when will a partition table not have a check constraint?
i'd contend this deserves a clearly defined exception raised, as in:

```
  raise <<-EXCEPTION
    You've tried to create a partitioned table on the parent table #{@parent_table_class.name} with
    the partitioned_key_values: #{partition_key_values.inspect} but no check constraint could
    be discerned.  it is possible your Partitioned model does not specify a constraint, as:
      partitioned do |partition|
         partition.constraint 'company_id = #{field_value}'
      end
    or:
     partitioned do |partition|
        partition.constraint lambda {|model, value|
             return "#{model.field_to_partition} = #{value}"
         }
     end
   or maybe your lambda is returning nil for some reason?
  EXCEPTION
```