feat(model): add mergePlugins option to discriminator

lib/helpers/model/discriminator.js

@@ -184,8 +184,9 @@ module.exports = function discriminator(model, name, schema, tiedValue, applyPlu
     if (mergeHooks) {
       schema.s.hooks = model.schema.s.hooks.merge(schema.s.hooks);
     }
-
-    schema.plugins = Array.prototype.slice.call(baseSchema.plugins);
+    if (applyPlugins) {
+      schema.plugins = Array.prototype.slice.call(baseSchema.plugins);
+    }
     schema.callQueue = baseSchema.callQueue.concat(schema.callQueue);
     delete schema._requiredpaths; // reset just in case Schema#requiredPaths() was called on either schema
   }

lib/model.js

@@ -1215,6 +1215,7 @@ Model.exists = function exists(filter, options, callback) {
  * @param {Boolean} [options.clone=true] By default, `discriminator()` clones the given `schema`. Set to `false` to skip cloning.
  * @param {Boolean} [options.overwriteModels=false] by default, Mongoose does not allow you to define a discriminator with the same name as another discriminator. Set this to allow overwriting discriminators with the same name.
  * @param {Boolean} [options.mergeHooks=true] By default, Mongoose merges the base schema's hooks with the discriminator schema's hooks. Set this option to `false` to make Mongoose use the discriminator schema's hooks instead.
+ * @param {Boolean} [options.mergePlugins=true] By default, Mongoose merges the base schema's plugins with the discriminator schema's plugins. Set this option to `false` to make Mongoose use the discriminator schema's plugins instead.
  * @return {Model} The newly created discriminator model
  * @api public
  */
@@ -1232,6 +1233,7 @@ Model.discriminator = function(name, schema, options) {
   options = options || {};
   const value = utils.isPOJO(options) ? options.value : options;
   const clone = typeof options.clone === 'boolean' ? options.clone : true;
+  const mergePlugins = typeof options.mergePlugins === 'boolean' ? options.mergePlugins : true;
 
   _checkContext(this, 'discriminator');
 
@@ -1242,7 +1244,7 @@ Model.discriminator = function(name, schema, options) {
     schema = schema.clone();
   }
 
-  schema = discriminator(this, name, schema, value, true, options.mergeHooks);
+  schema = discriminator(this, name, schema, value, mergePlugins, options.mergeHooks);
   if (this.db.models[name] && !schema.options.overwriteModels) {
     throw new OverwriteModelError(name);
   }

test/model.discriminator.test.js

@@ -2039,4 +2039,42 @@ describe('model', function() {
       schema.pre('save', function testHook12472() {});
     }
   });
+
+  it('supports `mergePlugins` option to use the discriminator schema\'s plugins over the base schema\'s (gh-12604)', function() {
+    let pluginTimes = 0;
+    const shapeDef = { name: String };
+    const shapeSchema = Schema(shapeDef, { discriminatorKey: 'kind' });
+    shapeSchema.plugin(myPlugin, { opts1: true });
+
+    const Shape = db.model('Test', shapeSchema);
+
+    const triangleSchema = Schema({ ...shapeDef, sides: { type: Number, enum: [3] } });
+    triangleSchema.plugin(myPlugin, { opts2: true });
+    const Triangle = Shape.discriminator(
+      'Triangle',
+      triangleSchema
+    );
+
+    const squareSchema = Schema({ ...shapeDef, sides: { type: Number, enum: [4] } });
+    squareSchema.plugin(myPlugin, { opts3: true });
+    const Square = Shape.discriminator(
+      'Square',
+      squareSchema,
+      { mergeHooks: false, mergePlugins: false }
+    );
+
+    assert.equal(Triangle.schema.s.hooks._pres.get('save').filter(hook => hook.fn.name === 'testHook12604').length, 2);
+    assert.equal(Square.schema.s.hooks._pres.get('save').filter(hook => hook.fn.name === 'testHook12604').length, 1);
+
+    const squareFilteredPlugins = Square.schema.plugins.filter((obj) => obj.fn.name === 'myPlugin');
+    assert.equal(squareFilteredPlugins.length, 1);
+    assert.equal(squareFilteredPlugins[0].opts['opts3'], true);
+
+    assert.equal(pluginTimes, 3);
+
+    function myPlugin(schema) {
+      pluginTimes += 1;
+      schema.pre('save', function testHook12604() {});
+    }
+  });
 });

test/types/discriminator.test.ts

@@ -21,7 +21,7 @@ doc.email = 'hello';
 const Disc2 = Base.discriminator<IDiscriminatorTest>(
   'Disc2',
   new Schema({ email: { type: String } }),
-  { value: 'test', mergeHooks: false }
+  { value: 'test', mergeHooks: false, mergePlugins: false }
 );
 
 function test(): void {

types/models.d.ts

@@ -6,6 +6,7 @@ declare module 'mongoose' {
     clone?: boolean;
     overwriteModels?: boolean;
     mergeHooks?: boolean;
+    mergePlugins?: boolean;
   }
 
   export interface AcceptsDiscriminator {