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    ReQL command: concat_map

    Command syntax

    stream.concat_map(function) → stream

    array.concat_map(function) → array

    Description

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

    concat_map 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 { |x| [x, x.mul(2)] }.run(conn)
    

    Result:

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

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

    r.expr([1, 2, 3]).concat_map { |x| [x, x.mul(2)] }.run(conn)
    

    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').concat_map { |hero|
        hero[:defeated_monsters]
    }.run(conn)
    

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

    r.table('posts').concat_map { |post|
        r.table('comments').get_all(
            post['id'], :index => 'post_id'
        ).map { |comment| { :left => post, :right => comment} }
    }.run(conn)
    

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