One-to-many relationship

As explained in API Design Patterns

A one-to-many relationship is an association in which one entity may be related to multiple entities while each related entity has at most one corresponding entity on the other side.

A one-to-many relationship associates one resource with multiple resources while each related resource points to at most one resource on the other side. In a chat API, one User can author many Message resources, but each message has exactly one author. section 4.1.1

Chat messages and authors

Represent the relationship with an author field on each Message resource that references one specific User. Read in the opposite direction, the same structure means that a User may have many associated messages. This differs from many-to-many membership, where both sides can reference multiple resources. section 4.1.1

When modeling a relationship, clarify these points:

• Identify which resource contains the reference and which resource may be associated with many instances. section 4.1.1
• Distinguish one-to-many structure from one-to-one, many-to-many, and optional one-to-many relationships when drawing an entity relationship diagram. section 4.4

Figure 4.3 A message resource contains a reference to a specific user who authored the message.

Use the message-and-user diagram to visualize how a single Message references one User resource. section 4.1.1

How API Design Patterns covers it

How deep it goes

Defines reference direction with a chat example and contrasts many-to-many relationships, while deferring relationship mechanics.

Examples

Uses User and Message resources, a message author field, and a bookmark-and-folder entity relationship exercise.

What you can do after

Evaluate resource models and draw entity relationship diagrams using one-to-many connectors.

How it connects

Connects foreign keys, references, many-to-many relationships, one-to-one relationships, and optional one-to-many relationships.

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Also explained in

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Related concepts

• foreign keyincludes · A foreign key is a column or set of columns in a relational table whose values reference a candidate key in the same or another table, establishing a relationship and supporting referential integrity.
• @manytooneconfigured by · @ManyToOne is a JPA annotation that maps an association from multiple source entities to one target entity.
• minimum cardinalityconfigured by · Minimum cardinality is the smallest number of related entity instances required or permitted for an entity instance in a relationship.
• ForeignKeyconfigured by · ForeignKey is a database field or construct that represents a relationship by referencing a key in another table or model.

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