1 files changed, 40 insertions, 0 deletions
diff --git a/config/initializers/0_duplicate_migrations.rb b/config/initializers/0_duplicate_migrations.rb
new file mode 100644
index 000000000..4ab806587
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+++ b/config/initializers/0_duplicate_migrations.rb
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+# Some migrations have been present in glitch-soc for a long time and have then
+# been merged in upstream Mastodon, under a different version number.
+#
+# This puts us in an uneasy situation in which if we remove upstream's
+# migration file, people migrating from upstream will end up having a conflict
+# with their already-ran migration.
+#
+# On the other hand, if we keep upstream's migration and remove our own,
+# any current glitch-soc user will have a conflict during migration.
+#
+# For lack of a better solution, as those migrations are indeed identical,
+# we decided monkey-patching Rails' Migrator to completely ignore the duplicate,
+# keeping only the one that has run, or an arbitrary one.
+
+ALLOWED_DUPLICATES = [20180410220657, 20180831171112].freeze
+
+module ActiveRecord
+  class Migrator
+    def self.new(direction, migrations, target_version = nil)
+      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 || non_migrated.length == 0
+            # 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
+
+      super(direction, migrations, target_version)
+    end
+  end
+end