Data access is contextual, a generic approach will fail
In my previous post, I discussed the following scenario, a smart client application for managing Machines, Parts & Maintenance History:
From the client perspective, Machine,Parts & Maintenance History where all part of the same entity, and they wanted to be able to be able return that to the user in a single call. They run into several problems in doing that, mostly, from what I could see, because they tried to do something that I strongly discourage, sending entities on the wire so they could be displayed on the smart client application. The client uses CSLA, so the actual business logic is sitting in Factory objects (I discussed my objection to that practice here), but the entities are still doing triple duty here. They need to be saved to the database, serve as DTOs on the wire and also handle being presented in the UI.
When I objected to that practice, I was asked: “Are we supposed to have three duplicates of everything in the application? One for the database, one for the wire and one for the UI? That is insane!”
The answer to this question is:
This is the symbol for Mu, which is the answer you give when the question is meaningless.
The problem with the question as posed is that it contains an invalid assumption. The assumption is that the three separate models in the application are identical to one another. They aren’t, not by a long shot.
Let me see if I can explain it better with an example. Given the Machine –>> Part –>> Maintenance History model, and an application to manage those as our backdrop. We will take two sample scenarios to discuss, the first would be to record replacing a part in a machine, and the second would be looking at a list of machines that need fixing.
For the first scenario, we need to show the user the Maintenance screen for the machine, which looks like this:
And on the second one, machines requiring fixes, we have:
Now, both of those screens are driven by data in the Machine –>> Part –>> Maintenance History model. But they have very different data requirements.
In the first case, we need the Machine.Name, Machine.NextMaintenanceDate, and a list of Part.SKU, Part.Name, Part.Broken, Part.Notes.
In the second case, we need Machine.Owner, Machine.Car, Machine.Age, Machine.Preferred.
The data set that they require is completely different, not only that, but sending the full object graph to each of those is a waste. If you noticed, no one actually even needed Maintenance History for any of those screens. It is likely that Maintenance History will be only needed rarely, but by saying that there is only one model that needs to serve all those different scenarios, we are tying ourselves into knots, because we need to serve Maintenance History every time that we are asked for a machine, even though it is not needed.
The way I would design the system, I would have:
Entities: Machine, Part, Maintenance History
This is the full data model that we use. Note that I am not discussing issues like CQRS here, because the application doesn’t warrant it. A single model for everything would work well here.
DTO: MachineHeaderDTO, MachineAndPartsDTO, PartAndMaintenanceHistoryDTO, etc.
We have specific DTOs for each scenario, only giving us the data that we need, so we can spare a lot of effort in the server side in deciding how to fill the data.
The view models would be built on top of the DTOs, usually providing some additional services like INotifyPropertyChanged, calculated properties, client side validation, etc.
I am in favor of each screen having its own View Models, because that lead to a self contained application, but I wouldn’t have a problem with common view models shared among the entire application.
And because I know you’ll want to see some code, the Alexandria sample application demonstrate those concepts quite well: http://github.com/ayende/alexandria