ReQL command: concatMap

Command syntax

stream.concatMap(function) → stream
array.concatMap(function) → array

Description

Concatenate one or more elements into a single sequence using a mapping function.

concatMap works in a similar fashion to map, applying the given function to each element in a sequence, but it will always return a single sequence. If the mapping function returns a sequence, map would produce a sequence of sequences:

r.expr([1, 2, 3]).map(function(x) { return [x, x.mul(2)] }).run(conn, callback)

Result:

[[1, 2], [2, 4], [3, 6]]

Whereas concatMap with the same mapping function would merge those sequences into one:

r.expr([1, 2, 3]).concatMap(function(x) { return [x, x.mul(2)] }).run(conn, callback)

Result:

[1, 2, 2, 4, 3, 6]

The return value, array or stream, will be the same type as the input.

Example: Construct a sequence of all monsters defeated by Marvel heroes. The field “defeatedMonsters” is an array of one or more monster names.

r.table('marvel').concatMap(function(hero) {
    return hero('defeatedMonsters')
}).run(conn, callback)

Example: Simulate an eqJoin using concatMap. (This is how ReQL joins are implemented internally.)

r.table("posts").concatMap(function(post) {
	return r.table("comments").getAll(
		post("id"),
		{ index:"postId" }
	).map(function(comment) {
		return { left: post, right: comment }
	})
}).run(conn, callback)

Related commands

• map

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