Projecting hierarchies with AutoMapper+QueryableExtensions - entity-framework

Question

I'm trying to project a hierarchy of entities to some DTOs using AutoMapper. I'm using EntityFramework Core (latest stable version to date).
The hierarchy is really simple:
I have a Calculation that has a collection of Parameters. Parameters optionally have a Calculation, hence the recursion.
class Calculation
{
int Id {get; set; }
ICollection<Parameter> Parameters { get; set }
}
class Parameter
{
int Id {get; set; }
Calculation Calculation { get; set }
}
I would like to project this to a tree like
Calc1
Param1
Param2
Param3
Calc2
Param4
Param4
I've defined AutoMapper maps for both Calculation and Parameter, but when I project the root Entity, I only get the first level of the hierarchy.
How do I make AutoMapper recurse projections automatically? Is there a way to do that?

Answer 1

Hierarchical queries are possible with ProjectTo but generally discouraged. Remember, ProjectTo only creates a LINQ Select expression. We can't create an infinite depth of projections, so you'll want to tell AutoMapper a MaxDepth when you configure the map for building out the hierarchy.
In practice, we do one of two things:
Explicitly model the hierarchy levels (ParentDto, GrandParentDto) for the depth we want
Use recursive CTEs to create a flat list of the entire hierarchy, then re-assemble the hierarchy in-memory
The latter won't use AutoMapper at all, but at least results in SQL that won't make your eyes bleed.