backend/src/models/InventoryHistory.js

import mongoose from "mongoose";

const InventoryHistorySchema = new mongoose.Schema(
  {
    product: { type: mongoose.Schema.Types.ObjectId, ref: "Product", required: true, index: true },
    oldQty: { type: Number, required: true },
    newQty: { type: Number, required: true },
    changedBy: { type: String, default: "system" }, // you can store email/userId later
  },
  { timestamps: true }
);

// Fast lookups: latest first for a product
InventoryHistorySchema.index({ product: 1, createdAt: -1 });

export default mongoose.model("InventoryHistory", InventoryHistorySchema);