Bump acts-as-taggable-on to 6.0.0

Rails 5.1 introduced certain changes in the way a record is touched when
the counter cache option is enabled in a belongs to association.

We need to upgrade acts-as-taggable-on so it keeps changing the
`updated_at` attribute when a new tag is added to a record.

Note we now need to reload the records in some cases to get the
`context_tag_list` method to return what we expect. Methods like
`context_tags` however work properly with no need to reload the record.

db/migrate/20190619133544_add_missing_unique_indices.acts_as_taggable_on_engine.rb

@@ -0,0 +1,17 @@
+# 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
+else
+  class AddMissingUniqueIndices < ActiveRecord::Migration; end
+end
+AddMissingUniqueIndices.class_eval do
+  def self.up
+    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 :taggings, name: "taggings_idx"
+  end
+end

db/migrate/20190619133547_add_missing_indexes_on_taggings.acts_as_taggable_on_engine.rb

@@ -0,0 +1,22 @@
+# This migration comes from acts_as_taggable_on_engine (originally 6)
+if ActiveRecord.gem_version >= Gem::Version.new("5.0")
+  class AddMissingIndexesOnTaggings < ActiveRecord::Migration[4.2]; end
+else
+  class AddMissingIndexesOnTaggings < ActiveRecord::Migration; end
+end
+AddMissingIndexesOnTaggings.class_eval do
+  def change
+    add_index :taggings, :taggable_id unless index_exists? :taggings, :taggable_id
+    add_index :taggings, :taggable_type unless index_exists? :taggings, :taggable_type
+    add_index :taggings, :tagger_id unless index_exists? :taggings, :tagger_id
+    add_index :taggings, :context unless index_exists? :taggings, :context
+
+    unless index_exists? :taggings, [:tagger_id, :tagger_type]
+      add_index :taggings, [:tagger_id, :tagger_type]
+    end
+
+    unless index_exists? :taggings, [:taggable_id, :taggable_type, :tagger_id, :context], name: "taggings_idy"
+      add_index :taggings, [:taggable_id, :taggable_type, :tagger_id, :context], name: "taggings_idy"
+    end
+  end
+end

db/schema.rb

@@ -1390,8 +1390,15 @@ ActiveRecord::Schema.define(version: 20191108173350) do
     t.integer "tagger_id"
     t.string "context", limit: 128
     t.datetime "created_at"
+    t.index ["context"], name: "index_taggings_on_context"
+    t.index ["tag_id", "taggable_id", "taggable_type", "context", "tagger_id", "tagger_type"], name: "taggings_idx", unique: true
     t.index ["tag_id"], name: "index_taggings_on_tag_id"
     t.index ["taggable_id", "taggable_type", "context"], name: "index_taggings_on_taggable_id_and_taggable_type_and_context"
+    t.index ["taggable_id", "taggable_type", "tagger_id", "context"], name: "taggings_idy"
+    t.index ["taggable_id"], name: "index_taggings_on_taggable_id"
+    t.index ["taggable_type"], name: "index_taggings_on_taggable_type"
+    t.index ["tagger_id", "tagger_type"], name: "index_taggings_on_tagger_id_and_tagger_type"
+    t.index ["tagger_id"], name: "index_taggings_on_tagger_id"
   end
 
   create_table "tags", id: :serial, force: :cascade do |t|