EF Core and ProjectToType eager loader known issue

[savornicesei]

Hi all,

Given the AutoMapper recent drama I've found out about this known issue and found the mention in the docs.

I'm looking for more context to it, if it's possible - root cause and limitations required to make it work properly.

Is it specific just to EF Core or it's affecting EF6 as well? We're using EF6 and didn't notice it.

Looking at Maperly, their ProjectToType extension has a pretty long list of constraints

Since queryable projection mappings use System.Linq.Expressions.Expression under the hood, such mappings have several limitations:

Object factories are not applied
Constructors with unmatched optional parameters are ignored
ThrowOnPropertyMappingNullMismatch is ignored
AllowNullPropertyAssignment is ignored
Enum mappings do not support the ByName strategy
Reference handling is not supported
Nullable reference types are disabled
Deep cloning is not applied

Thanks.

All the best,
Simo

[DocSvartz]

@savornicesei
In the specified issue I believe there are at least two problems, perhaps they are even related.

1. Loading related entities similar to .Include(x=>x.Property) when there was no manual indication of this.
2. Custom .Include() is not working.

example of custom Include:

.Include(x => 
                x.SomeOtherEntity
                    .OrderByDescending(p => p.Date)
                    .Take(1))

[DocSvartz]

On first case

1. Loading related entities similar to .Include(x=>x.Property) when there was no manual indication of this.

I'm not sure, but if EF6 doesn't have this problem, then it looks like EFCore changed the handling of these expressions.

Using this .ProjectToType()
If you don't explicitly specify to Ignore any member of MyModel.
You in fact say - Give me all the members of the source (which matched by name) that are so that I can get the result.

And it actually starts doing that)

[savornicesei]

I can double check it on EF6. Didn't noticed it but I also did not look for it

[DocSvartz]

Yes, if possible double check.
I only have EFCore projects now

[DocSvartz]

On 2 case:
It wil be work when it use this in config

TypeAdapterConfig<Case, Case>
      .NewConfig()
      .Map(x => x.Statusstorages,y=>y.Statusstorages.OrderByDescending(x=>x.StatusId).Take(1))
      .IgnoreNonMapped(true);

Then you will get a SQL query that does both OrderByDescending and Select

It seems EFcore can't merge its own settings with the additional ones from ProjectToType.

(It's more of an override, but if you look at the IQueryable debugging, there is a custom Include function there, but it seems to be located in the wrong place)

Although if it didn't work in EF6 - then it's a ProjectToType problem (we need to think about what can be done here)

[DocSvartz]

ProjectToType equal .Select() in EF6 and EFCore

Both described problems are related to how .Select() works:

1. Everything you specify will be loaded;
2. If you need to perform any actions (Sorting and other) on Related Entities, this must be specified in the Select call;

[DocSvartz]

@andrerav @stagep
I think You can make a separate extension .ProjectToType in Mapster.EFCore that can fix this.

1) By default it will map only primitive types and those members that are directly specified in the Mapping configuration;

1. By default, Collections are not automatically mapped. (need using Include or Explicitly specify the mapping in the configuration) [Updated]
2. Use the expression from Include as a source for the mapping configuration - it Work
   (maybe and other loading) - not testing

(It will be necessary to recognize types from EF, so it will not be possible to include this in the main Mapster) - It's private Type,
but we need to think about the transfer in base lib Mapster [Updated]

What do you think about this?

[DocSvartz]

MVP The prototype is basically ready :)
PR #787