prepare_update_statement

# File activerecord/lib/arel/visitors/postgresql.rb, line 38
        def prepare_update_statement(o)
          if o.key && has_join_sources?(o) && !has_group_by_and_having?(o) && !has_limit_or_offset_or_orders?(o)
            # Join clauses cannot reference the target table, so alias the
            # updated table, place the entire relation in the FROM clause, and
            # add a self-join (which requires the primary key)
            stmt = o.clone
            stmt.relation, stmt.wheres = o.relation.clone, o.wheres.clone
            stmt.relation.right = [stmt.relation.left, *stmt.relation.right]
            stmt.relation.left = stmt.relation.left.alias("__active_record_update_alias")
            Array.wrap(o.key).each do |key|
              stmt.wheres << key.eq(stmt.relation.left[key.name])
            end
            stmt
          else
            super
          end
        end