feat: error out when encountering unknown meta schemas

BREAKING CHANGE: Unknown $schema have previously have been treated as draft 4

Author: kylef

CHANGELOG.md

@@ -1,5 +1,13 @@
 # JSONSchema Changelog
 
+## TBD
+
+### Breaking Changes
+
+- When a schema references an unknown meta schema via `$schema`, a reference
+  error will now be thrown instead of the schema being silently treated as a
+  draft 4 schema.
+
 ## 0.6.0
 
 ### Breaking Changes

Sources/JSONSchema.swift

@@ -52,22 +52,27 @@ public struct Schema {
   }
 
   public func validate(_ data: Any) throws -> ValidationResult {
-    let validator = JSONSchema.validator(for: schema)
+    let validator = try JSONSchema.validator(for: schema)
     return try validator.validate(instance: data)
   }
 
   public func validate(_ data: Any) throws -> AnySequence<ValidationError> {
-    let validator = JSONSchema.validator(for: schema)
+    let validator = try JSONSchema.validator(for: schema)
     return try validator.validate(instance: data)
   }
 }
 
 
-func validator(for schema: [String: Any]) -> Validator {
-  guard let schemaURI = schema["$schema"] as? String else {
+func validator(for schema: [String: Any]) throws -> Validator {
+  guard schema.keys.contains("$schema") else {
+    // Default schema
     return Draft4Validator(schema: schema)
   }
 
+  guard let schemaURI = schema["$schema"] as? String else {
+    throw ReferenceError.notFound
+  }
+
   if let id = DRAFT_2020_12_META_SCHEMA["$id"] as? String, schemaURI == id {
     return Draft202012Validator(schema: schema)
   }
@@ -84,11 +89,11 @@ func validator(for schema: [String: Any]) -> Validator {
     return Draft6Validator(schema: schema)
   }
 
-  if let id = DRAFT_04_META_SCHEMA["$id"] as? String, schemaURI == id {
+  if let id = DRAFT_04_META_SCHEMA["id"] as? String, schemaURI == id {
     return Draft4Validator(schema: schema)
   }
 
-  return Draft4Validator(schema: schema)
+  throw ReferenceError.notFound
 }
 
 

Tests/JSONSchemaTests/JSONSchemaTests.swift

@@ -108,11 +108,42 @@ class ValidateTests: XCTestCase {
     try XCTAssertFalse(validate(["price": 34.99], schema: schema).valid)
   }
 
-  func testDraft6ValidatorIsAvailable() {
-    let result = validator(for: ["$schema": "http://json-schema.org/draft-06/schema#"])
+  func testDefaultValidator() throws {
+    let result = try validator(for: [:])
+    XCTAssertTrue(result is Draft4Validator, "Unexpected type of validator \(result)")
+  }
+
+  func testDraft4ValidatorIsAvailable() throws {
+    let result = try validator(for: ["$schema": "http://json-schema.org/draft-04/schema#"])
+    XCTAssertTrue(result is Draft4Validator, "Unexpected type of validator \(result)")
+  }
+
+  func testDraft6ValidatorIsAvailable() throws {
+    let result = try validator(for: ["$schema": "http://json-schema.org/draft-06/schema#"])
     XCTAssertTrue(result is Draft6Validator, "Unexpected type of validator \(result)")
   }
 
+  func testDraft7ValidatorIsAvailable() throws {
+    let result = try validator(for: ["$schema": "http://json-schema.org/draft-07/schema#"])
+    XCTAssertTrue(result is Draft7Validator, "Unexpected type of validator \(result)")
+  }
+
+  func testDraft201909ValidatorIsAvailable() throws {
+    let result = try validator(for: ["$schema": "https://json-schema.org/draft/2019-09/schema"])
+    XCTAssertTrue(result is Draft201909Validator, "Unexpected type of validator \(result)")
+  }
+
+  func testDraft202012ValidatorIsAvailable() throws {
+    let result = try validator(for: ["$schema": "https://json-schema.org/draft/2020-12/schema"])
+    XCTAssertTrue(result is Draft202012Validator, "Unexpected type of validator \(result)")
+  }
+
+  func testUnknownValidator() throws {
+    XCTAssertThrowsError(
+      try validator(for: ["$schema": "https://example.com/schema"])
+    )
+  }
+
   func testValidateDraft7() throws {
     let schema: [String: Any]  = [
       "$schema": "http://json-schema.org/draft-07/schema#",