$accumulator (聚合)错误mongo数据库

Question

需要帮助的mongo与双重说明，我复制了这一部分：

db.books.aggregate([
{
  $group :
  {
    _id : "$author",
    avgCopies:
    {
      $accumulator:
      {
        init: function() {                        // Set the initial state
          return { count: 0, sum: 0 }
        },

        accumulate: function(state, numCopies) {  // Define how to update the state
          return {
            count: state.count + 1,
            sum: state.sum + numCopies
          }
        },

        accumulateArgs: ["$copies"],              // Argument required by the accumulate function

        merge: function(state1, state2) {         // When the operator performs a merge,
          return {                                // add the fields from the two states
            count: state1.count + state2.count,
            sum: state1.sum + state2.sum
          }
        },

        finalize: function(state) {               // After collecting the results from all documents,
          return (state.sum / state.count)        // calculate the average
        },
        lang: "js"
      }
    }
  }
}
])

但是我得到了一个错误"errmsg“：”未知的组运算符'$accumulator'"，https://docs.mongodb.com/manual/reference/operator/aggregation/accumulator/#grp._S_accumulator

Answer 1

我也面临着同样的问题。检查您的MongoDB版本。$accumulator是4.4版中的新功能。

> mongo --version

附注：

我的MongoDB版本是4.2.5