select_related not preserved in ValuesQuery

[laggron42]

Hello!

I noticed that the optimization done in ValuesQuery is a bit too agressive when it comes to related selects.

Take the following example:

from tortoise.models import Model
from tortoise import Tortoise, fields

class Author(Model):
    age = fields.IntField()

class Book(Model):
    author = fields.ForeignKeyField("models.Author")

async def main():
    await Tortoise.init(db_url='sqlite://:memory:', modules={"models": ["example"]})
    query = (
        Book.all()
        .select_related("author")
        .annotate(custom=RawSQL("book__author.age * 2 :: BIGINT"))
        .order_by("custom")
    )
    print(query.sql())
    # SELECT "book"."id","book"."author_id",book__author.age * 2 :: BIGINT "custom","book__author"."id" "book__author.id","book__author"."age" "book__author.age" FROM "book" LEFT OUTER JOIN "author" "book__author" ON "book__author"."id"="book"."author_id" ORDER BY "custom" ASC

    print(query.values("id").sql())
    # SELECT "id" "id" FROM "book" ORDER BY book__author.age * 2 :: BIGINT ASC

As you can see in the second query, the JOIN is removed which results in an operational error, as book__author doesn't exist anymore. It seems Tortoise isn't aware that I need the join, despite having requested it explicitely via select_related.

I can imagine it's hard for the library to understand which related fields are needed when RawSQL is being used (I'm forced to use it for the cast, there's no way to my knowledge to cast types ORM-style), is there a way for me to force Tortoise to make that join?

Edit: I'd rather avoid having more than one column in the .values, I'm actually using .values_list("id", flat=True), which I can't do with two values.