```ruby
class AddMissingUniqueIndices < ActiveRecord::Migration

  def self.up
    add_index :tags, :name, unique: true

    remove_index :taggings, :tag_id
    remove_index :taggings, [:taggable_id, :taggable_type, :context]
    add_index :taggings, [:tag_id, :taggable_id, :taggable_type, :context, :tagger_id, :tagger_type], unique: true, name: 'tagging_idx'
   end

  def self.down
    remove_index :tags, :name

    remove_index :taggings, name: 'tagging_idx'
    add_index :taggings, :tag_id
    add_index :taggings, [:taggable_id, :taggable_type, :context]
  end

end
```

Format new migration so easier to read
user
```ruby
class AddMissingUniqueIndices < ActiveRecord::Migration

  def self.up
    add_index :tags, :name, unique: true

    remove_index :taggings, :tag_id
    remove_index :taggings, [:taggable_id, :taggable_type, :context]
    add_index :taggings,
      [:tag_id, :taggable_id, :taggable_type, :context, :tagger_id, :tagger_type],
      unique: true, name: 'taggings_idx'
   end

  def self.down
    remove_index :tags, :name

    remove_index :taggings, name: 'tagging_idx'
    add_index :taggings, :tag_id
    add_index :taggings, [:taggable_id, :taggable_type, :context]
  end

end
```
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