def _(expr)
      expr = Arel.sql(expr.to_sql) if expr.is_a? ::ActiveRecord::Relation
      Nodes.wrap Arel::Nodes::Grouping.new(expr)
    end
def _(expr)
      expr = Arel.sql(expr.to_sql) if expr.is_a? ::ActiveRecord::Relation
      Nodes.wrap Arel::Nodes::Grouping.new(expr)
    end
Create a Grouping node. This allows you to set balanced
 pairs of parentheses around your SQL.

 ==== Arguments

 * +expr+ - The expression to group.

 ==== Example
     Post.where.has{_([summary, description]).in(...)}
     #=> SELECT "posts".* FROM "posts" WHERE ("posts"."summary", "posts"."description") IN (...)"
     Post.select{[id, _(Comment.where.has{post_id == posts.id}.selecting{COUNT(id)})]}.as('comment_count')}
     #=> SELECT "posts"."id", (SELECT COUNT("comments"."id") FROM "comments" WHERE "comments.post_id" = "posts"."id") AS "comment_count" FROM "posts"