IQueryable 投影到 DTO - 在这种情况下自动映射器可以提供帮助吗？

Question

I'm looking at some C# WebAPI code for a project I just got brought onto and notice the general pattern is like this for data access:

1. The controller makes a call directly to a repository asking for data. The controller's return type is IQueryable where T is a DTO (not an entity)
2. The repository uses dbContext to select one or more entities (joins, etc) and then does a projection using select to cast it inline into an iqueryable of the relevant DTO. This is all done inline and sometimes it takes up quite a lot of lines of code.
3. This IQueryable is returned to the controller, who then returns the data back out.

So I'm concerned about a few things here. For one, shouldn't something like AutoMapper be used to handle this so each specific repository isn't flooded with these rather large projections from one (or several) entities to one DTO?

Again, keep in mind most of the time, it's not one entity (ie User -> UserDto) but several entities mapping to one flattened DTO. So for example Class1, Class2, Class3, Class4 all map to AggregatedClassDto. Should this be done in the repository layer at all, and if so, is Automapper appropriate in this case?

If I move the mapping logic out somewhere else, I think it's all going to be custom (almost nothing from a property/field perspective translates 1-1). Doesn't that just move all the code to lots of custom converters or something?

Finally, any other areas of concern?

Thanks!

Answer 1

AutoMapper can certainly handle that using AutoMapper.QueryableExtensions .

You can define pretty complex mappings even if your classes and properties don't map one-to-one. You can read more about queryable extensions on the AutoMapper Wiki .

As far as having the mappings done inside the repository, speaking from a single-responsibility point of view, you probably shouldn't perform mappings inside your repository and return entities instead.

Answer 2

I'm adding this as a separate answer since I do not fully agree with the first answer.

I agree on the point that mappings should not be done inside the repository, but I do not agree that all mappings should be done by AutoMapper since there is a conceptual difference between a DTO and an object , being that a DTO is a data transfer object and strictly speaking isn't always created by you, and therefore should not be trusted. In my experience it is better to convert DTOs to POCOs manually rather than automap them (the other way, sure, go ahead). This blog-post quite well describes my opinion on the matter.