Joined fields (spec/02 §1.4): a reader of an input reads each per-identity source's answer for its own identity. Its rows carry, beside their own fields, every output field but the identity, named "<id>.<field>", a string, never an atom, nil where the output holds nothing for the row's entity; its schema describes them as the outputs do.

Which sources a reader sees is its owner's to decide; on a Macaw page, macaw_compose's. Ported from visualize's Visualize.VirtualSource.Join, whose names were atoms.

iex> alias Macaw.Data.Schema
iex> hosts = Schema.new!(%{"host" => %{type: :category, identity: true}, "cpu" => %{type: :number}})
iex> health = Schema.new!(%{"host" => %{type: :category, identity: true}, "state" => %{type: :category}})
iex> outputs = [{"health", health, [%{"host" => "a", "state" => "ok"}]}]
iex> Macaw.Data.Derived.Join.join([%{"host" => "a", "cpu" => 3}, %{"host" => "b", "cpu" => 9}], hosts, outputs)
[%{"host" => "a", "cpu" => 3, "health.state" => "ok"}, %{"host" => "b", "cpu" => 9, "health.state" => nil}]