One-to-many "flattened" projection?

[ChrisSimmons]

Imagine I rent vehicles. I'm writing a software system to keep track of the vehicles I'm renting. As well, I want to be able to keep some informative information about the types of vehicles that I rent, both for my internal purposes as well as for display on my website. I would like to use Marten for my system. Also it should be noted that I am an event-sourcing newbie so go easy with me if I'm doing something really wrong.

I'm starting out with two aggregates:

• VehicleType, representing the information about a type of vehicle like Make, Model, and Year
• RentalVehicle, representing an actual physical car out on my lot that I rent out to people

If my fleet of rental vehicles comprises ten 2022 Ford Mustangs and eleven 2021 Nissan Sentras, I will have two VehicleType and twenty-one RentalVehicle aggregates in my system.

I would like to represent RentalVehicle in an event-sourced manner, with events such as:

• VehicleAcquired, which is the "creation" event for the RentalVehicle and carries the Id of the respective VehicleType, the Vehicle Identification Number (VIN), and the acquisition date
• VehicleRented
• VehicleSold
• VehicleLost

The VehicleType aggregate does not really need to be event-sourced and can be stored in a document. However, for the purposes of my problem here, imagine it is event sourced and only has one event:

• VehicleTypeCreated functions as the "creation" event for VehicleType, carrying values for Make, Model, and Year

My problem arises when I want to have a projection that combines the information from these aggregates. Let's assume I want a projection VehicleInformation (ignoring how horrible this name is). The documents in this projection need to carry the information for each RentalVehicle as well as the associated VehicleType. If I were using an RDBMS, I'd just have tables RentalVehicle and VehicleType with a materialized view VehicleInformation like

SELECT 
  rv.Vin,
  rv.DateAcquired,
  rv.Status,
  vt.Make,
  vt.Model,
  vt.Year
FROM 
  RentalVehicle rv
  INNER JOIN 
  VehicleType vt 
  ON rv.VehicleTypeId = vt.Id

Of course that's not possible here since we're not looking at this with tables and relations and views. I know my Marten projection will need to be multi-streamed but I don't see a way to accomplish this with the provided MultiStreamProjection, basically because:

• The VehicleType event occurs before any associated RentalVehicle aggregates even exist so there's nothing to do in that case anyway
• Also, even if we had VehicleType events like VehicleMakeUpdated, then there's the challenge of "broadcasting" that one event to all the existing RentalVehicle aggregates
• RentalVehicle events (I'm assuming) will need to "look up" information on the associated VehicleType in order to apply that to the associated VehicleInformation document

I think I can accomplish what I've set out to do here with a custom IProjection but the implementation seems rather complex in my head. And this kind of scenario seems like it would be pretty common (essentially a flattened one-to-many view/projection) that there has to be a better way.

Any better recommendations? Maybe I'm looking at the problem the wrong way and there is some better and more elegant way to model this that will solve the problem?

[ChrisSimmons]

I don't see a way around this except through a custom IProjection.