierMaterial.supplier_id == supplier_id,
            SupplierMaterial.material_id == material_id,
        )
    )
    if link is None:
        raise ValidationError("این متریال به این تأمین‌کننده متصل نیست", field="material_id")
    await db.delete(link)
    await db.flush()