The peculiarities of Relation#distinct and Relation#count

generate.rb

require 'active_record'

ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')

ActiveRecord::Schema.define do
  create_table :activities do |t|
    t.string :day
    t.string :name
  end
end

class Activity < ActiveRecord::Base
end

{
  monday: 'running',
  tuesday: 'biking',
  wednesday: 'swimming',
  thursday: 'running',
  friday: 'biking',
  saturday: nil,
  sunday: nil,
}.each do |day, name|
  4.times do
    Activity.create! day: day, name: name
  end
end

relations = {
  'select(:name)' => -> { Activity.select(:name) },
  'select(:name).distinct' => -> { Activity.select(:name).distinct },
  'all' => -> { Activity.all },
  'distinct' => -> { Activity.distinct },
  'select(:name, :day)' => -> { Activity.select(:day, :name) },
  'select(:name, :day).distinct' => -> { Activity.select(:day, :name).distinct }
}

methods = {
  'count' => -> (r) { r.count },
  'count(:all)' => -> (r) { r.count(:all) },
  'size' => -> (r) { r.size },
  'to_a.size' => -> (r) { r.to_a.size },
}

print "| %-028s |" % ''
methods.keys.each do |key|
  print " %-11s |" % key
end
puts

relations.each do |relation_name, relation|
  print "| %-028s |" % relation_name
  methods.each do |method_name, method|
    begin
      print "%12d |" % method.call(relation.call)
    rescue
      print "%12s |" % '<fail>'
    end
  end
  puts
end

output.txt

|                              | count       | count(:all) | size        | to_a.size   |
| select(:name)                |          20 |          28 |          28 |          28 |
| select(:name).distinct       |           3 |          28 |          28 |           4 |
| all                          |          28 |          28 |          28 |          28 |
| distinct                     |          28 |          28 |          28 |          28 |
| select(:name, :day)          |      <fail> |          28 |          28 |          28 |
| select(:name, :day).distinct |      <fail> |          28 |          28 |           7 |