javascript - 在 CouchDB 的 MapReduce 中选择属于一个用户的 n 个元素

Question

请多多包涵，我对整个 CouchDb 都是新手。

数据库看起来像:

** item ** count ** user **
   A       20       bob
   B       30       bob
   C       10       bob
   D       15       john

我想写一个 MapReduce 来选择属于 bob 的所有项目并且只返回前 2 个，排序。所以它应该返回 [{item:"B",count:"30"},{item:"A",count:"20}]

我不确定如何做到这一点？好像我必须发出(doc.item，doc.count)，但我怎么知道用户是否拥有该文档？如何运行另一个 MapReduce 来选择顶部元素？

Answer 1

一个解决方案是编写您的 View 以使用复杂的键，例如:

function (doc) {
    emit([doc.user, doc.count], doc.item);
}

如果您将 descending=true 添加到您的查询字符串中，您将得到如下 View 结果:

{"total_rows":4,"offset":0,"rows":[
    {"id":"53f359b7cd360da296dd9aab3d0029bd","key":["john",15],"value":"D"},
    {"id":"53f359b7cd360da296dd9aab3d001a0e","key":["bob",30],"value":"B"},
    {"id":"53f359b7cd360da296dd9aab3d000fec","key":["bob",20],"value":"A"},
    {"id":"53f359b7cd360da296dd9aab3d002668","key":["bob",10],"value":"C"}
]}

它已经按用户排序，然后计数。 (以项目类型为值)

然后你可以使用 _list function做剩下的事。下面的代码基本上遍历 View ，并为每个用户返回前 2 个结果。如果您在查询字符串中指定 user=bob，您将只获得 bob 的结果。

function (head, req) {
    // specify that we're sending JSON as our response
    provides('json', function () {
        var results = [],
            result, user, count, row;

        while (row = getRow()) {
            // if the user doesn't match the last iteration, reset our counter
            if (user != row.key[0]) {
                user = row.key[0];
                count = 0;
            }

            // we only need the top 2
            if (count++ >= 2) {
                continue;
            }

            // start building a result object
            result = {
                item: row.value,
                count: row.key[1]
            };

            // if we provide user=?
            if (req.query.user) {
                // check to see if it matches the current user
                if (req.query.user === user) {
                    // if so, add it to the results
                    results.push(result);
                }
            // by default, we'll return the top 2 for every user
            } else {
                // add the user key to the result object
                result.user = row.key[0];
                // and add it to the result set
                results.push(result);
            }
        }

        // send outside the loop, since it needs to be sent as valid JSON
        send(JSON.stringify(results));
    });
}