fix(schema): avoid applying default write concern to operations that are in a transaction

lib/helpers/schema/applyWriteConcern.js

@@ -6,6 +6,12 @@ module.exports = function applyWriteConcern(schema, options) {
   if (options.writeConcern != null) {
     return;
   }
+  // Don't apply default write concern to operations in transactions,
+  // because setting write concern on an operation in a transaction is an error
+  // See: https://www.mongodb.com/docs/manual/reference/write-concern/
+  if (options && options.session && options.session.transaction) {
+    return;
+  }
   const writeConcern = get(schema, 'options.writeConcern', {});
   if (Object.keys(writeConcern).length != 0) {
     options.writeConcern = {};

test/docs/transactions.test.js

@@ -477,4 +477,20 @@ describe('transactions', function() {
 
     assert.equal(i, 3);
   });
+
+  it('doesnt apply schema write concern to transaction operations (gh-11382)', async function() {
+    db.deleteModel(/Test/);
+    const Test = db.model('Test', Schema({ status: String }, { writeConcern: { w: 'majority' } }));
+
+    await Test.createCollection();
+    await Test.deleteMany({});
+
+    const session = await db.startSession();
+
+    await session.withTransaction(async function() {
+      await Test.findOneAndUpdate({}, { name: 'test' }, { session });
+    });
+
+    await session.endSession();
+  });
 });