Schema

A Schema declares a governed type — the itemtype URL it applies to and the properties, validators, and constraints that are enforced when items of that type are written or read.

When to reach for it

Declare a schema for any itemtype that needs cardinality enforcement, type validation, defaults, resolvers, uniqueness, referential integrity, or @validate rules. Items whose itemtype has no matching schema are stored unchanged, without any of those checks.

Shape

A schema is an HTML element with itemscope itemtype="https://pagelove.org/Schema" and a type child that names the governed URL. Everything else is optional.

<div itemscope itemtype="https://pagelove.org/Schema">
  <meta itemprop="type" content="https://example.com/Person">
  <meta itemprop="parent" content="https://example.com/Entity">

  <div itemprop="property" itemscope itemtype="https://pagelove.org/Property">
    <!-- See the Property reference -->
  </div>

  <div itemprop="constraint" itemscope itemtype="https://pagelove.org/GroupConstraint">
    <!-- See the Required combinations reference -->
  </div>

  <script type="text/sessel" itemprop="@validate">
    <!-- Schema-level validator -->
  </script>
</div>

Fields

Field	Cardinality	Accepts	Purpose
`type`	`1..1`	URL	The `itemtype` URL this schema governs. A schema with no `type` is silently skipped.
`name`	`0..1`	Text	Human-readable name. Used in error messages and tooling.
`description`	`0..1`	Text	Human-readable description of what the type represents.
`parent`	`0..1`	URL	Governed type URL of a parent schema to inherit from.
`property`	`0..n`	Nested Property item	A property declaration.
`constraint`	`0..n`	Nested Required combinations item	A group-level cardinality rule across several properties.
`@validate`	`0..1`	Sessel or JavaScript binding	Schema-level validator. Must return truthy. See below.

Inheritance

A schema with a parent field inherits everything the parent declares — properties, group constraints, resolvers, and validators. The chain is walked from root to leaf; a child property with the same name as a parent property overrides the parent's declaration for that property.

When parent is omitted, user-defined schemas implicitly inherit from https://pagelove.org/Instance. Every user schema participates in the Instance type hierarchy unless it declares a different explicit parent.

Cycles are detected and rejected — a cyclic chain produces a 422 on every write to items of any schema in the cycle. An unknown parent URL is not rejected: the inheritance chain silently stops at the last resolvable ancestor, and the schema behaves as if it had no further parent. This is different from a cycle, which is always an error.

How inherited members combine

Inherited member	Merge rule
Properties	All parent properties are included. Child declarations override parent declarations by `name`.
Group constraints	Parent constraints apply unless the child declares a constraint under the same `group` name.
`@read` resolvers	Run ancestor-first — parents transform before children.
`@write` resolvers	Run child-first — children transform before parents.
`@validate` expressions	Every validator in the chain runs, ancestor-first. All must return `true`.
`unique`	If any schema in the chain marks a property as `unique`, it is unique for the leaf type.

Schema-level `@validate`

Use schema-level @validate for rules that span more than one property — "start date must be before end date", "total must equal the sum of line items".

Schema-level @validate accepts either a Sessel expression or a JavaScript module. As with the other binding slots, the language is selected by the wrapper's itemtype URL, not by the <script type> attribute.

Sessel

The expression's self binding is the [itemscope] element being written, so self.microdata() gives read access to every property on the item.

<script type="text/sessel" itemprop="@validate">
  let md = self.microdata();
  md["start-date"].first() < md["end-date"].first()
</script>

JavaScript

Wrap an ES module in a JavaScript/Module item. The module's default export must be a function, and the schema-level slot binds the instance to this as a serialized element string (it is not passed as a positional argument; the positional context argument is null for this slot). A function form is required so this is observable; an arrow function in a strict-mode ES module does not bind its own this. Return truthy to accept, falsy or thrown to reject.

<div itemprop="@validate" itemscope itemtype="https://pagelove.org/JavaScript/Module">
  <script itemprop="source" type="module">
    export default function () {
      // `this` is the serialized instance HTML.
      return this.includes('itemprop="start-date"');
    }
  </script>
</div>

Note: the schema-level slot's this-as-serialized-HTML convention differs from a property-level JavaScript @validate, where the property's value is passed as the first positional argument. See JavaScript bindings for the full contract, supported language features, errors, and resource limits.

Schema-level @validate runs only after cardinality, type, and property-level @validate have all passed, and before group constraints are checked. This guarantees per-property structural correctness before the cross-property rule evaluates against it; group constraints (which span properties, the same category of check) run last.

Examples

Minimal

<div itemscope itemtype="https://pagelove.org/Schema">
  <meta itemprop="type" content="https://example.com/Tag">
  <div itemprop="property" itemscope itemtype="https://pagelove.org/Property">
    <meta itemprop="name" content="label">
    <meta itemprop="type" content="https://schema.host/Text">
    <meta itemprop="cardinality" content="1..1">
    <meta itemprop="unique" content="true">
  </div>
</div>

With inheritance and a cross-property validator

<div itemscope itemtype="https://pagelove.org/Schema">
  <meta itemprop="type" content="https://example.com/Entity">
  <div itemprop="property" itemscope itemtype="https://pagelove.org/Property">
    <meta itemprop="name" content="identifier">
    <meta itemprop="type" content="https://schema.host/Text">
    <meta itemprop="cardinality" content="1..1">
    <meta itemprop="unique" content="true">
  </div>
</div>

<div itemscope itemtype="https://pagelove.org/Schema">
  <meta itemprop="type" content="https://example.com/Person">
  <meta itemprop="parent" content="https://example.com/Entity">
  <div itemprop="property" itemscope itemtype="https://pagelove.org/Property">
    <meta itemprop="name" content="email">
    <meta itemprop="type" content="https://schema.host/Text">
    <meta itemprop="cardinality" content="1..1">
  </div>
  <script type="text/sessel" itemprop="@validate">
    let md = self.microdata();
    md["email"].first().matches("^[^@]+@[^@]+\\.[^@]+$")
  </script>
</div>

Error cases

Condition	Result
Schema with no `type`	Silently skipped during parsing. No writes are validated against it.
Unknown `parent` URL	Not an error — the inheritance chain silently stops at the last resolvable ancestor.
Circular inheritance chain	422 on every write to items of any schema in the cycle.
`@validate` fails to compile	422 on every write to items of the governed type.
`@validate` returns anything other than `true`	422 with `check: "@validate"` in the violation.

The 422 response body is HTML with Microdata, using the https://pagelove.org/SchemaViolation itemtype.