command.rb

require "event_sourcing/command"
require "aggregates/mathtrade"

module Commands
  RequestMathtrade = EventSourcing::Command.new(:mathtrade_id, :wantlist) do |event_store|
    stream = event_store.get_stream(mathtrade_id)
    mathtrade = Aggregates::Mathtrade.new(stream.events)
    event_store.append(mathtrade_id, stream.version, mathtrade.request(wantlist: wantlist))
  end
end

command_spec.rb

require "unit_helper"
require "commands/mathtrade"

describe Commands::RequestMathtrade do
  let(:command)  { Commands::RequestMathtrade.new(mathtrade_id: "some-id", wantlist: wantlist) }

  describe "execute" do
    let(:wantlist)    { double("Wantlist") }
    let(:event_store) { double("EventStore") }
    let(:mathtrade)   { instance_double("Aggregates::Mathtrade") }
    let(:history)     { double("History") }
    let(:event)       { double("Event")}
    let(:stream)      { double("Stream", events: history, version: 123)}

    before do
      allow(event_store).to receive(:get_stream).with("some-id").and_return(stream)
      allow(Aggregates::Mathtrade).to receive(:new).with(history).and_return(mathtrade)
      allow(mathtrade).to receive(:request).with(wantlist: wantlist).and_return(event)
    end

    after { command.execute(event_store) }

    it "appends to the event store whatever the aggregate returns" do
      expect(event_store).to receive(:append).with("some-id", 123, event)
    end
  end
end