Cast JSON POCO to Derived Type?

[TimFeldmann-Work]

I'm not entirely sure if this is feasible, but I wanted to ask anyway just in case I was missing something obvious.

I am building a MassTransit-like package for my work. I want to give developers who use my package the ability to query a JsonDocument instance like they would with LINQ instead of relying on methods such as JsonDocument.RootElement.GetProperty("x")... for cases where Saga data needs to be manually manipulated.

I am currently relying on the JsonDocument type but wanted to see if I could make this work with the POCO mapping approach.

Here is a sample of what I am trying to achieve:

public class JsonDerivedEntity : JsonBaseEntity
{
    public int IntTest { get; set; } = 5;
}

public class BaseEntity
{
    public Guid Id { get; set; }
    [Column(TypeName = "jsonb")]
    public JsonBaseEntity JsonEntity { get; set; }

    public BaseEntity()
    {
        Id = Guid.NewGuid();
    }
}

var baseEntity = new BaseEntity();

baseEntity.JsonEntity = new JsonDerivedEntity() 
{ 
    IntTest = 11 
};

dbContext.JsonBaseEntity.Add(baseEntity);

await dbContext.SaveChangesAsync();

dbContext.ChangeTracker.Clear();

var result = await dbContext.JsonBaseEntity
    .FirstOrDefaultAsync(x => ((JsonDerivedEntity)x.JsonEntity).IntTest == 11);

The insertion portion of this code works as expected, the issue I'm having is I can't figure out if it's possible to cast the JsonBaseEntity to a derived type. My plan was to create a generic method which would cast JsonEntity to the generic type provided by the user. This would allow developers to query the JSON entity with JSON-supported LINQ as is provided by example here.

My issue is with specifying a base type on the entity, and trying to cast that type to the actual type the dev is wanting to query for. When attempting to cast the entity to a derived type, I receive the following exception:

The LINQ expression could not be translated.

Thanks for any help and suggestions on this!

[shravan2x]

@roji Perhaps the framework could check for the presence of the JsonPolymorphic attribute, and implicitly translate the query above to WHERE JsonEntity->>'type' == '<JsonDerivedEntity's_Type_from_JsonDerivedAttribute>' AND JsonEntity->>'IntTest' == 11?

[roji]

Duplicate of dotnet/efcore#27779

[roji]

This is definitely on the roadmap on the EF side (at which point it'll work also on PG), but is not implemented at this point (there's a good chance it will get done for 10).