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class AddIndexToExerciseChoice < ActiveRecord::Migration[5.2]
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def change
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change_column_default :exercise_answers, :exercise_choice_id, from: nil, to: -1
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ExerciseAnswer.where(exercise_choice_id: nil).update_all(exercise_choice_id: -1)
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sql = %Q(delete from exercise_answers where (exercise_question_id, user_id, exercise_choice_id) in
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(select * from (select exercise_question_id, user_id, exercise_choice_id from exercise_answers group by exercise_question_id, user_id, exercise_choice_id having count(*) > 1) a)
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and id not in (select * from (select min(id) from exercise_answers group by exercise_question_id, user_id, exercise_choice_id having count(*) > 1 order by id) b))
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ActiveRecord::Base.connection.execute sql
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add_index :exercise_answers, [:exercise_question_id, :user_id, :exercise_choice_id], name: 'exercise_choice_index', unique: true
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remove_index :exercise_answers, name: :index_on_question_id_user_id
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end
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end
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