Efcore: SQLite Query: Cast from decimal to double not translated

[NArnott]

When using Sqlite as a database for EFCore, the following expression doesn't work:

```c#
Where(x => x.Price > -1)

This expression will return items where Price is -5. One workaround is to use Decimal.Negate and reverse the expression, but that shouldn't be needed. Both methods work fine with using SQL as the database. 

### Steps to reproduce
Given Db Model Item:

```c#
class Item
{
    public long Id { get; set; }

    public string Name { get; set; }

    public decimal Price { get; set; }
}

and db context

```c#
class TestDbContext : DbContext
{
public TestDbContext(DbContextOptions options) : base(options) { }

public DbSet<Item> Items { get; set; }

}

and the following repo:

```c#
static async Task Main()
{
    var connection = new SqliteConnection("DataSource=:memory:");

    connection.Open();

    var options = new DbContextOptionsBuilder<TestDbContext>()
                        .UseSqlite(connection)
                        .Options;

    var context = new TestDbContext(options);

    context.Database.EnsureCreated();

    var items = new List<Item>
    {
        new Item{Id = 1, Name = "Item 1", Price = 100},
        new Item{Id = 2, Name = "Item 2", Price = 10},
        new Item{Id = 3, Name = "Item 3", Price = 0},
        new Item{Id = 4, Name = "Item 4", Price = -1},
        new Item{Id = 5, Name = "Item 5", Price = -5}
    };

    context.Items.AddRange(items);
    context.SaveChanges();

    var query1 = context.Items.Where(x => x.Price > 0).Select(x => x.Name);
    var results1 = await query1.ToArrayAsync();
    Console.WriteLine($"Items with price greater than 0 - {string.Join(',', results1)}");

    var query2 = context.Items.Where(x => x.Price > -1m).Select(x => x.Name);
    var results2 = await query2.ToArrayAsync();
    Console.WriteLine($"Items with price greater than -1 - {string.Join(',', results2)}");

    var query3 = context.Items.Where(x => decimal.Negate(x.Price) < 1).Select(x => x.Name);
    var results3 = await query3.ToArrayAsync();
    Console.WriteLine($"Items with price greater than -1 - {string.Join(',', results3)}");

    Console.ReadLine();
}

The result of query2 produces Item 1, 2, 3 and 5, where only Item 1, 2, 3 are expected.

c# Console.WriteLine("Hello World!");

Further technical details

EF Core version: 2.0.01
Database Provider: Microsoft.EntityFrameworkCore.Sqlite 2.0.1)
Operating system: Windows 10
IDE: Visual Studio 2017 15.5.2

[bricelam]

Duplicate of #10265

You can mitigate it by casting to a double.

.Where(x => (double)x.Price > -1)

[bricelam]

See the discussion in #10249 for more context.

[NArnott]

@bricelam casting to double doesn't seem to work. In fact, it makes it worse. I get all 5 items returned instead of the expected 3 items (or the unexpected 4 items when I don't cast as double).

[bricelam]

You're right. 🙁 The cast is getting lost. (Same for OrderBy().)

SELECT "x"."Name"
FROM "Items" AS "x"
WHERE "x"."Price" > -1

[bricelam]

Workaroud

context.Items.FromSql("SELECT * FROM Items WHERE CAST(Price AS REAL) > -1").Select(x => x.Name);

[ajcvickers]

Notes from triage: this is a case where the convert should not be removed since it contains semantic information needed for an appropriate translation. This is a specific case of the more general issue described in #10265 where, depending on how a type/value is mapped to the store, the translation may need to be changed or client eval may need to be triggered.

[smitpatel]

8861 added ability to introduce casting for projection but not for other places.

[divega]

I discussed this issue with @smitpatel. It seems we have only thought about fixing for 3.0 by preserving the convert node if you have an explicit cast to double in .Where(x => (double)x.Price > -1) but without the cast this would still return incorrect results.

@smitpatel tells me that if there is no explicit cast, to avoid returning incorrect results (we can client-eval, throw or add the cast to REAL automatically), we would need additional information from the type mapping to know that decimal properties cannot be correctly compared on the server on SQLite, which is covered by #10249.

I have removed #10249 from the backlog to discuss it.

[bricelam]

I think I already updated this to client-eval in 2.2 when there’s no cast.

[smitpatel]

Generated SQL in new pipeline

SELECT "p"."ProductID", "p"."Discontinued", "p"."ProductName", "p"."SupplierID", "p"."UnitPrice", "p"."UnitsInStock"
FROM "Products" AS "p"
WHERE CAST("p"."UnitPrice" AS REAL) > -1.0

[bricelam]

My hero! lol, I'm so happy with all the improvements around types in 3.0