fix: sets primary read preference for writes

Uses a primary read preference for write / DDL operations, needed for server selection.

NODE-2784

Author: Thomas Reggi

lib/operations/command_v2.js

@@ -17,7 +17,9 @@ class CommandOperationV2 extends OperationBase {
 
     this.ns = parent.s.namespace.withCollection('$cmd');
     const propertyProvider = this.hasAspect(Aspect.NO_INHERIT_OPTIONS) ? undefined : parent;
-    this.readPreference = ReadPreference.resolve(propertyProvider, this.options);
+    this.readPreference = this.hasAspect(Aspect.WRITE_OPERATION)
+      ? ReadPreference.primary
+      : ReadPreference.resolve(propertyProvider, this.options);
     this.readConcern = resolveReadConcern(propertyProvider, this.options);
     this.writeConcern = resolveWriteConcern(propertyProvider, this.options);
     this.explain = false;

test/functional/collection.test.js

@@ -1,9 +1,11 @@
 'use strict';
+const Topology = require('../../lib/core/sdam/topology').Topology;
 const setupDatabase = require('./shared').setupDatabase;
 const chai = require('chai');
 const expect = chai.expect;
 const sinonChai = require('sinon-chai');
 const mock = require('mongodb-mock-server');
+const ReadPreference = require('../../lib/core/topologies/read_preference');
 chai.use(sinonChai);
 
 describe('Collection', function() {
@@ -1307,4 +1309,42 @@ describe('Collection', function() {
       });
     }
   });
+
+  context('DDL methods with serverSelection readPreference primary', () => {
+    const collectionMethodSet = {
+      createIndex: [{ quote: 'text' }]
+    };
+
+    Object.keys(collectionMethodSet).forEach(operation => {
+      it(`should ${operation} with serverSelection readPreference primary`, {
+        metadata: {
+          requires: { topology: 'replicaset' }
+        },
+        test: function(done) {
+          const opArgs = collectionMethodSet[operation];
+          const configuration = this.configuration;
+          const client = configuration.newClient(configuration.writeConcernMax(), {
+            useUnifiedTopology: true,
+            readPreference: 'primaryPreferred'
+          });
+          client.connect((err, client) => {
+            expect(err).to.not.exist;
+            const db = client.db(configuration.db);
+            const collection = db.collection('db-two');
+            const TopologySpy = this.sinon.spy(Topology.prototype, 'selectServer');
+            const callback = err => {
+              expect(err).to.not.exist;
+              expect(TopologySpy.called).to.equal(true);
+              expect(TopologySpy)
+                .nested.property('args[0][0].readPreference.mode')
+                .to.equal(ReadPreference.PRIMARY);
+              client.close(done);
+            };
+            opArgs.push(callback);
+            collection[operation].apply(collection, opArgs);
+          });
+        }
+      });
+    });
+  });
 });