om/node_modules/mongoose/lib/helpers/schema/merge.js

'use strict';

module.exports = function merge(s1, s2) {
  s1.add(s2.obj);

  s1.callQueue = s1.callQueue.concat(s2.callQueue);
  s1.method(s2.methods);
  s1.static(s2.statics);

  for (const query in s2.query) {
    s1.query[query] = s2.query[query];
  }

  for (const virtual in s2.virtuals) {
    s1.virtuals[virtual] = s2.virtuals[virtual].clone();
  }

  s1.s.hooks.merge(s2.s.hooks, false);
};