sequence.reduce(function) → value r.reduce(sequence, function) → value
Produce a single value from a sequence through repeated application of a reduction function.
The reduction function can be called on:
The reduction function can be called on the results of two previous reductions because the reduce command is distributed and parallelized across shards and CPU cores. A common mistaken when using the reduce command is to suppose that the reduction is executed from left to right. Read the map-reduce in RethinkDB article to see an example.
If the sequence is empty, the server will produce a ReqlRuntimeError that 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 post has a field comments that 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 post has a field comments that 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.
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https://rethinkdb.com/api/ruby/reduce/