Remove dangling changesets

As the removed const requires us to delete filesystem repositories
directly in SQL, we need to manually remove associated changesets
that would otherwise remain broken in the database and may cause errors
when trying to access them (e.g., throught the revisions API endpoint).

This commit destroys all changesets of a filesystem repository before
destroying the repository itself.

db/migrate/20150716163704_remove_filesystem_repositories.rb

@@ -30,12 +30,31 @@
 # Removes all remaining Repository::Filesystem entries.
 #
 class RemoveFilesystemRepositories < ActiveRecord::Migration
+  include Migration::Utils
+
   def up
-    # Circumvent Repository.where(...) since this tries to load
-    # the Repository::Filesystem constant and fails.
-    ActiveRecord::Base.connection.execute <<-SQL
-      DELETE FROM repositories
-      WHERE type = #{quote_value("Repository::Filesystem")}
-    SQL
+
+    ActiveRecord::Base.transaction do
+      # Delete any changesets belonging to filesystem repositories
+      ActiveRecord::Base.connection.execute <<-SQL
+        DELETE FROM changesets
+        WHERE repository_id IN (
+          SELECT id
+          FROM repositories
+          WHERE type = #{filesystem_type}
+        )
+      SQL
+
+      # Circumvent Repository.where(...) since this tries to load
+      # the Repository::Filesystem constant and fails.
+      ActiveRecord::Base.connection.execute <<-SQL
+        DELETE FROM repositories
+        WHERE type = #{filesystem_type}
+      SQL
+    end
+  end
+
+  def filesystem_type
+    quote_value('Repository::Filesystem')
   end
 end