How to Select record with the maximum value in a GROUP BY?

MyModel.where("place_id=?",place_id).
        group("place_id,user_id").order("count(user_id) DESC").first

MyModel.select("DISTINCT(place_id)").
        merge(MyModel.group("place_id,user_id").order("count(user_id) DESC"))

Movie.joins(:datapoints).where(datapoints: {timestamp: (Time.now)..(Time.now+1.year)}).group(:genre).maximum(:cumulative_downloads)

rel = Movie.joins(:datapoints).where(datapoints: {timestamp: (Time.now)..(Time.now+1.year)}).group(:genre)
mins = rel.minimum(:cumulative_downloads)
maxs = rel.maximum(:cumulative_downloads)
res = {}
maxs.each{|k,v| res[k] = v-mins[k]}

# get the genre and diff per movie
result = Movie.select('movies.genre, MAX(datapoints.cumulative_downloads)-MIN(datapoints.cumulative_downloads) as diff').joins(:datapoints).group(:movie_id) 
# sum the diffs per genre
per_genre = Hash.new(0)
result.each{|m| per_genre[m.genre] += m.diff}

# get the genre and diff per movie
result = Movie
    .select('movies.movie_id, movies.genre, MAX(datapoints.cumulative_downloads)-MIN(datapoints.cumulative_downloads) as diff')
    .joins(:datapoints)
    .group('movies.movie_id, movies.genre') 
# sum the diffs per genre
per_genre = Hash.new(0)
result.each{|m| per_genre[m.genre] += m.diff}

SELECT user_id, MAX(score) FROM table GROUP BY user_id ORDER BY MAX(score) DESC LIMIT 3;

With CTE AS (SELECT ProjectName, Origin, Week, count(projectname) as myCount
    FROM FOO
    GROUP BY ProjectName, Origin, week)
SELECT A.ProjectName, A.Origin, A.Week, A.MyCount
FROM CTE A
INNER JOIN (SELECT ProjectName, Origin, Max(myCount) MaxMyCount 
            FROM CTE
            GROUP BY ProjectName, Origin) B
   on A.Projectname = B.ProjectName
  and A.Origin = B.Origin
  and A.myCount = B.MaxMyCount
ORDER BY ProjectName, Origin, Week

SELECT x.* 
  FROM my_table x 
  JOIN 
     ( SELECT discount,MIN(price) min_price FROM my_table GROUP BY discount) y 
    ON y.discount = x.discount 
   AND y.min_price = x.price;