fix migration error

db/migrate/20181102125923_add_constraints_to_most_recent_for_form_answer_transitions.rb

@@ -2,7 +2,7 @@ class AddConstraintsToMostRecentForFormAnswerTransitions < ActiveRecord::Migrati
   disable_ddl_transaction!
 
   def up
-    Rake::Task['statesman:backfill_most_recent'].invoke("FormAnswer")
+    Rake::Task['statesman:backfill_most_recent_fixed'].invoke("FormAnswer")
     FormAnswerTransition.where(most_recent: nil).update_all(most_recent: false)
     add_index :form_answer_transitions, [:form_answer_id, :most_recent], unique: true, where: "most_recent", name: "index_form_answer_transitions_parent_most_recent", algorithm: :concurrently rescue ""
     change_column_null :form_answer_transitions, :most_recent, false

lib/tasks/statesman.rake

@@ -0,0 +1,62 @@
+namespace :statesman do
+  desc "Set most_recent to false for old transitions and to true for the " \
+       "latest one. Safe to re-run"
+  task :backfill_most_recent_fixed, [:parent_model_name] => :environment do |_, args|
+    parent_model_name = args.parent_model_name
+    abort("Parent model name must be specified") unless parent_model_name
+
+    parent_class = parent_model_name.constantize
+    transition_class = parent_class.transition_class
+    parent_fk = "#{parent_model_name.demodulize.underscore}_id"
+    updated_at = if ActiveRecord.default_timezone == :utc
+                   Time.now.utc
+                 else
+                   Time.now
+                 end
+
+    total_models = parent_class.count
+    done_models = 0
+    batch_size = 500
+
+    parent_class.find_in_batches(batch_size: batch_size) do |models|
+      ActiveRecord::Base.transaction(requires_new: true) do
+        if Statesman::Adapters::ActiveRecord.database_supports_partial_indexes?
+          # Set all transitions' most_recent to FALSE
+          transition_class.where(parent_fk => models.map(&:id)).
+            update_all(most_recent: false, updated_at: updated_at)
+        else
+          transition_class.where(parent_fk => models.map(&:id)).
+            update_all(most_recent: nil, updated_at: updated_at)
+        end
+
+        # Set current transition's most_recent to TRUE
+        initial_t = transition_class.arel_table
+        subsequent_t = initial_t.alias
+
+        later_row_for_same_parent = initial_t[parent_fk].
+          eq(subsequent_t[parent_fk]).
+          and(initial_t[:sort_key].
+                                    lt(subsequent_t[:sort_key]))
+
+        no_later_row = subsequent_t[:id].eq(nil)
+        in_current_parent_batch = initial_t[parent_fk].in(models.map(&:id))
+
+        latest_ids_query = initial_t.join(subsequent_t, Arel::Nodes::OuterJoin).
+          on(later_row_for_same_parent).
+          where(no_later_row.and(in_current_parent_batch)).
+          project(initial_t[:id]).to_sql
+
+        latest_ids = transition_class.find_by_sql(latest_ids_query).
+          to_a.collect(&:id)
+
+        transition_class.where(id: latest_ids).
+          update_all(most_recent: true, updated_at: updated_at)
+      end
+
+      done_models += batch_size
+      puts "Updated #{transition_class.name.pluralize} for " \
+           "#{[done_models, total_models].min}/#{total_models} " \
+           "#{parent_model_name.pluralize}"
+    end
+  end
+end