types: automatically infer discriminator type

test/types/discriminator.test.ts

@@ -1,4 +1,5 @@
 import mongoose, { Document, Model, Schema, SchemaDefinition, SchemaOptions, Types, model } from 'mongoose';
+import { expectType } from 'tsd';
 
 const schema: Schema = new Schema({ name: { type: 'String' } });
 
@@ -76,3 +77,42 @@ function test(): void {
 
   const sampleCardDb: CardDb = sampleLandDb;
 }
+
+function gh15535() {
+  const ParentSchema = new Schema({
+    field1: {
+      type: String,
+      required: true
+    },
+    field2: Number
+  }, {
+    discriminatorKey: 'field1',
+    methods: {
+      getField2() {
+        return this.field2;
+      }
+    }
+  });
+
+  const ParentModel = mongoose.model('Parent', ParentSchema);
+
+  const ChildSchema = new Schema({
+    field3: String
+  }, {
+    methods: {
+      getField3() {
+        return this.field3;
+      }
+    }
+  });
+
+
+  const ChildModel = ParentModel.discriminator('child', ChildSchema);
+
+  const doc = new ChildModel({});
+  expectType<string>(doc.field1);
+  expectType<number | null | undefined>(doc.field2);
+  expectType<number | null | undefined>(doc.getField2());
+  expectType<string | null | undefined>(doc.field3);
+  expectType<string | null | undefined>(doc.getField3());
+}

types/models.d.ts

@@ -266,7 +266,6 @@ declare module 'mongoose' {
     THydratedDocumentType = HydratedDocument<TRawDocType, TVirtuals & TInstanceMethods, TQueryHelpers, TVirtuals>,
     TSchema = any> extends
     NodeJS.EventEmitter,
-    AcceptsDiscriminator,
     IndexManager,
     SessionStarter {
     new <DocType = Partial<TRawDocType>>(doc?: DocType, fields?: any | null, options?: boolean | AnyObject): THydratedDocumentType;
@@ -419,6 +418,33 @@ declare module 'mongoose' {
       TInstanceMethods & TVirtuals
     >;
 
+    /** Adds a discriminator type. */
+    discriminator<TDiscriminatorSchema extends Schema<any>>(
+      name: string,
+      schema: TDiscriminatorSchema,
+      value?: string | number | ObjectId | DiscriminatorOptions
+    ): Model<
+      TRawDocType & InferSchemaType<TDiscriminatorSchema>,
+      TQueryHelpers & ObtainSchemaGeneric<TDiscriminatorSchema, 'TQueryHelpers'>,
+      TVirtuals & ObtainSchemaGeneric<TDiscriminatorSchema, 'TVirtuals'>,
+      HydratedDocument<
+        TRawDocType & InferSchemaType<TDiscriminatorSchema>,
+        TVirtuals & ObtainSchemaGeneric<TDiscriminatorSchema, 'TVirtuals'> & TInstanceMethods & ObtainSchemaGeneric<TDiscriminatorSchema, 'TInstanceMethods'>,
+        TQueryHelpers & ObtainSchemaGeneric<TDiscriminatorSchema, 'TQueryHelpers'>,
+        TVirtuals & ObtainSchemaGeneric<TDiscriminatorSchema, 'TVirtuals'>
+      >
+    > & ObtainSchemaGeneric<TDiscriminatorSchema, 'TStaticMethods'>;
+    discriminator<D>(
+      name: string | number,
+      schema: Schema,
+      value?: string | number | ObjectId | DiscriminatorOptions
+    ): Model<D>;
+    discriminator<T, U>(
+      name: string | number,
+      schema: Schema<T, U>,
+      value?: string | number | ObjectId | DiscriminatorOptions
+    ): U;
+
     /**
      * Delete an existing [Atlas search index](https://www.mongodb.com/docs/atlas/atlas-search/create-index/) by name.
      * This function only works when connected to MongoDB Atlas.