Arel: active relation from Arel::SelectManager with join - ruby-on-rails-4

Question

Let us we have a Rails 4.2.x app and we have two tables posts and authors, and we want to use Arel to get the posts authored by an author with name == 'Karl'.
(In this case we could be happy with Active Record joins but this is just to keep the example simple.)
posts = Arel::Table.new :posts
authors = Arel::Table.new :authors
my_query = posts.project(Arel.star)
.join(authors)
.on(posts[:author_id].eq(authors[:id]))
.where(authors[:name].eq('Karl'))
> my_query.class
=> Arel::SelectManager
Now we could get back an array (of class Array) of posts by doing:
> Post.find_by_sql my_query
[master] Post Load (3.1ms) SELECT * FROM "posts" INNER JOIN "authors"
ON "posts"."author_id" = "authors"."id"
WHERE "authors"."name" = 'Karl'
=> [#<Post:0x005612815ebdf8
id: 7474,
...
]
So we do get an array of posts, not an active record relation:
> Post.find_by_sql(my_query).class
=> Array
Also injecting the manager into Post.where won't work
> Post.where my_query
=> #<Post::ActiveRecord_Relation:0x2b13cdc957bc>
> Post.where(my_query).first
ActiveRecord::StatementInvalid: PG::SyntaxError:
ERROR: subquery must return only one column
SELECT "posts".* FROM "posts"
WHERE ((SELECT * FROM "posts" INNER JOIN "authors" ON "posts"."author_id" = "authors"."id" WHERE "authors"."name" = 'Karel'))
ORDER BY "posts"."id" ASC LIMIT 1
I am thinking I must be missing something. In short: how do you get an active record relation from a select manager like my_query above (or another select manager accomplishing the same thing).

Answer 1

You can't get ActiveRecord::Relation from Arel::SelectManager neither from sql string. You have two ways to load data through ActiveRecord:
Do all query logic in Arel. In this case you can't use any of ActiveRecord::Relation methods. But you have same functionality in Arel. In your example you may set limit through Arel:
my_query.take(10)
Other way is to use Arel in ActiveRecord::Relation methods. You may rewrite your query like this:
posts = Arel::Table.new :posts
authors = Arel::Table.new :authors
join = posts.join(authors).
on(posts[:author_id].eq(authors[:id])).
join_sources
my_query = Post.
joins(join).
where(authors[:name].eq('Karl'))
> my_query.class
=> ActiveRecord::Relation
In this case you may use my_query as ActiveRecord::Relation