How to find records that have duplicate data using Active Record

What is the best way to find 开发者_JAVA百科records with duplicate values in a column using ruby and the new Activerecord?

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Translating @TuteC into ActiveRecord:

sql = 'SELECT id, 
         COUNT(id) as quantity 
         FROM types 
         GROUP BY name 
       HAVING quantity > 1'
#=>
Type.select("id, count(id) as quantity")
  .group(:name)
  .having("quantity > 1")

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Here's how I solved it with the AREL helpers, and no custom SQL:

Person.select("COUNT(last_name) as total, last_name")
  .group(:last_name)
  .having("COUNT(last_name) > 1")
  .order(:last_name)
  .map{|p| {p.last_name => p.total} }

Really, it's just a nicer way to write the SQL. This finds all records that have duplicate last_name values, and tells you how many and what the last names are in a nice hash.

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I was beating my head against this problem with a 2016 stack (Rails 4.2, Ruby 2.2), and got what I wanted with this:

> Model.select([:thing]).group(:thing).having("count(thing) > 1").all.size
 => {"name1"=>5, "name2"=>4, "name3"=>3, "name4"=>2, "name5"=>2}

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With custom SQL, this finds types with same values for name:

sql = 'SELECT id, COUNT(id) as quantity FROM types
         GROUP BY name HAVING quantity > 1'
repeated = ActiveRecord::Base.connection.execute(sql)

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In Rails 2.x, select is a private method of AR class. Just use find():

klass.find(:all, 
  :select => "id, count(the_col) as num", 
  :conditions => ["extra conditions here"], 
  :group => 'the_col', 
  :having => "num > 1")

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Here is a solution that extends the other answers to show how to find and iterate through the records grouped by the duplicate field:

duplicate_values = Model.group(:field).having(Model.arel_table[:field].count.gt(1)).count.keys
Model.where(field: duplicate_values).group_by(&:field).each do |value, records|
  puts "The records with ids #{records.map(&:id).to_sentence} have field set to #{value}"
end

It seems a shame this has to be done with two queries but this answer confirms this approach.