fix(model): make subclassed models handle discriminators correctly

Fix Automattic#7547

lib/model.js

@@ -51,6 +51,7 @@ const VERSION_ALL = VERSION_WHERE | VERSION_INC;
 const modelCollectionSymbol = Symbol.for('mongoose#Model#collection');
 const modelSymbol = require('./helpers/symbols').modelSymbol;
 const schemaMixedSymbol = require('./schema/symbols').schemaMixedSymbol;
+const subclassedSymbol = Symbol('mongoose#Model#subclassed');
 
 /**
  * A Model is a class that's your primary tool for interacting with MongoDB.
@@ -1027,6 +1028,14 @@ Model.discriminator = function(name, schema, value) {
   applyMethods(d, schema);
   applyStatics(d, schema);
 
+  if (this[subclassedSymbol] != null) {
+    for (const submodel of this[subclassedSymbol]) {
+      submodel.discriminators = submodel.discriminators || {};
+      submodel.discriminators[name] =
+        model.__subclass(model.db, schema, submodel.collection.name);
+    }
+  }
+
   return d;
 };
 
@@ -4656,6 +4665,16 @@ Model.__subclass = function subclass(conn, schema, collection) {
   Model.prototype.__proto__ = _this.prototype;
   Model.db = Model.prototype.db = conn;
 
+  _this[subclassedSymbol] = _this[subclassedSymbol] || [];
+  _this[subclassedSymbol].push(Model);
+  if (_this.discriminators != null) {
+    Model.discriminators = {};
+    for (const key of Object.keys(_this.discriminators)) {
+      Model.discriminators[key] = _this.discriminators[key].
+        __subclass(_this.db, _this.discriminators[key].schema, collection);
+    }
+  }
+
   const s = schema && typeof schema !== 'string'
     ? schema
     : _this.prototype.schema;