Chained map/reduce in couchDB

问题

In couchDB, I have a set of items like the following (simplified for example's sake):

{_id: 1, date: "Jul 1", user: "user1"}
{_id: 2, date: "Jul 2", user: "user1"}
{_id: 3, date: "Jul 3", user: "user2"}
...etc...

I'd like to get a list of "most recent activity", sorted by date, with no duplicate user _ids. I can create a view with results like so:

{key: "July 3", _id: 3, user: "user2"}
{key: "July 2", _id: 2, user: "user1"}
{key: "July 1", _id: 1, user: "user1"}

but this contains duplicate entries for the same user. Or I can create a view that maps {key: user, value: date} and reduces to

{key: "user1", mostRecentDate: "July 2"}
{key: "user2", mostRecentDate: "July 3"}

but that isn't sorted by "most recent".

I know that the obvious solution - reducing over the results of another view isn't supported. BigCouch supports chained map/reduce, but appears to be rather out of date / unsupported (last release 2012).

This seems like a rather common problem - what are some existing solutions (beyond "switch databases")?

回答1:

Here is a general idea of how you can do chained map reduce with couchdb 1.xxx. What we want is the ability to pass the the results of one map/reduce to another.

Subscribe to the _changes feed filtered by the view. This will give you a list of docs that will actually be emitted by the map function.
Next we need to call the view function for these filtered docs. It's simple since we can pass a list of keys to the view so we simply pass the keys and get the desired result subset of the view.
Next we push this result either in a separate database or in the same one. We can use bulk inserts to perform the inserts faster. If you use a separate database you can even reuse the _id's from the view results so the bulk updates would be a lot easier.
Within this database we define another view that sorts our results based on value.

{key: "user1", mostRecentDate: "July 2"} {key: "user2", mostRecentDate: "July 3"}

since you have already gotten to this step all you need to do is create a view on mostRecentDate in the second database and you will get user activity sorted by date.

I hope you are using a dummy reduce though. One that returns null and is only used for group=true.

using a list function in step 4 can make your life easier. As bulk updates require the list of docs to be in the form {"docs":[....]} you can easily get it in one go with a list function.

来源：https://stackoverflow.com/questions/25270276/chained-map-reduce-in-couchdb

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couchdb