Jsontron 0.1.1

Overview

JSON Schema is excellent at shape: types, required fields, formats, enums. It is awkward at business rules that cross fields, compare against policy at the document root, or select a subset of nodes and assert something about each.

Jsontron adds that missing layer on top of JsonSchema.Net: Schematron-style rules / assert keywords whose context and test expressions are jq, evaluated by Devlooped.JQSharp.

• Structural validation stays with JSON Schema.
• Cross-cutting rules stay in the schema, next to the data they describe.
• jq expressions are parsed once when the schema is built, then reused.

Quick start

using System.Text.Json;
using Json.Schema;
using Jsontron;

// Once per process — extends Dialect.Default with rules/assert
MetaSchemas.Register();

var schema = JsonSchema.FromText(
    """
    {
      "type": "object",
      "required": [ "orders", "policy", "approvedCustomers" ],
      "rules": [
        {
          "context": ".orders[] | select(.total > 1000)",
          "asserts": [
            {
              "test": ".discount >= ($root.policy.minHighValueDiscount // 0.1)",
              "message": "High-value order \\(.id // \"?\") discount \\(.discount) is below policy"
            },
            {
              "test": ".customer.id | IN($root.approvedCustomers[])",
              "message": "Customer \\(.customer.id) is not on the approved list"
            }
          ]
        }
      ]
    }
    """);

using var doc = JsonDocument.Parse(
    """
    {
      "policy": { "minHighValueDiscount": 0.15 },
      "approvedCustomers": [ "acme", "globex" ],
      "orders": [
        {
          "id": "O-1001",
          "total": 2500,
          "discount": 0.05,
          "customer": { "id": "initech" }
        },
        {
          "id": "O-1002",
          "total": 80,
          "discount": 0,
          "customer": { "id": "unknown" }
        }
      ]
    }
    """);

var result = schema.Evaluate(
    doc.RootElement,
    new EvaluationOptions { OutputFormat = OutputFormat.List });

// O-1001 fails both asserts (discount + customer).
// O-1002 is ignored by the rule (total ≤ 1000).
Console.WriteLine(result.IsValid); // False

Why jq?

jq already knows how to walk JSON: filter arrays, default missing fields, compare values, and build messages with string interpolation. Jsontron reuses that instead of inventing another expression language.

An assert fails when the test is not jq-truthy (false, null, or an empty stream) — the same idea as Schematron assert.

Keywords

rules

Full form: an array of rules. Each rule picks context nodes, then runs one or more asserts.

{
  "rules": [
    {
      "context": ".lineItems[] | select(.sku | startswith(\"PROMO-\"))",
      "asserts": [
        {
          "test": ".qty <= ($root.promotions[.sku].maxQty // 1)",
          "message": "Promo \\(.sku) allows at most \\($root.promotions[.sku].maxQty // 1), got \\(.qty)"
        }
      ]
    }
  ]
}

assert (sugar for context: ".")

When the rule is “about this node,” skip the full rules array:

{
  "type": "object",
  "properties": {
    "email": { "type": "string", "format": "email" },
    "age": { "type": "integer", "minimum": 0 }
  },
  "assert": ".age >= 18 or (.guardianEmail | type) == \"string\""
}

Forms accepted:

{
  "assert": [
    ".status | IN([\"draft\", \"submitted\", \"approved\"])",
    {
      "test": ".status != \"approved\" or .approver != null",
      "message": "Approved documents require an approver"
    }
  ]
}

If both assert and rules appear on the same schema object, sugar asserts are merged into the rule with context: "." (created if needed) and evaluated once.

Nested schema locations

assert / rules apply to the instance at that schema location. Under properties / items, . is the nested value; $root remains the full document.

{
  "type": "object",
  "properties": {
    "shipments": {
      "type": "array",
      "items": {
        "type": "object",
        "required": [ "weightKg", "method" ],
        "assert": {
          "test": ".method != \"air\" or .weightKg <= $root.limits.maxAirKg",
          "message": "Air shipment \\(.id // \"?\") exceeds max air weight"
        }
      }
    }
  }
}

Messages

message is a jq expression that should produce a string. If it does not start with ", Jsontron wraps it as a jq string for you so plain text just works:

"message": "Discount too low"

Use jq interpolation when you want values in the text (escape backslashes in JSON):

"message": "Line \\(.sku): qty \\(.qty) exceeds cap"

Or start with " yourself for a full jq string expression.

Registration

MetaSchemas.Register();

That:

1. Extends Dialect.Default (and BuildOptions.Default.Dialect) with rules / assert
2. Registers the vocabulary and meta-schema
   (https://www.schemastore.org/jsontron-0.1.json)

After Register(), ordinary JsonSchema.FromText / Build calls pick up the keywords with no extra BuildOptions plumbing.

Keyword syntax is also published as schemas/jsontron-0.1.json.

Design notes

• Build-time compile: invalid keyword shape or invalid jq fails when the schema is built, not on the first instance.
• No rule shadowing (v1): every rule runs (closer to Schematron 2025 group than classic pattern shadowing).
• Empty context: if context matches nothing, asserts do not run — vacuously valid.
• Stack: JsonSchema.Net 9.x + Devlooped.JQSharp 1.0.2+ (evaluation-time variables for $root).

Open Source Maintenance Fee

To ensure the long-term sustainability of this project, users of this package who generate revenue must pay an Open Source Maintenance Fee. While the source code is freely available under the terms of the License, this package and other aspects of the project require adherence to the Maintenance Fee.

To pay the Maintenance Fee, become a Sponsor at the proper OSMF tier. A single fee covers all of Devlooped packages.

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