[RUBY] [Rails] Solution when migration error occurs in acts-as-taggable-on

Development environment

・ Ruby: 2.5.7 Rails: 5.2.4 ・ Vagrant: 2.2.7 -VirtualBox: 6.1 ・ OS: macOS Catalina


When deploying with $ rails db: migrate or Capistrano An error occurs in the migration file of ʻacts-as-taggable-on` as shown below.


StandardError: An error has occurred, all later migrations canceled:

Mysql2::Error: Cannot drop index 'index_taggings_on_tag_id': needed in a foreign key constraint: DROP INDEX `index_taggings_on_tag_id` ON `taggings`

20200525095250/db/migrate/20200516111153_add_missing_unique_indices.acts_as_taggable_on_engine.rb:11:in `up'


It seems that the cause is that the Gem creator is simply skipping. In short, this error will continue to appear for the rest of your life unless the Gem creator modifies the Gem itself to prevent this error.


ʻActs-as-taggable-on` of the migration file created at the time of installation, Comment out unnecessary code.

** * Pay attention to the file name! ** **


# This migration comes from acts_as_taggable_on_engine (originally 2)
if ActiveRecord.gem_version >= Gem::Version.new('5.0')
  class AddMissingUniqueIndices < ActiveRecord::Migration[4.2]; end
  class AddMissingUniqueIndices < ActiveRecord::Migration; end
AddMissingUniqueIndices.class_eval do
  def self.up
    # add_index ActsAsTaggableOn.tags_table, :name, unique: true

    # remove_index ActsAsTaggableOn.taggings_table, :tag_id if index_exists?(ActsAsTaggableOn.taggings_table, :tag_id)
    # remove_index ActsAsTaggableOn.taggings_table, name: 'taggings_taggable_context_idx'
    # add_index ActsAsTaggableOn.taggings_table,
    #           [:tag_id, :taggable_id, :taggable_type, :context, :tagger_id, :tagger_type],
    #           unique: true, name: 'taggings_idx'

  def self.down
    # remove_index ActsAsTaggableOn.tags_table, :name

    # remove_index ActsAsTaggableOn.taggings_table, name: 'taggings_idx'

    # add_index ActsAsTaggableOn.taggings_table, :tag_id unless index_exists?(ActsAsTaggableOn.taggings_table, :tag_id)
    # add_index ActsAsTaggableOn.taggings_table, [:taggable_id, :taggable_type, :context], name: 'taggings_taggable_context_idx'


# This migration comes from acts_as_taggable_on_engine (originally 4)
if ActiveRecord.gem_version >= Gem::Version.new('5.0')
  class AddMissingTaggableIndex < ActiveRecord::Migration[4.2]; end
  class AddMissingTaggableIndex < ActiveRecord::Migration; end
AddMissingTaggableIndex.class_eval do
  def self.up
    # add_index ActsAsTaggableOn.taggings_table, [:taggable_id, :taggable_type, :context], name: 'taggings_taggable_context_idx'

  def self.down
    # remove_index ActsAsTaggableOn.taggings_table, name: 'taggings_taggable_context_idx'

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