How to find records that have duplicate data using Active Record

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刺人心
刺人心 2021-01-31 04:12

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

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  • 2021-01-31 04:13

    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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  • 2021-01-31 04:17

    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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  • 2021-01-31 04:21

    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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  • 2021-01-31 04:32

    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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  • 2021-01-31 04:32

    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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  • 2021-01-31 04:33

    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.

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