OPDA Knowledge base
Concept tier foundation

Role Mixin

opda:RoleMixin

Role Mixin

A Role Mixin is a part you play — but unlike a Role, it can be borne by more than one Kind of party. A Seller is a Role Mixin borne by either a Person or an Organisation; the model does not assume Sellers are always natural persons.

Why it matters

Property transactions routinely involve sellers and buyers that are corporate vehicles (Limited Companies, LLPs), trusts, or individuals — sometimes mixed in the same chain. Role Mixin is the modelling pattern that lets one role Seller be borne by either kind of bearer without splitting into PersonSeller and OrganisationSeller everywhere downstream.

If you have ever needed to ask “who is the Seller, and what kind of Seller are they?” without enumerating every combination, this is the entity that buys you that flexibility.

Hard cases

  • Mixed-bearer chain. A transaction chain where one link’s Seller is a Person and the next link’s Seller is an Organisation. Both wear the Seller Role Mixin; downstream consumers can read the bearer kind to decide what’s required next.
  • Specialised sub-Role. A consumer needs to distinguish PersonSeller from OrganisationSeller for one specific use case. The model allows specialisation without forcing every consumer to use it.
  • Role Mixin keyed by bearer. A downstream system tries to assign an ID to the Seller independent of the (Transaction, bearer) tuple. The IC says: the Seller’s identity is parasitic on the Transaction it sits in plus the party bearing it — never give a Role Mixin its own ID.

Identity Criterion

A Role Mixin instance is identified by its (Relator-context, bearer) tuple — the Transaction (or other Relator) it sits in, plus the Person or Organisation bearing the role. See the Logical tier → for the typed structure.

  • Role — the sortal variant: a Role is borne by a single Kind
  • Relator — Role Mixins sit within a Relator’s context
  • Seller — the canonical OPDA Role Mixin (Person or Organisation → Seller in a Transaction context)
  • Buyer — the mirror Role Mixin (Person or Organisation → Buyer in a Transaction context)
%%{init: {"theme": "base"}}%% flowchart LR accTitle: Role Mixin related-Kinds neighbourhood graph accDescr: Role Mixin as the pattern Kind specialised by Seller and Buyer (the canonical OPDA Role Mixins); contrasts with Role (single-Kind sortal); sits within a Relator's context. classDef centre fill:#E1BEE7,stroke:#6A1B9A,stroke-width:3px,color:#4A148C classDef cls fill:#B3E5FC,stroke:#0277BD,stroke-width:2px,color:#01579B RoleMixin["RoleMixin
(pattern)"]:::centre Role["Role
(single-Kind)"]:::cls Relator["Relator"]:::cls Seller["Seller
(canonical)"]:::cls Buyer["Buyer
(mirror)"]:::cls RoleMixin -->|"sitsWithin"| Relator RoleMixin -.->|"contrast with"| Role Seller -.->|"specialises"| RoleMixin Buyer -.->|"specialises"| RoleMixin

Source ODR

ODR-0006 — Agents and roles §Q2

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