fix(query): add writeConcern() method to avoid writeConcern deprecation warning

Fix #10009

Author: vkarpov15

lib/query.js

@@ -1083,6 +1083,53 @@ Query.prototype.session = function session(v) {
   return this;
 };
 
+/**
+ * Sets the 3 write concern parameters for this query:
+ *
+ * - `w`: Sets the specified number of `mongod` servers, or tag set of `mongod` servers, that must acknowledge this write before this write is considered successful.
+ * - `j`: Boolean, set to `true` to request acknowledgement that this operation has been persisted to MongoDB's on-disk journal.
+ * - `wtimeout`: If [`w > 1`](/docs/api.html#query_Query-w), the maximum amount of time to wait for this write to propagate through the replica set before this operation fails. The default is `0`, which means no timeout.
+ *
+ * This option is only valid for operations that write to the database:
+ *
+ * - `deleteOne()`
+ * - `deleteMany()`
+ * - `findOneAndDelete()`
+ * - `findOneAndReplace()`
+ * - `findOneAndUpdate()`
+ * - `remove()`
+ * - `update()`
+ * - `updateOne()`
+ * - `updateMany()`
+ *
+ * Defaults to the schema's [`writeConcern` option](/docs/guide.html#writeConcern)
+ *
+ * ####Example:
+ *
+ *     // The 'majority' option means the `deleteOne()` promise won't resolve
+ *     // until the `deleteOne()` has propagated to the majority of the replica set
+ *     await mongoose.model('Person').
+ *       deleteOne({ name: 'Ned Stark' }).
+ *       writeConcern({ w: 'majority' });
+ *
+ * @method writeConcern
+ * @memberOf Query
+ * @instance
+ * @param {Object} writeConcern the write concern value to set
+ * @see mongodb https://mongodb.github.io/node-mongodb-native/3.1/api/global.html#WriteConcern
+ * @return {Query} this
+ * @api public
+ */
+
+Query.prototype.writeConcern = function writeConcern(val) {
+  if (val == null) {
+    delete this.options.writeConcern;
+    return this;
+  }
+  this.options.writeConcern = val;
+  return this;
+};
+
 /**
  * Sets the specified number of `mongod` servers, or tag set of `mongod` servers,
  * that must acknowledge this write before this write is considered successful.

test/query.test.js

@@ -3776,4 +3776,16 @@ describe('Query', function() {
 
     assert.deepEqual(q._fields, { doesntpopulate: 0, populatescorrectly: 0 });
   });
+
+  it('sets `writeConcern` option correctly (gh-10009)', function() {
+    const testSchema = new mongoose.Schema({
+      name: String
+    });
+    const Test = db.model('Test', testSchema);
+
+    const q = Test.find();
+    q.writeConcern({ w: 'majority', wtimeout: 1000 });
+
+    assert.deepEqual(q.options.writeConcern, { w: 'majority', wtimeout: 1000 });
+  });
 });