Support $apply for EF Core

[kosinsky]

Issues

This pull request fixes issues #1154 and #1257. Contributes to #669

Description

We are generating expressions that look like

Next = new AggregationPropertyContainer() {
       Name = "Prop2",
       Value = $it.Prop2,
       Next = …
}                    

Value type is object, as a result proper .NET expression must look like as

Next = new AggregationPropertyContainer() {
       Name = "Prop2",
       Value = (object) $it.Prop2,
       Next = …
}    

or AccessViolationException will be thrown when executing on real objects. On other hand, casts to object isn't translatable by EF6 as a result skipping (object) in that case.
We used to do that adjustment for Linq to Objects, EF Core could execute some parts client side and uses linq to object as result.

Also had to tune how aggregation on top of IGrouping works. Linq to Objects/EF Core and EF 6 has different limitations

Checklist (Uncheck if it is not completed)

• Test cases added
• Build and test with one-click build and test script passed

[jannikbuschke]

@kosinsky @robward-ms is there anything stopping from releasing this fix? Im eagerly waiting for $apply 👍

[madansr7]

@jannikbuschke, we have been working to release a few important updates in ODL so haven't reviewed this one yet. WebAPI's next release would be in march and i am hoping this PR will be part of that release.

[xuzhg]

It seems good to me. @mikepizzo @KanishManuja-MS Would you take a look?

[mikepizzo]

Note that we effectively switch the default here, since _linqToObjectMode has the same default value, but the opposite behavior, of _classicEF. That's fine, as long as bind is called to explicitly set this value. I assume there are no scenarios where this value is used without first calling bind? #Closed

[kosinsky]

New default behavior is correct form .NET point of view. Expression like: object a = 15; must look like object a = (object) 15; on MSIL to do proper boxing or AccessViolationException will be thrown. Unfortunately, EF6 can't translate that to SQL. So we have to do hack for EF6. The same for IEnumerable and IGroupingBy

#Closed

[kosinsky]

@mikepizzo do you have any other feedback to this PR?

[mikepizzo]

[TehWardy]

Is this part of the current version of Microsoft.AspNetCore.OData ?
I'm using version 7.4.1 at the moment and I can't seem to get any form aggregation / apply operation working.

[TehWardy]

It looks like the limitation here may be in EF in my case ...

based on the above my query:

?$apply=groupby((DebtorName),aggregate($count as Count))

should be fine within odata, the exception given (as shown below) implies that this is a known issue with EF ...

2020-08-07 20:08:29,196 [9] ERROR  Api.IAppBuilderExtensions - Processing of the LINQ expression '(GroupByShaperExpression:
KeySelector: new GroupByWrapper{ GroupByContainer = new LastInChain{ 
        Name = (N'DebtorName'), 
        Value = (a.DebtorName) 
    }
     }
, 
ElementSelector:(EntityShaperExpression: 
    EntityType: ActiveTransaction
    ValueBufferExpression: 
        (ProjectionBindingExpression: EmptyProjectionMember)
    IsNullable: False
)
)' by 'RelationalProjectionBindingExpressionVisitor' failed. This may indicate either a bug or a limitation in EF Core. See https://go.microsoft.com/fwlink/?linkid=2101433 for more detailed information.