fix: prevent overwriting user defined writeConcern

lib/helpers/schema/applyWriteConcern.js

@@ -3,9 +3,11 @@
 const get = require('../get');
 
 module.exports = function applyWriteConcern(schema, options) {
-  const writeConcern = get(schema, 'options.writeConcern', {});
+  const writeConcern = options.writeConcern ? options : get(schema, 'options.writeConcern', {});
   if (Object.keys(writeConcern).length != 0) {
-    options.writeConcern = {};
+    if (!options.writeConcern) {
+      options.writeConcern = {};
+    }
     if (!('w' in options) && writeConcern.w != null) {
       options.writeConcern.w = writeConcern.w;
     }

lib/query.js

@@ -1954,7 +1954,6 @@ Query.prototype._optionsForExec = function(model) {
   if (!model) {
     return options;
   }
-
   // Apply schema-level `writeConcern` option
   applyWriteConcern(model.schema, options);
 

test/helpers/applyWriteConcern.test.js

@@ -0,0 +1,33 @@
+'use strict';
+
+const assert = require('assert');
+const applyWriteConcern = require('../../lib/helpers/schema/applyWriteConcern');
+const start = require('../common');
+const mongoose = start.mongoose;
+
+describe('applyWriteConcern', function() {
+  let db;
+  before(function() {
+    db = start();
+  });
+  after(async function() {
+    await db.close();
+  });
+  it('should not overwrite user specified writeConcern options gh-13592', async function() {
+    const options = { writeConcern: { w: 'majority' } };
+    const testSchema = new mongoose.Schema({ name: String }, { writeConcern: { w: 0 } });
+    const Test = db.model('Test', testSchema);
+    await Test.create({ name: 'Test Testerson' });
+    applyWriteConcern(testSchema, options);
+    assert.deepStrictEqual({ writeConcern: { w: 'majority' } }, options);
+    await Test.deleteMany({}, options);
+    assert.deepStrictEqual({ writeConcern: { w: 'majority' } }, options);
+    await Test.deleteMany({});
+    /**
+     * Because no options were passed in, it is using the schema level writeConcern options.
+     * However, because we are ensuring that user specified options are not being overwritten,
+     * this is the only reasonable way to test this case as our options object should not match the schema options.
+     */
+    assert.deepStrictEqual({ writeConcern: { w: 'majority' } }, options);
+  });
+});