MongoDB 3.2 的 aggregate 的 lookup，联合查询功能

文档地址： https://docs.mongodb.com/manual/reference/operator/aggregation/lookup/#pipe._S_lookup

db.people_news.aggregate([
      {
        $lookup:
          {
            from: "news_id",
            localField: "_id",
            foreignField: "news_id",
            as: "inventory_docs"
          }
    },
    { $match : { inventory_docs : { $exists:1 } } },
    {$project: {  inventory_docs:1 ,match_size: { $size: "$inventory_docs" }  }},
] 
)
​