```ruby
class UniqueIndexPostVisibilities < ActiveRecord::Migration
  def self.up
    sql = <<-SQL
    SELECT `post_visibilities`.post_id, `post_visibilities`.aspect_id FROM `post_visibilities`
      GROUP BY post_id, aspect_id
        HAVING COUNT(*)>1;
    SQL

    result = execute(sql)
    dup_pvs = result.to_a
    undesired_ids = []

    dup_pvs.each do |arr|
      post_id, aspect_id = arr
      pv_ids = execute("
        SELECT `post_visibilities`.id FROM `post_visibilities`
        WHERE `post_visibilities`.post_id = #{post_id}
          AND `post_visibilities`.aspect_id = #{aspect_id};"
      ).to_a.flatten!
      pv_ids.pop
      undesired_ids.concat(pv_ids)
    end
    execute("DELETE FROM `post_visibilities` WHERE `post_visibilities`.id IN (#{undesired_ids.join(",")});") unless undesired_ids.empty?


    remove_index :post_visibilities, [:aspect_id, :post_id]
    add_index :post_visibilities, [:aspect_id, :post_id], :unique => true
  end

  def self.down
    remove_index :post_visibilities, [:aspect_id, :post_id]
    add_index :post_visibilities, [:aspect_id, :post_id]
  end
end
```

Abort migration if there are post-visibilities left, so that the db is not left with no index
user
```ruby
class UniqueIndexPostVisibilities < ActiveRecord::Migration
  def self.up
    sql = <<-SQL
    SELECT `post_visibilities`.post_id, `post_visibilities`.aspect_id FROM `post_visibilities`
      GROUP BY post_id, aspect_id
        HAVING COUNT(*)>1;
    SQL

    result = execute(sql)
    dup_pvs = result.to_a
    undesired_ids = []

    dup_pvs.each do |arr|
      post_id, aspect_id = arr
      pv_ids = execute("
        SELECT `post_visibilities`.id FROM `post_visibilities`
        WHERE `post_visibilities`.post_id = #{post_id}
          AND `post_visibilities`.aspect_id = #{aspect_id};"
      ).to_a.flatten!
      pv_ids.pop
      undesired_ids.concat(pv_ids)
    end
    execute("DELETE FROM `post_visibilities` WHERE `post_visibilities`.id IN (#{undesired_ids.join(",")});") unless undesired_ids.empty?

    new_result = execute(sql)
    raise "Not all violating visibilities deleted" unless new_result.to_a.empty?

    remove_index :post_visibilities, [:aspect_id, :post_id]
    add_index :post_visibilities, [:aspect_id, :post_id], :unique => true
  end

  def self.down
    remove_index :post_visibilities, [:aspect_id, :post_id]
    add_index :post_visibilities, [:aspect_id, :post_id]
  end
end
```
assistant