Difference between active record and sql for with/without request

benchmark_active_record_VS_sql.rb

#In pry-rails on my rails project

require 'benchmark/ips'

Benchmark.ips do |x|
     x.report("sql request") { User.where("EXISTS (SELECT 1 FROM groups_users WHERE groups_users.user_id = users.id AND groups_users.group_id IN (?))", [8939, 8950]).where("NOT EXISTS (SELECT 1 FROM groups_users WHERE groups_users.user_id = users.id AND groups_users.group_id IN (?))", [8942]).count
     x.report("active record") { (User.joins(:groups).where(groups: {id: ["8939","8950"]}) - User.joins(:groups).where(groups: {id: 8942})).count }
     x.compare!
end

# Comparison:
#         sql request:      299.4 i/s
#       active record:      123.6 i/s - 2.42x slower

=> #<Benchmark::IPS::Report:0x007fe0517c1008
 @data=nil,
 @entries=
  [#<Benchmark::IPS::Report::Entry:0x007fe04f7a9158 @ips=299.353570090315, @ips_sd=41, @iterations=1479, @label="sql request", @measurement_cycle=29, @microseconds=5037948.6083984375, @show_total_time=false>,
   #<Benchmark::IPS::Report::Entry:0x007fe050e004b0 @ips=123.57053401687592, @ips_sd=13, @iterations=612, @label="active record", @measurement_cycle=12, @microseconds=5004271.2688446045, @show_total_time=false>]>