合并不同集合的两个对象，获取Mongodb中的具体数据

db.collection1.aggregate([
  {
    $lookup: {
      from: "collection2",
      localField: "field1",
      foreignField: "field2",
      as: "mergedData"
    }
  }
])

db.collection1.aggregate([
  {
    $lookup: {
      from: "collection2",
      localField: "field1",
      foreignField: "field2",
      as: "mergedData"
    }
  },
  {
    $unwind: "$mergedData"
  }
])

db.collection1.aggregate([
  {
    $lookup: {
      from: "collection2",
      localField: "field1",
      foreignField: "field2",
      as: "mergedData"
    }
  },
  {
    $unwind: "$mergedData"
  },
  {
    $project: {
      "mergedData.field3": 1,
      "mergedData.field4": 1
    }
  }
])