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'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) {  const keys = Object.keys(from);  let i = 0;  const len = keys.length;  let key;
  path = path || '';
  while (i < len) {    key = keys[i++];    if (key === 'discriminators' || key === 'base' || key === '_applyDiscriminators') {      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);    }  }};