Metamodel Concepts (Historical)

Note: This document contains historical design notes from the original development. For current documentation, see Metamodel Reference.

Datatype System Evolution

This section describes the conceptual evolution of the datatype system.

The Self-Referential Type

The metamodel's core insight is that :dt/type (the "type" property) is itself a typed entity. This creates a self-referential foundation:

;; :dt/type is both:
;; 1. An attribute (with :db/valueType, :db/cardinality)
;; 2. An instance of :dt/Property
;; 3. A slot of :dt/Resource

{:db/ident :dt/type
 :db/valueType :db.type/ref
 :db/cardinality :db.cardinality/one
 :dt/type :dt/Property          ;; Self-reference!
 :dt/domain :dt/Resource
 :dt/range :dt/Class}

Fn Types

Database functions in Datomic are compiled Clojure code stored in the database. The metamodel embraces this by defining :dt/Fn as a first-class type:

F(x): D → R

Where:

• D is the domain type (:fn/domain)
• R is the range type (:fn/range)

This allows functions to be typed members of the metamodel, enabling:

• Version-controlled code in the database
• Type-checked function composition
• First-class workflow/pipeline definitions

The "Any" Variant Type

The :dt/Any type solves the literal/reference bifurcation in Datomic's data model. While Datomic requires attributes to be either literal values OR references, dt/Any can hold either:

Slot	Description
:value/string	A literal string value
:value/long	A 64-bit integer
:value/ref	A reference to another entity
:value/instant	A point in time
...	(other literal types)

Important: dt/Any is a variant type, not a top type. It doesn't participate in inheritance like dt/Resource.

Aggregate References

For any type T, the metamodel provides:

• T* - 1D aggregate (array of T)
• T** - 2D aggregate (matrix of T)

Accessed via the :dt/list property:

(-> (entity :model/User) :dt/list)       ;; => :model/User*
(-> (entity :model/User) :dt/list :dt/list)  ;; => :model/User**

This enables typed collection handling within the metamodel.

Original Schema Example

;; Historical Datatype Definition
[
  {:db/id #db/id[:db.part/db]
   :db/ident :dt/type
   :db/valueType :db.type/ref
   :db/cardinality :db.cardinality/one
   :db/doc "A reference to the data type of an entity"
   :db.install/_attribute :db.part/db}

  {:db/id :dt/type
   :dt/type :dt/Class
   :dt/context "system"
   :dt/name "Datatype"
   :dt/slots [:dt/context
              :dt/name
              :dt/parent
              :dt/list
              :dt/component
              :dt/slots]}
]

See Metamodel Reference for current schema documentation.