fix: add missing file

lib/helpers/schema/applyReadConcern.js

@@ -0,0 +1,22 @@
+'use strict';
+
+const get = require('../get');
+
+module.exports = function applyReadConcern(schema, options) {
+  if (options.readConcern !== undefined) {
+    return;
+  }
+
+  // Don't apply default read concern to operations in transactions,
+  // because you shouldn't set read concern on individual operations
+  // within a transaction.
+  // See: https://www.mongodb.com/docs/manual/reference/read-concern/
+  if (options && options.session && options.session.transaction) {
+    return;
+  }
+
+  const level = get(schema, 'options.readConcern.level', null);
+  if (level != null) {
+    options.readConcern = { level };
+  }
+};