some progress

node_modules/mongoose/lib/helpers/processConnectionOptions.js

@@ -0,0 +1,64 @@
+'use strict';
+
+const clone = require('./clone');
+const MongooseError = require('../error/index');
+
+function processConnectionOptions(uri, options) {
+  const opts = options ? options : {};
+  const readPreference = opts.readPreference
+    ? opts.readPreference
+    : getUriReadPreference(uri);
+
+  const resolvedOpts = (readPreference && readPreference !== 'primary' && readPreference !== 'primaryPreferred')
+    ? resolveOptsConflicts(readPreference, opts)
+    : opts;
+
+  return clone(resolvedOpts);
+}
+
+function resolveOptsConflicts(pref, opts) {
+  // don't silently override user-provided indexing options
+  if (setsIndexOptions(opts) && setsSecondaryRead(pref)) {
+    throwReadPreferenceError();
+  }
+
+  // if user has not explicitly set any auto-indexing options,
+  // we can silently default them all to false
+  else {
+    return defaultIndexOptsToFalse(opts);
+  }
+}
+
+function setsIndexOptions(opts) {
+  const configIdx = opts.config && opts.config.autoIndex;
+  const { autoCreate, autoIndex } = opts;
+  return !!(configIdx || autoCreate || autoIndex);
+}
+
+function setsSecondaryRead(prefString) {
+  return !!(prefString === 'secondary' || prefString === 'secondaryPreferred');
+}
+
+function getUriReadPreference(connectionString) {
+  const exp = /(?:&|\?)readPreference=(\w+)(?:&|$)/;
+  const match = exp.exec(connectionString);
+  return match ? match[1] : null;
+}
+
+function defaultIndexOptsToFalse(opts) {
+  opts.config = { autoIndex: false };
+  opts.autoCreate = false;
+  opts.autoIndex = false;
+  return opts;
+}
+
+function throwReadPreferenceError() {
+  throw new MongooseError(
+    'MongoDB prohibits index creation on connections that read from ' +
+            'non-primary replicas.  Connections that set "readPreference" to "secondary" or ' +
+            '"secondaryPreferred" may not opt-in to the following connection options: ' +
+            'autoCreate, autoIndex'
+  );
+}
+
+module.exports = processConnectionOptions;