has_and_belongs_to_many build is not persising the relationship

[jcoyne]

class Part < ActiveFedora::Base
end
class Whole < ActiveFedora::Base
  has_and_belongs_to_many :parts, predicate: ::RDF::URI('http://example.com/foo')
end

whole = Whole.create
whole.parts.build
whole.save
whole.reload
whole.parts
# => []

we expect that whole.parts should contain one Part

[jcoyne]

It's actually only doing this when you don't have an inverse has_many on the Part model. Perhaps we can raise a warning if you do whole.parts.build and there isn't an inverse relationship.

[jcoyne]

Seems like its happening when one of these blocks don't match:
https://github.com/projecthydra/active_fedora/blob/master/lib/active_fedora/associations/has_and_belongs_to_many_association.rb#L21-L28 I suspect is should look more like this.