Mongodb 映射减少 2 个集合
假设我们有用户和帖子集合。在后期收集中，投票将用户名存储为密钥。
db.user.insert({name:'a', age:12});
db.user.insert({name:'b', age:12});
db.user.insert({name:'c', age:22});
db.user.insert({name:'d', age:22});
db.post.insert({Title:'Title1', vote:[a]});
db.post.insert({Title:'Title2', vote:[a,b]});
db.post.insert({Title:'Title3', vote:[a,b,c]});
db.post.insert({Title:'Title4', vote:[a,b,c,d]});
我们想按帖子标题进行分组，并找出不同用户年龄的投票数。
> {_id:'Title1', value:{ ages:[{age:12, Count:1},{age:22, Count:0}]} }
> {_id:'Title2', value:{ ages:[{age:12, Count:2},{age:22, Count:0}]} }
> {_id:'Title3', value:{ ages:[{age:12, Count:2},{age:22, Count:1}]} }
> {_id:'Title4', value:{ ages:[{age:12, Count:2},{age:22, Count:2}]} }
我已经搜索过，但没有找到访问 mongodb mapreduce 中的 2 个集合的方法。 是否可以通过re-reduce来实现？
我知道在帖子中嵌入用户文档非常简单，但这不是一个好方法，因为真正的用户文档有很多属性。如果我们包含用户文档的简化版本，则会限制分析的维度。
{Title:'Title1', vote:[{name:'a', age:12}]}
Let say we have user and post collection. In post collection, vote store the user name as a key.
db.user.insert({name:'a', age:12});
db.user.insert({name:'b', age:12});
db.user.insert({name:'c', age:22});
db.user.insert({name:'d', age:22});
db.post.insert({Title:'Title1', vote:[a]});
db.post.insert({Title:'Title2', vote:[a,b]});
db.post.insert({Title:'Title3', vote:[a,b,c]});
db.post.insert({Title:'Title4', vote:[a,b,c,d]});
We would like to group by the post.Title and find out the count of vote in different user age.
> {_id:'Title1', value:{ ages:[{age:12, Count:1},{age:22, Count:0}]} }
> {_id:'Title2', value:{ ages:[{age:12, Count:2},{age:22, Count:0}]} }
> {_id:'Title3', value:{ ages:[{age:12, Count:2},{age:22, Count:1}]} }
> {_id:'Title4', value:{ ages:[{age:12, Count:2},{age:22, Count:2}]} }
I have searched through and doesn't find a way to access 2 collection in mongodb mapreduce.
Could it be possible to achieve in re-reduce?
I know it is much simple to embedded the user document in post, but it is not a nice way to do as the real user document have many properties. If we include the simplify version of user document, it will limit the dimension of analysis.
{Title:'Title1', vote:[{name:'a', age:12}]}
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MongoDB没有多集合Map/Reduce。 MongoDB 没有任何 JOIN 语法，可能不太适合临时连接。您需要以某种方式对该数据进行非规范化。
您有几个选项：
选项#1：将年龄嵌入投票。
选项#2：保留年龄计数器
选项#3：执行“手动”加入
您的最后一个选择是编写脚本/代码，对两个集合执行
for循环并正确合并数据。因此，您将循环
post并输出一个包含标题和投票列表的集合。然后，您将循环遍历新集合并通过查找每个用户来更新年龄。我的建议
选择#1或#2。
MongoDB does not have a multi-collection Map / Reduce. MongoDB does not have any JOIN syntax and may not be very good for ad-hoc joins. You will need to denormalize this data in some way.
You have a few options:
Option #1: Embed the age with the vote.
Option #2: Keep a counter of the ages
Option #3: Do a "manual" join
Your last option is to write script/code that does a
forloop over both collections and merges the data correctly.So you would loop over
postand output a collection with the title and the list of votes. Then you would loop through the new collection and update the ages by looking up eachuser.My suggestion
Go with #1 or #2.
相反，
向用户文档添加新字段并在每次投票更新中维护它会更容易。当然，您可以享受使用 MapReduce 来分析您的数据。
Instead of
It is easier to add a new field to user document and maintain it in each vote update.Of course, you can enjoy to use map reduce to analysis your data.