Using ViewModels instead DTOs as the result of a CQRS query

Question

Reading a SO question, I realized that my Read services could provide some smarter object like ViewModels instead plain DTOs. This makes me reconsider what information should be provided by the objects returned by the Read Services

Before, using just DTOs, my Read Service just made flat view mapping of a database query into hash like structure with minimum normalization and no behavior.

However I tend to think of a ViewModel as something "smarter" that can have generated information not provided by the database, like status icon, calculated values, reformatted values, default values, etc.

I am starting to see that the construction of some ViewModel objects might get more complicated and has potential downsides if I made my generic ReadServiceInterface return ViewModels only:

(1) Should I plan some design restriction for the ViewModels returned by my CQRS? Like making sure that their construction is almost as fast as a plain DTO?

(2) DTOs by nature are easily serialized and ready to be sent to an external system in a SOA architecture or embedded into a message. Does this mean that using ViewModels will have a negative impact on my architecture?

(3) Which type of ViewModels should I keep outside my Read Services?

(4) Should I expect all ViewModels to be retrieved from Read Services?

In the past I implemented some ViewModels that needed more than one query. In a CQRS I suppose, that is a design smell, since everything they provide, should be in only one query.

I am starting a new project, where I thought that any query will return either aggregate objects or DTOs. Since now ViewModels come into play. I am wondering:

(5) Should I plan that queries within my architecture will yield two type of objects (ViewModels+Aggregates) or three (+DTO)?

Answer 1

View Models (VM) serve a single master: the View. We're usually consider the VM a pretty dumb object so in this regard, there's no technical difference between a VM and a DTO, only their purpose and semantics are different.

How you build a VM is an implementation detail. Some VM are pre generated and stored in a VM repository. Others are built in real-time by a service (or a query handler) either by querying the db directly or querying other repos/services then assembling the results. There's no right or wrong and no rules about how to do it. It comes down to preference.

In CQRS the important part is separation of commands from queries i.e more than one model. There's no rule about how many queries you should do or if you should return a view model or dto. As long as you have at least one read model dedicated for queries, it's CQRS.

Don't let technicalities complicate your design. Proper design is more about high level structure and not low level implementation. Use CQRS because having a read model simplifies your app, not for other reasons. Aim for simplification and clean code, not for rigid rules that dictate a 'how to' recipe.