Side note before the answer, based on the structure of your models I would use has_and_belongs_to_many instead of using this explicit linking model since it appears the linking model doesn't add anything of value.
Either way though, the answer is the same, which is to use a custom validation. Depending on whether you go with things the way they are or simplify to a has_and_belongs_to many you'll want to validate slightly differently.
validate :has_project_disciplines
def has_project_disciplines
errors.add(:base, 'must add at least one discipline') if self.project_disciplinizations.blank?
end
or with has_and_belongs_to_many
validate :has_project_disciplines
def has_project_disciplines
errors.add(:base, 'must add at least one discipline') if self.project_disciplines.blank?
end