2 things:
it should be a belongs_to. Painting has a series_id column so it has
to be Painting belongs_to :series.
Secondly, the reason that a new instance of Series is created each
time is because you're asking for one. If you want the painting to be
appended to a given Series then you should be using Series.find ... to
retrieve the appropriate Series, rather than creating a new one with
Series.new