mongod关联表完整篇（解决objectid类型不匹配难度）

findById: async (db, id) => {
    return db.aggregate([
      {
        $lookup: {
          from: "collections",
          let: { jobId: "$_id" },
          pipeline: [
            { $addFields: { productId: { $toObjectId: "$goodsid" } } },
            { $match: { $expr: { $eq: ["$productId", "$$jobId"] } } },
          ],
          as: "product_collection",
        },
      },
      { $match: { $expr: { $eq: [{ $toString: "$_id" }, id] } } },
    ]);
  },

根据从表过滤主表数据

findCollectByUid: async (db, uid) => {
    return db.aggregate([
      {
        $lookup: {
          from: "collections",
          let: { jobId: "$_id" },
          pipeline: [
            { $addFields: { ProductId: { $toObjectId: "$goodsid" } } },
            { $match: { $expr: { $eq: ["$ProductId", "$$jobId"] }, uid: uid } },管道内过滤
          ],
          as: "collect",
        },
      },
      { $match: { "collect.0.uid": uid } },  // {$match:{"主表字段":"条件","collect.从表字段":"条件"} }])

    ]);
  },