gistfile1.rb

scope :in_collection, ->(collection) {
  where(Ddr::Index::Fields::IS_MEMBER_OF_COLLECTION => collection.respond_to?(:id) ? collection.id : collection)
}
scope :having_local_id, ->(local_id) { 
  where(Ddr::Index::Fields::LOCAL_ID => local_id)
}

def self.having_local_id!(local_id)
  results = having_local_id(local_id).to_a
  if results.size > 1
    raise Error
  end
  result.first
end

Collection.having_local_id!(local_id)

Item.in_collection(collection).having_local_id!(local_id)