resolve references before validating the discriminator

docs/json-schema.md

@@ -992,7 +992,7 @@ There are following requirements and limitations of using `discriminator` keywor
 - `mapping` in discriminator object is not supported.
 - [oneOf](#oneof) keyword must be present in the same schema.
 - discriminator property should be [requried](#required) either on the top level, as in the example, or in all `oneOf` subschemas.
-- each `oneOf` subschema must have [properties](#properties) keyword with discriminator property.
+- each `oneOf` subschema must have [properties](#properties) keyword with discriminator property. Here, the subschemas can also be included by reference (`$ref` keyword). 
 - schema for discriminator property in each `oneOf` subschema must be [const](#const) or [enum](#enum), with unique values across all subschemas.
 
 Not meeting any of these requirements would fail schema compilation.

lib/vocabularies/discriminator/index.ts

@@ -2,6 +2,7 @@ import type {CodeKeywordDefinition, AnySchemaObject, KeywordErrorDefinition} fro
 import type {KeywordCxt} from "../../compile/validate"
 import {_, getProperty, Name} from "../../compile/codegen"
 import {DiscrError, DiscrErrorObj} from "../discriminator/types"
+import {resolveRef, SchemaEnv} from "../../compile"
 
 export type DiscriminatorError = DiscrErrorObj<DiscrError.Tag> | DiscrErrorObj<DiscrError.Mapping>
 
@@ -62,10 +63,24 @@ const def: CodeKeywordDefinition = {
       const topRequired = hasRequired(parentSchema)
       let tagRequired = true
       for (let i = 0; i < oneOf.length; i++) {
-        const sch = oneOf[i]
-        const propSch = sch.properties?.[tagName]
-        if (typeof propSch != "object") {
-          throw new Error(`discriminator: oneOf schemas must have "properties/${tagName}"`)
+        let sch = oneOf[i]
+        let propSch
+        if (sch?.["$ref"] && Object.keys(sch).length === 1) {
+          sch = resolveRef.call(it.self, it.schemaEnv, it.baseId, sch?.["$ref"])
+          if (sch instanceof SchemaEnv) {
+            sch = sch.schema
+          }
+          propSch = sch?.properties?.[tagName]
+          if (typeof propSch != "object") {
+            throw new Error(
+              `discriminator: referenced oneOf schemas must have "properties/${tagName}"`
+            )
+          }
+        } else {
+          propSch = sch.properties?.[tagName]
+          if (typeof propSch != "object") {
+            throw new Error(`discriminator: oneOf schemas must have "properties/${tagName}"`)
+          }
         }
         tagRequired = tagRequired && (topRequired || hasRequired(sch))
         addMappings(propSch, i)

spec/discriminator.spec.ts

@@ -81,6 +81,84 @@ describe("discriminator keyword", function () {
     })
   })
 
+  describe("validation with referenced schemas", () => {
+    const definitions1 = {
+      schema1: {
+        properties: {
+          foo: {const: "x"},
+          a: {type: "string"},
+        },
+        required: ["foo", "a"],
+      },
+      schema2: {
+        properties: {
+          foo: {enum: ["y", "z"]},
+          b: {type: "string"},
+        },
+        required: ["foo", "b"],
+      },
+    }
+    const mainSchema1 = {
+      type: "object",
+      discriminator: {propertyName: "foo"},
+      oneOf: [
+        {
+          $ref: "#/definitions/schema1",
+        },
+        {
+          $ref: "#/definitions/schema2",
+        },
+      ],
+    }
+
+    const definitions2 = {
+      schema1: {
+        properties: {
+          foo: {const: "x"},
+          a: {type: "string"},
+        },
+        required: ["a"],
+      },
+      schema2: {
+        properties: {
+          foo: {enum: ["y", "z"]},
+          b: {type: "string"},
+        },
+        required: ["b"],
+      },
+    }
+    const mainSchema2 = {
+      type: "object",
+      discriminator: {propertyName: "foo"},
+      required: ["foo"],
+      oneOf: [
+        {
+          $ref: "#/definitions/schema1",
+        },
+        {
+          $ref: "#/definitions/schema2",
+        },
+      ],
+    }
+
+    const schema = [
+      {definitions: definitions1, ...mainSchema1},
+      {definitions: definitions2, ...mainSchema2},
+    ]
+
+    it("should validate data", () => {
+      assertValid(schema, {foo: "x", a: "a"})
+      assertValid(schema, {foo: "y", b: "b"})
+      assertValid(schema, {foo: "z", b: "b"})
+      assertInvalid(schema, {})
+      assertInvalid(schema, {foo: 1})
+      assertInvalid(schema, {foo: "bar"})
+      assertInvalid(schema, {foo: "x", b: "b"})
+      assertInvalid(schema, {foo: "y", a: "a"})
+      assertInvalid(schema, {foo: "z", a: "a"})
+    })
+  })
+
   describe("valid schemas", () => {
     it("should have oneOf", () => {
       invalidSchema(