Using Func<> for where predicate does not work

[bastien-besson]

I'm trying to retrieve specific data from my table using a predicate. The researched sql query is as follow :
SELECT [plan].[Id] FROM [Plan] AS [plan] WHERE [plan].[IdCampagne] = 39

Plan is a table with columns

• Id [int]
• IdCampagne[int]
• OtherStuff [int]

Steps to reproduce

1. WORKING CODE

 var a = _context.Plan.AsNoTracking()
                    .Where(plan => plan.IdCampagne == 39)
                    .Select(plan => new { Id = plan.Id })
                    .ToList();

gives :
SELECT [plan].[Id] FROM [Plan] AS [plan] WHERE [plan].[IdCampagne] = 39

2. NOT WORKING CODE

Func<Plan, bool> predicate = p => p.IdCampagne == 39;
var a = _context.Plan.AsNoTracking()
        .Where(predicate)
        .Select(plan => new { Id = plan.Id })
        .ToList();

gives :
SELECT [p].[Id], [p].[IdCampagne], [p].[OtherStuff] FROM [Plan] AS [p]

Further technical details

EF Core version: 2.1.1
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows Server 2012 R2 Standard
IDE: Visual Studio 2017 15.7.5

[smitpatel]

You need to create lambda of Expression<Func<Plan, bool>>. Raw Func<> are opaque in expression tree and we cannot look what is inside. Using Expression<Func<>> integrate the lambda inside the expression with inner body.

[bastien-besson]

Thanks @smitpatel for your fast answer.

I feel like the understanding of this behavior is far from evident, and as in one case the whole table is retrieved and in the other not, I guess this will cause problems in many architecture.
Isn't there maybe a way to promote a better understanding of this difference ?

[smitpatel]

The difference has nothing specific to EF Core as such. It is about difference in IQueryable vs IEnumerable. For any translation framework to work correctly, you would require IQueryable (Expression<Func<>>) type constructs. Above query would fail to work correctly even in other ORMs or linq to SQL translations.
If you have ideas about how to educate customers about this then please share.