I wanted to do a UNION query in Active Record, combining the results of two subqueries.

This is how I did it in Rails 4.1.5 with Postgres:

category = Category.find(123)
items = category.items
query1 = items.some_scope
query2 = items.other_scope

# Get a real category_id instead of "$1" in the generated SQL.
sql = Item.connection.unprepared_statement {
  "((#{query1.to_sql}) UNION (#{query2.to_sql})) AS items"
}

Item.from(sql).order("we_can_add_an_order_if_we_like ASC")

The generated query will be something like

SELECT items.*
  FROM ((SELECT …) UNION (SELECT …)) AS items
  ORDER BY we_can_add_an_order_if_we_like ASC;

This is of course not guaranteed to be fast just because it's raw SQL.

Consider UNION vs. UNION ALL, benchmark as needed, and use multiple queries or Ruby code if that's faster or more readable in your situation.