前言：Produce a single value from a sequence through repeated application of a reduction function.

reduce

ReQL command: reduce

Command syntax

sequence.reduce(function) → value r.reduce(sequence, function) → value

Description

Produce a single value from a sequence through repeated application of a reduction function.

The reduction function can be called on:

two elements of the sequence
one element of the sequence and one result of a previous reduction
two results of previous reductions

The reduction function can be called on the results of two previous reductions because the reducecommand is distributed and parallelized across shards and CPU cores. A common mistaken when using the reducecommand is to suppose that the reduction is executed from left to right. Read the map-reduce in RethinkDBarticle to see an example.

If the sequence is empty, the server will produce a ReqlRuntimeErrorthat can be caught with default.
If the sequence has only one element, the first element will be returned.

Example:Return the numbers of documents in the table posts.

r.table("posts").map{|doc| 1 }
    .reduce{ |left, right| left+right }
    .default(0).run(conn)

A shorter way to execute this query is to use count.

Example:Suppose that each posthas a field commentsthat is an array of comments.
Return the number of comments for all posts.

r.table("posts").map{|doc|
    doc["comments"].count()
}.reduce{ |left, right|
    left+right
}.default(0).run(conn)

Example:Suppose that each posthas a field commentsthat is an array of comments.
Return the maximum number comments per post.

r.table("posts").map{|doc|
    doc["comments"].count()
}.reduce{ |left, right|
    r.branch(
        left > right,
        left,
        right
    )
}.default(0).run(conn)

A shorter way to execute this query is to use max.