create_join_table uses same logic as HABTM reflections

Before this change, create_join_table would not remove the common prefix
in the join table name, unlike ActiveRecord::Reflections. A HABTM
between Music::Artist and Music::Record would use a table
music_artists_records, while create_join table would create
music_artists_music_records.

activerecord/CHANGELOG.md

+*   `create_join_table` removes a common prefix when generating the join table.
+    This matches the existing behavior of HABTM associations.
+
+    Fixes #13683.
+
+    *Stefan Kanev*
+
 *   Dont swallow errors on compute_type when having a bad alias_method on
     a class.
 

activerecord/lib/active_record/migration/join_table.rb

@@ -8,7 +8,7 @@ def find_join_table_name(table_1, table_2, options = {})
       end
 
       def join_table_name(table_1, table_2)
-        [table_1.to_s, table_2.to_s].sort.join("_").to_sym
+        ModelSchema.derive_join_table_name(table_1, table_2).to_sym
       end
     end
   end

activerecord/lib/active_record/model_schema.rb

@@ -55,6 +55,17 @@ module ModelSchema
       delegate :type_for_attribute, to: :class
     end
 
+    # Derives the join table name for +first_table+ and +second_table+. The
+    # table names appear in alphabetical order. A common prefix is removed
+    # (useful for namespaced models like Music::Artist and Music::Record):
+    #
+    #   artists, records => artists_records
+    #   records, artists => artists_records
+    #   music_artists, music_records => music_artists_records
+    def self.derive_join_table_name(first_table, second_table) # :nodoc:
+      [first_table.to_s, second_table.to_s].sort.join("\0").gsub(/^(.*_)(.+)\0\1(.+)/, '\1\2_\3').gsub("\0", "_")
+    end
+
     module ClassMethods
       # Guesses the table name (in forced lower-case) based on the name of the class in the
       # inheritance hierarchy descending directly from ActiveRecord::Base. So if the hierarchy

activerecord/lib/active_record/reflection.rb

@@ -567,7 +567,7 @@ def derive_foreign_key
         end
 
         def derive_join_table
-          [active_record.table_name, klass.table_name].sort.join("\0").gsub(/^(.*_)(.+)\0\1(.+)/, '\1\2_\3').gsub("\0", "_")
+          ModelSchema.derive_join_table_name active_record.table_name, klass.table_name
         end
 
         def primary_key(klass)

activerecord/test/cases/migration/create_join_table_test.rb

@@ -119,6 +119,30 @@ def test_drop_join_table_with_column_options
 
         assert !connection.tables.include?('artists_musics')
       end
+
+      def test_create_and_drop_join_table_with_common_prefix
+        with_table_cleanup do
+          connection.create_join_table 'audio_artists', 'audio_musics'
+          assert_includes connection.tables, 'audio_artists_musics'
+
+          connection.drop_join_table 'audio_artists', 'audio_musics'
+          assert !connection.tables.include?('audio_artists_musics'), "Should have dropped join table, but didn't"
+        end
+      end
+
+      private
+
+        def with_table_cleanup
+          tables_before = connection.tables
+
+          yield
+        ensure
+          tables_after = connection.tables - tables_before
+
+          tables_after.each do |table|
+            connection.execute "DROP TABLE #{table}"
+          end
+        end
     end
   end
 end