Store Computed Data存储计算数据

Insert sample data插入示例数据

Create the screenings collection:创建screenings（放映）集合：

db.screenings.insertMany( [
   {
      theater: "Alger Cinema",
      location: "Lakeview, OR",
      movie_title: "Lost in the Shadows",
      movie_id: 1,
      num_viewers: 344,
      revenue: 3440
   },
   {
      theater: "City Cinema",
      location: "New York, NY",
      movie_title: "Lost in the Shadows",
      movie_id: 1,
      num_viewers: 1496,
      revenue: 22440
   },

] )

Insert computed data插入计算数据

Users often want to know how many people saw a certain movie and how much money that movie made. In the current schema, to add num_viewers and revenue, you must perform a read for theaters that screened a movie with the title "Lost in the Shadows" and sum the values of those fields.用户经常想知道有多少人看过某部电影，这部电影赚了多少钱。在当前模式中，要添加num_viewers和revenue（收入），您必须对放映了标题为“迷失在阴影中”的电影的影院进行读取，并将这些字段的值相加。

To avoid performing that computation every time the information is requested, you can compute the total values and store them in a movies collection with the movie record itself:为了避免每次请求信息时都执行该计算，您可以计算总值并将其与movies记录本身一起存储在电影集合中：

db.movies.insertOne(
   {
      _id: 1,
      title: "Lost in the Shadows",
      total_viewers: 1840,
      total_revenue: 25880
   }
)

Updated computed data更新的计算数据

Consider a new screening is added to the screenings collection:考虑在screenings（放映）集合中添加一个新的筛选：

db.screenings.insertOne(
   {
      theater: "Overland Park Cinema",
      location: "Boise, ID",
      movie_title: "Lost in the Shadows",
      movie_id: 1,
      num_viewers: 760,
      revenue: 7600
   }
)

The computed data in the movies collection no longer reflects the current screening data. How often you update computed data depends on your application:movies集合中的计算数据不再反映当前的放映数据。更新计算数据的频率取决于应用程序：

• In a low write environment, the computation can occur in conjunction with any update of the screenings data.在低写环境中，计算可以与screenings数据的任何更新同时进行。
• In an environment with more regular writes, the computations can be done at defined intervals (every hour for example). The source data in screenings isn't affected by writes to the movies collection, so you can run calculations at any time.在具有更规则写入的环境中，可以按定义的间隔（例如每小时）进行计算。screenings中的源数据不受写入电影集的影响，因此您可以随时运行计算。

To update the computed data based on the screenings data, you can run the following aggregation at a regular interval:要根据筛选数据更新计算数据，可以定期运行以下聚合：

db.screenings.aggregate( [
   {
      $group: {
         _id: "$movie_id",
         total_viewers: {
            $sum: "$num_viewers"
         },
         total_revenue: {
            $sum: "$revenue"
         }
      }
   },
   {
      $merge: {
         into: { db: "test", coll: "movies" },
         on: "_id",
         whenMatched: "merge"
      }
   }
] )

Confirm results确认结果

To confirm that the movies collection was updated, query the collection:要确认movies集合已更新，请查询集合：

db.movies.find()

Output:输出：

[
   {
      _id: 1,
      title: 'Lost in the Shadows',
      total_viewers: 2600,
      total_revenue: 33480
   }
]