Add support for nested Relation

[vmihailenco]

Instead of

q.Relation("Foo").Relation("Foo.Bar")

You should be able to write

q.Relation("Foo", func(q *SelectQuery) *SelectQuery {
    return q.Relation("Bar")
}).

[wavded]

@vmihailenco I ran into this when trying to migration from go-pg. I have the statement:

Relation("Task.User.id")

And it now gives me an error in "bun":

Attachment does not have relation="Task.User.id"

This is a one to one to one relationship. Is this possible with bun?

[vmihailenco]

@wavded it should be (AFAIR it is the same in pg v10)

q = db.Relation("Task.User", func(q *SelectQuery) *SelectQuery {
    return q.Column("id")
})

[wavded]

That works! Thx @vmihailenco

[metalrex100]

@vmihailenco hey! Any progress on this issue?