Metamodel reference

Metamodel — relationship types

Each section below is one relationship.<subType>. The Attributes table lists the subtype's own + concern-contributed attributes (provider-tagged); universal documentation attributes are omitted here (see providers.md). Allowed children lists the structural child rules with their cardinality (min..max, * = unbounded).

relationship.aggregation

A shared/independent containment — the parent groups the target but does not own its lifecycle (default @onDelete set-null).

Owning provider: metaobjects-core-types

Rules: M:N is expressed by @cardinality:'many' + @objectRef + @through: @through names a junction entity that MUST declare two identity.reference children (one per FK side), and the relationship's FK fields are DERIVED from those references — never restated. @sourceRefField disambiguates a DIRECTED self-join by naming the source-side FK field on the junction; @symmetric marks an UNDIRECTED self-join (union-on-read) valid only when @objectRef == the declaring entity; the two are mutually exclusive. Aggregation is shared/independent — the target outlives the parent (default @onDelete set-null).

When to use: One entity groups others it does NOT own (children outlive the parent; delete sets the FK null). Use instead of composition when there is no ownership.

Attributes

Allowed children

No structural children.

relationship.association

A plain reference to another entity — no ownership; the target has an independent lifecycle (default @onDelete restrict).

Owning provider: metaobjects-core-types

Rules: M:N is expressed by @cardinality:'many' + @objectRef + @through: @through names a junction entity that MUST declare two identity.reference children (one per FK side), and the relationship's FK fields are DERIVED from those references — never restated. @sourceRefField disambiguates a DIRECTED self-join by naming the source-side FK field on the junction; @symmetric marks an UNDIRECTED self-join (union-on-read) valid only when @objectRef == the declaring entity; the two are mutually exclusive. Association is a plain reference — the target's lifecycle is independent (default @onDelete restrict).

When to use: A plain directed reference to another entity, no ownership or cascade. The lightest link — when you just need to point at another entity.

Attributes

Allowed children

No structural children.

relationship.base

Abstract relationship base — shared shape for the concrete association/aggregation/composition subtypes; not authored directly. A relationship.base node fails to load (ERR_ABSTRACT_SUBTYPE_AUTHORED): this subtype is a registry anchor concrete subtypes inherit from, never a node in a document.

Owning provider: metaobjects-core-types

Rules: @cardinality is an open string at the metamodel level ('one'/'many', and Java-canonical composite forms such as 'many-to-one'); @objectRef names the target entity. M:N is expressed by @cardinality:'many' + @objectRef + @through: @through names a junction entity that MUST declare two identity.reference children (one per FK side), and the relationship's FK fields are DERIVED from those references — never restated. @sourceRefField disambiguates a DIRECTED self-join by naming the source-side FK field on the junction; @symmetric marks an UNDIRECTED self-join (union-on-read) valid only when @objectRef == the declaring entity; the two are mutually exclusive. @onDelete/@onUpdate carry referential actions (cascade/set-null/restrict/no-action).

Attributes

Allowed children

No structural children.

relationship.composition

An owned containment — the parent owns the target's lifecycle; deleting the parent deletes the children (default @onDelete cascade).

Owning provider: metaobjects-core-types

Rules: M:N is expressed by @cardinality:'many' + @objectRef + @through: @through names a junction entity that MUST declare two identity.reference children (one per FK side), and the relationship's FK fields are DERIVED from those references — never restated. @sourceRefField disambiguates a DIRECTED self-join by naming the source-side FK field on the junction; @symmetric marks an UNDIRECTED self-join (union-on-read) valid only when @objectRef == the declaring entity; the two are mutually exclusive. Composition is owned lifecycle — the children do not outlive the parent (default @onDelete cascade).

When to use: You need a parent that OWNS a child collection (one-to-many, cascade on delete). Declare it to generate the FK + typed navigation instead of a bare FK field + hand-written joins.

Attributes

Allowed children

No structural children.