There are significant problems when executing queries that either convert nulls in the database to non-nulls in code or vice-versa. Therefore, we have marked this feature as internal for EF Core 6.0. You can still use it, but you will get a compiler warning. The warning can be disabled with a #pragma. See #26230 for more information.

When using FromSql to create a custom filtered data set, any column conversions defined on the DbQuery object don't get applied.

I think the column conversions should be included automatically around the sub-query supplied to FromSql, similar to how a sub-query is used if more predicates are added to the IQueryable.

    public class Model
    {
        public int Id { get; set; }
        public bool Flag { get; set; }
    }

    public partial class MyDbContext : DbContext
    {
        public DbQuery<Model> Models { get; set; }

        protected override void OnModelCreating(ModelBuilder modelBuilder)
        {
                    modelBuilder.Query<Models>(
                        entity =>
                        {
                            entity.ToView("dbo.Models");
                            // Convert null Flag to false on way in
                            entity.Property(e => e.Flag).HasConversion<bool?>(f => f, t => t ?? false);
                        });
        }
    }

    // ...

    var models = dbContext.Models.FromSql("SELECT * FROM dbo.Models").ToList();

This, I think, should result in SQL similar to this being executed...

SELECT [e].[Id], COALESCE([e].[Flag], 0) AS [Flag]
FROM (
    SELECT * FROM dbo.Models) AS [e];

But, in reality, the plain SQL from FromSql is executed

SELECT * FROM dbo.Models

EF Core version: 2.1.4
Database Provider: Microsoft.EntityFrameworkCore.SqlServer
Operating system: Windows 7
IDE: Visual Studio 2017 15.8.5