Monkey-patch Rails' migrator to ignore duplicate migrations across Mastodon and glitch-soc

config/initializers/0_duplicate_migrations.rb

@@ -0,0 +1,28 @@
+ALLOWED_DUPLICATES = [20180410220657, 20180831171112].freeze
+
+module ActiveRecord
+  class Migrator
+    old_initialize = instance_method(:initialize)
+
+    define_method(:initialize) do |direction, migrations, target_version|
+      migrated = Set.new(Base.connection.migration_context.get_all_versions)
+
+      migrations.group_by(&:name).each do |name, duplicates|
+        if duplicates.length > 1 && duplicates.all? { |m| ALLOWED_DUPLICATES.include?(m.version) }
+          # We have a set of allowed duplicates. Keep the migrated one, if any.
+          non_migrated = duplicates.reject { |m| migrated.include?(m.version.to_i) }
+
+          if duplicates.length == non_migrated.length
+            # There weren't any migrated one, so we have to pick one “canonical” migration
+            migrations = migrations - duplicates[1..-1]
+          else
+            # Just reject every duplicate which hasn't been migrated yet
+            migrations = migrations - non_migrated
+          end
+        end
+      end
+
+      old_initialize.bind(self).(direction, migrations, target_version)
+    end
+  end
+end