# 2 associations

Let's add the one-to-many association between questions and answers.

association QuestionHasAnswer
{
    question: Question[1..1] final;
    answers: Answer[0..*];

    relationship this.id == Answer.questionId
}

Associations have two ends, and a direction.

The source end is named question and has type Question. The lower-bound of the multiplicity is 1 and the upper bound is also 1.

The target end is named answers and has type Answer. The lower-bound of the multiplicity is 0 and the upper bound is * which means "many".

Question[1..1] means that every answer requires a question. An optional, or nullable property has the multiplicity [0..1]. The syntax [1] is shorthand for [1..1].

Answer[0..*] means that every question has many answers, and that the set of answers is allowed to be empty. A non-empty set has the multiplicity [1..*]. The syntax [*] is shorthand for [0..*].

Colloquially, we call this a one-to-many relationship. Keep in mind that this shorthand is vague. It only refers to the upper-bounds of the association ends.

Klass doesn't enforce singular and plural names. Take care to use singular names for to-one ends and plural names for to-many ends.

The question end has the modifier final which means that an answer cannot be repointed to a different question. In the data layer, this means the foreign key questionId is immutable. It doesn't mean that the question's text is immutable.