ruby on rails - ActiveRecord - How to `select` polymorphic join as a boolean? -

i've been banging head against wall attempting come query, figured time post here.

i have class called flair -- it's polymorphic, since lots of things can have flair:

class flair < activerecord::base   belongs_to :flairable, polymorphic: true, touch: true   belongs_to :user end 

and comment has many flairs:

class comment < activerecord::base   has_many :flairs, as: :flairable end 

when getting list of comments, want know ones have flair belongs given user.

the best i've been able come far is

# don't worry interpolation; example # , assume have valid @comments , user_id   @comments.select('comments.*').     select('flairs.id has_flaired').     joins("left join flairs on flairable_id = comments.id , flairs.user_id = #{user_id}") 

but returns 1 result every flair on comment (as expected left join), multiplying number of appearances of each comment in array number of flairs has.

i've tried using distinct on (user_id), applying limit, etc., efforts turn syntax errors.

can offer guidance? useful pointer somewhere in docs has examples bit more involved ones offered here: http://www.postgresql.org/docs/9.4/static/sql-select.html. i've tried suggestions @ http://www.postgresql.org/docs/9.4/static/queries-table-expressions.html, nothing seems stick.

thanks!

section 7.2.1.3 in www.postgresql.org/docs/9.4/static/queries-table-expressions.html , first example here, , extremely patient boss ended helping problem. in end (after banging on keyboard while), managed please postgres/activerecord gods syntactically correct query (we're sanitizing user_id before passing joins statement.

@comments.select(   "comments.*,    flairs.id has_flaired" ).joins(   "left join    flairs on flairs.id = (      select id flairs flairs.user_id = #{user_id}      , flairs.flairable_id = comments.id      limit 1    )" ) 

this works because i'm interested in whether or not row user , flairable_id exists in flairs table, check table in subquery , use results in main query.

it makes sense i've written out, i'll leave here in case else gets flummoxed.