some progress

node_modules/mongoose/lib/helpers/discriminator/areDiscriminatorValuesEqual.js

@@ -0,0 +1,16 @@
+'use strict';
+
+const isBsonType = require('../isBsonType');
+
+module.exports = function areDiscriminatorValuesEqual(a, b) {
+  if (typeof a === 'string' && typeof b === 'string') {
+    return a === b;
+  }
+  if (typeof a === 'number' && typeof b === 'number') {
+    return a === b;
+  }
+  if (isBsonType(a, 'ObjectId') && isBsonType(b, 'ObjectId')) {
+    return a.toString() === b.toString();
+  }
+  return false;
+};

node_modules/mongoose/lib/helpers/discriminator/checkEmbeddedDiscriminatorKeyProjection.js

@@ -0,0 +1,12 @@
+'use strict';
+
+module.exports = function checkEmbeddedDiscriminatorKeyProjection(userProjection, path, schema, selected, addedPaths) {
+  const userProjectedInPath = Object.keys(userProjection).
+    reduce((cur, key) => cur || key.startsWith(path + '.'), false);
+  const _discriminatorKey = path + '.' + schema.options.discriminatorKey;
+  if (!userProjectedInPath &&
+      addedPaths.length === 1 &&
+      addedPaths[0] === _discriminatorKey) {
+    selected.splice(selected.indexOf(_discriminatorKey), 1);
+  }
+};

node_modules/mongoose/lib/helpers/discriminator/getConstructor.js

@@ -0,0 +1,26 @@
+'use strict';
+
+const getDiscriminatorByValue = require('./getDiscriminatorByValue');
+
+/**
+ * Find the correct constructor, taking into account discriminators
+ * @api private
+ */
+
+module.exports = function getConstructor(Constructor, value) {
+  const discriminatorKey = Constructor.schema.options.discriminatorKey;
+  if (value != null &&
+      Constructor.discriminators &&
+      value[discriminatorKey] != null) {
+    if (Constructor.discriminators[value[discriminatorKey]]) {
+      Constructor = Constructor.discriminators[value[discriminatorKey]];
+    } else {
+      const constructorByValue = getDiscriminatorByValue(Constructor.discriminators, value[discriminatorKey]);
+      if (constructorByValue) {
+        Constructor = constructorByValue;
+      }
+    }
+  }
+
+  return Constructor;
+};

node_modules/mongoose/lib/helpers/discriminator/getDiscriminatorByValue.js

@@ -0,0 +1,28 @@
+'use strict';
+
+const areDiscriminatorValuesEqual = require('./areDiscriminatorValuesEqual');
+
+/**
+ * returns discriminator by discriminatorMapping.value
+ *
+ * @param {Object} discriminators
+ * @param {string} value
+ * @api private
+ */
+
+module.exports = function getDiscriminatorByValue(discriminators, value) {
+  if (discriminators == null) {
+    return null;
+  }
+  for (const name of Object.keys(discriminators)) {
+    const it = discriminators[name];
+    if (
+      it.schema &&
+      it.schema.discriminatorMapping &&
+      areDiscriminatorValuesEqual(it.schema.discriminatorMapping.value, value)
+    ) {
+      return it;
+    }
+  }
+  return null;
+};

node_modules/mongoose/lib/helpers/discriminator/getSchemaDiscriminatorByValue.js

@@ -0,0 +1,27 @@
+'use strict';
+
+const areDiscriminatorValuesEqual = require('./areDiscriminatorValuesEqual');
+
+/**
+ * returns discriminator by discriminatorMapping.value
+ *
+ * @param {Schema} schema
+ * @param {string} value
+ * @api private
+ */
+
+module.exports = function getSchemaDiscriminatorByValue(schema, value) {
+  if (schema == null || schema.discriminators == null) {
+    return null;
+  }
+  for (const key of Object.keys(schema.discriminators)) {
+    const discriminatorSchema = schema.discriminators[key];
+    if (discriminatorSchema.discriminatorMapping == null) {
+      continue;
+    }
+    if (areDiscriminatorValuesEqual(discriminatorSchema.discriminatorMapping.value, value)) {
+      return discriminatorSchema;
+    }
+  }
+  return null;
+};

node_modules/mongoose/lib/helpers/discriminator/mergeDiscriminatorSchema.js

@@ -0,0 +1,75 @@
+'use strict';
+const schemaMerge = require('../schema/merge');
+const specialProperties = require('../../helpers/specialProperties');
+const isBsonType = require('../../helpers/isBsonType');
+const ObjectId = require('../../types/objectid');
+const isObject = require('../../helpers/isObject');
+/**
+ * Merges `from` into `to` without overwriting existing properties.
+ *
+ * @param {Object} to
+ * @param {Object} from
+ * @param {String} [path]
+ * @api private
+ */
+
+module.exports = function mergeDiscriminatorSchema(to, from, path, seen) {
+  const keys = Object.keys(from);
+  let i = 0;
+  const len = keys.length;
+  let key;
+
+  path = path || '';
+  seen = seen || new WeakSet();
+
+  if (seen.has(from)) {
+    return;
+  }
+  seen.add(from);
+
+  while (i < len) {
+    key = keys[i++];
+    if (!path) {
+      if (key === 'discriminators' ||
+        key === 'base' ||
+        key === '_applyDiscriminators' ||
+        key === '_userProvidedOptions' ||
+        key === 'options') {
+        continue;
+      }
+    }
+    if (path === 'tree' && from != null && from.instanceOfSchema) {
+      continue;
+    }
+    if (specialProperties.has(key)) {
+      continue;
+    }
+    if (to[key] == null) {
+      to[key] = from[key];
+    } else if (isObject(from[key])) {
+      if (!isObject(to[key])) {
+        to[key] = {};
+      }
+      if (from[key] != null) {
+        // Skip merging schemas if we're creating a discriminator schema and
+        // base schema has a given path as a single nested but discriminator schema
+        // has the path as a document array, or vice versa (gh-9534)
+        if ((from[key].$isSingleNested && to[key].$isMongooseDocumentArray) ||
+              (from[key].$isMongooseDocumentArray && to[key].$isSingleNested)) {
+          continue;
+        } else if (from[key].instanceOfSchema) {
+          if (to[key].instanceOfSchema) {
+            schemaMerge(to[key], from[key].clone(), true);
+          } else {
+            to[key] = from[key].clone();
+          }
+          continue;
+        } else if (isBsonType(from[key], 'ObjectId')) {
+          to[key] = new ObjectId(from[key]);
+          continue;
+        }
+      }
+      mergeDiscriminatorSchema(to[key], from[key], path ? path + '.' + key : key, seen);
+    }
+  }
+};