Support mapping ValueTuple with row constructors

[austindrenski]

Could we map otherwise unrecogized instances of ValueTuple to row constructors?

var test = ("red", 4, 2.2);
ctx.SomeEntity.Where(x => (x.Color, x.Price, x.Tax) == test);

SELECT *
FROM "SomeEntity" as x
WHERE ROW(x."Color", x."Price", x."Tax") = @__test_0;

/cc @roji

Related

https://www.postgresql.org/docs/10/static/functions-comparisons.html#ROW-WISE-COMPARISON

[roji]

That's a very cool idea, and it also connects with another idea I had: to map PostgreSQL composites to value tuples. For some reason I got a bit excited for 4.0 and implemented support for mapping composites as dynamic - but doing value tuple seems much more useful/interesting. It may be better to first look at what this means at the Npgsql ADO.NET level, before thinking about it in EF Core.

If we do end up mapping composites to value tuple, then ROW(...) very neatly fits as the literal SQL representation, which is definitely cool. PostgreSQL also allows you to treat a row of a regular table as a composite type - SELECT * FROM data WHERE data.* = ROW(1,2) - but I can't really see EF Core generating any SQL even close to that. However, composite literals would still be useful for filtering on a composite column of a table, i.e. WHERE my_table.my_column = ROW(1,2).

So I'd option an issue on http://github.com/npgsql/npgsql/ to track composite mapping to value tuples and concentrate on that first. Once that's done, we can (finally) look at composite support in the EF Core provider (#22), and do the ROW(...) literal representation as part of that. Sounds good?