Arel-ActiveRecord:Relation: OR clause, and correlated models help

Is there any way to do cleanly in Arel and/or ActiveRecord::Relation,
the following:

1. A *complex* OR clause
    I am aware of:
    t = A.arel_table (A === model, "as" = table)
    A.where(t[:colum1].lt(x).or(t[:column2].gt(y)).to_a

    but am looking for a case where I want to write something more
complex (multiple tables) like:

    some relation: r1 = A.includes(:lots of
stuff).where(...).where(....).where(...)....
    r2 = A.includes(:lots of stuff).where(...).where(...)

    and I want r1.or.r2 , or, written: r1 | r2 ...

    exactly like r1.merge(r2) or r1 & r2 for an AND chain...

2. Doing a group clause on, for example, r2 above, where the grouping is
done by a specific column (e.g. :item), but the item column comes from
*r1*, not *r2*, and is linked together using another column (e.g.
:position).

    That is, something like (writing pseudo Arel-like code):
    r2.includes(r1.results as
special_chain).where("special_chain.position ==
as.position).group("special_chain.item").sum

... is that possible/close?

Thanks.