Query: In TPH inheritance, int discriminator does not work, exception is thrown when compiling query

[Dixin]

In TPH inheritance, string discriminator works, but int discriminator does not work.

modelBuilder.Entity<SupplierTransaction>()
    .HasDiscriminator<int>("PaymentMethodID")
    .HasValue<CashTransactiony>(1)
    .HasValue<CheckTransaction>(2)
    .HasValue<CreditCardTransaction>(3)
    .HasValue<FundsTransferTransaction>(4);

When querying SupplierTansactions table, EF Core throws InvalidOperationException when compiling the query:

The binary operator Equal is not defined for the types 'System.Nullable`1[System.Int32]' and 'System.Int32'.
at System.Linq.Expressions.Expression.GetEqualityComparisonOperator(ExpressionType binaryType, String opName, Expression left, Expression right, Boolean liftToNull)
at System.Linq.Expressions.Expression.Equal(Expression left, Expression right)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.TypeIsExpressionTranslatingVisitor.<>c__DisplayClass3_0.b__0(IEntityType concreteEntityType) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 1247
at System.Linq.Enumerable.SelectListIterator`2.MoveNext()
at System.Linq.Enumerable.Aggregate[TSource](IEnumerable1 source, Func3 func)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.TypeIsExpressionTranslatingVisitor.VisitTypeBinary(TypeBinaryExpression typeBinaryExpression) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 1244
at Microsoft.EntityFrameworkCore.Query.ExpressionVisitors.ExpressionVisitorBase.Visit(Expression node) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore\Query\ExpressionVisitors\ExpressionVisitorBase.cs:line 26
at Remotion.Linq.Clauses.WhereClause.TransformExpressions(Func`2 transformation)
at Remotion.Linq.QueryModel.TransformExpressions(Func`2 transformation)
at Microsoft.EntityFrameworkCore.Query.RelationalQueryModelVisitor.VisitQueryModel(QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.Relational\Query\RelationalQueryModelVisitor.cs:line 371
at Microsoft.EntityFrameworkCore.Query.Internal.SqlServerQueryModelVisitor.VisitQueryModel(QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore.SqlServer\Query\Internal\SqlServerQueryModelVisitor.cs:line 92
at Microsoft.EntityFrameworkCore.Query.EntityQueryModelVisitor.CreateQueryExecutor[TResult](QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore\Query\EntityQueryModelVisitor.cs:line 259
at Microsoft.EntityFrameworkCore.Storage.Database.CompileQuery[TResult](QueryModel queryModel) in D:\Dixin\GitHub\EntityFramework\src\Microsoft.EntityFrameworkCore\Storage\Database.cs:line 67}

The reason is, in RelationalQueryModelVisitor.TypeIsExpressionTranslatingVisitor.VisitTypeBinary method:

var discriminatorPropertyExpression = CreatePropertyExpression(typeBinaryExpression.Expression, discriminatorProperty);
// CreatePropertyExpression calls property.ClrType.MakeNullable(), so the discriminatorPropertyExpression is of type **int?**, not int.

var discriminatorPredicate
    = concreteEntityTypes
        .Select(concreteEntityType =>
            Expression.Equal( // Exception.
                discriminatorPropertyExpression, // **int?** expression.
                Expression.Constant(_relationalAnnotationProvider.For(concreteEntityType).DiscriminatorValue))) // **int** expression.
        .Aggregate((current, next) => Expression.OrElse(next, current));

The discriminator property PaymentMethodID becomes an int? property, but the discriminator value is still int value as specified. So the Expression.Equal call throws the above exception.

Any workaround for int discriminator in TPH?

[divega]

@Dixin are you planning to send a PR with the fix?

[divega]

@Dixin also, does this repro with some specific query or with any query?

[divega]

Assigning to 1.1.1 based on the assumption that the whole scenario of integer discriminators could be broken, although we suspect there is something specific about this model or queries that make the bug surface. We can discuss moving it to a different milestone depending on those details.

[smitpatel]

Removing milestone for re-triage.
The issue mainly arises due to EF.Property expression being nullable type. If the discriminator column is of value type but in shadow state then materializer throws exception. For non-shadow discriminator, materializer would work fine but for the query with predicate Where(e => e is DerivedType) it would throw exception. The same would happen for any discriminator type as long as it is non-nullable.