What is required for EF Core's IMethodCallTranslator to work with `EF.Functions` to provide a custom function?

Question

I'm trying to implement a custom IMethodCallTranslator in EF Core 3.1 with the Sqlite provider.

I have created:

1. An extension method off of this DbFunctions which is called at query time
2. An implementation of IMethodCallTranslator which is Translate not called
3. A derived RelationalMethodCallTranslatorProvider which I'm passing an instance of my IMethodCallTranslator. This constructor is hit. I also tried overriding RelationalMethodCallTranslatorProvider.Translate and this was not hit.
4. An implementation of IDbContextOptionsExtension (and its info class) which registers the RelationalMethodCallTranslatorProvider as a singleton IMethodCallTranslatorProvider.

All of this is added via OnConfiguring by getting the IDbContextOptionsBuilderInfrastructure form of the options.

Am I missing something? I've tried following: https://github.com/tkhadimullin/ef-core-custom-functions/tree/feature/ef-3.1-version in my code, yet it's invoking the extension method, not my translator.

Answer 1

Apparently, you can't pass a reference to your full entity class to the function for further processing:

Doesn't work:

public static string DoWork(this DbFunctions _, object entity, string key)
//...
dbContext.Items.Select(i => new { Entity = i, String = EF.Functions.DoWork(i, i.StringValue) });

Where this will:

public static string DoWork(this DbFunctions _, string key)
//...
dbContext.Items.Select(i => new { Entity = i, String = EF.Functions.DoWork(i.StringValue) });

Also, generics are supported so it would be a nice way to introspect call and get the type of the entity for the model in a RelationalMethodCallTranslatorProvider.

public static string DoWork<T>(this DbFunctions _, string key)
//...
dbContext.Items.Select(i => new { Entity = i, String = EF.Functions.DoWork<Item>(i.StringValue) });