您需要先展开business_opportunities，然后再将它们与正则表达式匹配.以下是更新后的代码.

You need to unwind the business_opportunities before matching them with regular expression. Below is the updated code.

static async findRelatedTags(opts, params) {
    assert.object(params, 'params')
    assert.string(params.tag, 'params.tag')
    assert.number(params.limit, 'params.limit')

    assert.object(opts, 'opts')
    assert.object(opts.mongo_db, 'opts.mongo_db')

    const { mongo_db: db } = opts

    const query = [
      { $unwind : "$comments" },
      {
        $match: {
          business_opportunities: { '$regex': '^'+params.tag+'', $options: 'i'  }
        }
      },
      { "$group": {
         "_id": "$comments"
        }
      }
    ]

    const cursor = db.collection('business_opportunities').aggregate([
      ...query
    ]).limit(params.limit)

     var agg_results = cursor.toArray()
     return agg_results.map(obj => obj._id) 
  }

MongoDB 聚合将为您提供这样匹配的 business_opportunities

MongoDB aggregation will give you the matched business_opportunities like this

[{
    "_id" : "accounting"
},
{
    "_id" : "app design"
},
{
    "_id" : "aws"
}]

为了将其转换为 ["accounting", "app design", "aws"] 我在上面的示例中添加了 agg_results.map(obj => obj._id).

To convert it to ["accounting", "app design", "aws"] I have added agg_results.map(obj => obj._id) in example above.

除了在您的代码中正则表达式搜索区分大小写，所以我使用了 { '$regex': '^'+params.tag+'', $options: 'i' } 使其不区分大小写，如果不需要，请删除 $options.

Except that in your code regex search is Case Sensitive so I have used { '$regex': '^'+params.tag+'', $options: 'i' } to make it Case Insensitive, remove $options if you don't need it.