Order comments from certain users first, then by comment score - sql

Question

In a Rails 4 project I have articles which has_many comments.
I’m trying to show a list of comments under an article ordered by the comment score (an integer column in the comment table), but where comments from a small number of users are shown first (eg. admins & mods). These users would be passed as an array of user_ids to the query (user_id is also a column in the comment table):
class Comment < ActiveRecord::Base
# article_id :integer
# user_id :integer
# score :integer
belongs_to :article
belongs_to :user
scope :by_users, -> (user_ids) { order("user_id IN (?), score DESC", user_ids) }
Thus:
some_article.comments.by_users([1,2,3])
This scope gives a syntax error, and I can’t quite work out a query that orders comments so that ALL an article’s comments are returned, shown first from users with a user_id in the passed array, then secondarily ordered by comment score.
Update:
mahemoff’s great conditional idea doesn’t work for Postgres, but lead to this non-working attempt:
scope :by_users, -> (user_ids) { order("case when (user_id IN (?)) then 0 else 1 end, score DESC", user_ids) }
Which gives me a PostgreSQL syntax error relating to the placeholder ? that I haven’t been able to fix.

Answer 1

With mahemoff & mu is too short’s help and suggestions, this is my working scope:
scope :by_users, -> (user_ids) { order([sanitize_sql("case when (user_id IN (#{user_ids.join(',')})) then 0 else 1 end"), "score DESC" ]) }
It’s somewhat imperfect, as Rails’ order apparently doesn’t support the ? placeholder, so instead I’ve used string interpolation and (even though user_ids isn’t user exposed) sanitized the sql.
...a frustrating problem with this though, is that it’s impossible to use .last to retrieve the final record. some_article.by_users[1,2].last now throws an error as AR can’t work out where to reverse the ordering:
ORDER BY case when (user_id IN (1 DESC, 2)) then 0 else 1 end DESC, score ASC LIMIT 1
sigh...

Answer 2

Try this:
class Comment < ActiveRecord::Base
scope :by_users, -> (user_ids) { order("if(user_id IN (#{ids.join ','}), 0, 1), score DESC") }