db.collection.aggregate([
    { "$match": { "comment": buzzword } },
    {
        "$project": {
            "source": 1,
            "date": 1,
            "author": 1,
            "thread": 1,
            "commentID": 1,
            "title": 1,
            "comment": 1,
            "distinct_matched_comment": {
                "$arrayElemAt": [ 
                    {
                        "$filter": {
                            "input": "$comment",
                            "as": "word",
                            "cond": {
                                "$eq": ["$$word", buzzword]
                            }
                        }
                    }, 0
                ]
            }
        }
    }
])

解释

在上面的管道中，技巧是首先通过仅选择满足给定条件的元素来过滤评论数组。例如，为了演示这个概念，运行这个管道:

db.collection.aggregate([
    {
        "$project": {
            "filtered_comment": {
                "$filter": {
                    "input": ["This", "is", "a", "comment", "with", "a", "duplicate"], /* hardcoded input array for demo */
                    "as": "word", /* The variable name for the element in the input array. 
                                     The as expression accesses each element in the input array by this variable.*/
                    "cond": { /* this condition determines whether to include the element in the resulting array. */
                        "$eq": ["$$word", "a"] /* condition where the variable equals the buzzword "a" */
                    }
                }
            }
        }
    }
])

输出

{
    "_id" : ObjectId("57dbd747be80cdcab63703dc"),
    "filtered_comment" : [ 
        "a", 
        "a"
    ]
}

作为 $filter 的 input 参数接受解析为数组的表达式，您可以改用数组字段。

进一步利用上面的结果，我们可以展示 $arrayElemAt 运算符工作:

db.collection.aggregate([
    {
        "$project": {
            "distinct_matched_comment": {
                "$arrayElemAt": [ 
                    ["a", "a"], /* array produced by the above $filter expression */
                    0 /* the index position of the element we want to return, here being the first */
                ]   
            }
        }
    }
])

输出

{
    "_id" : ObjectId("57dbd747be80cdcab63703dc"),
    "distinct_matched_comment": "a"
}

由于 $arrayElemAt 中的表达式 运算符

{ "$arrayElemAt": [ <array>, <idx> ] } 

可以是任何有效的表达式，只要它解析为数组，您可以组合 $filter 此示例开头的表达式作为数组表达式，因为它返回一个数组，因此您的最终管道将如下所示:

db.collection.aggregate([
    {
        "$project": {
            "distinct_matched_comment": {
                "$arrayElemAt": [ 
                    {  /* expression that produces an array with elements that match a condition */
                        "$filter": {
                            "input": "$comment",
                            "as": "word",
                            "cond": {
                                "$eq": ["$$word", buzzword]
                            }
                        }
                    },                  
                    0 /* the index position of the element we want to return, here being the first */
                ]
            }
        }
    }
])