fix: use options for readPreference in client

NODE-2807

Author: Thomas Reggi

lib/mongo_client.js

@@ -169,6 +169,7 @@ function MongoClient(url, options) {
     dbCache: new Map(),
     sessions: new Set(),
     writeConcern: WriteConcern.fromOptions(options),
+    readPreference: ReadPreference.fromOptions(options) || ReadPreference.primary,
     namespace: new MongoDBNamespace('admin')
   };
 }
@@ -188,7 +189,7 @@ Object.defineProperty(MongoClient.prototype, 'writeConcern', {
 Object.defineProperty(MongoClient.prototype, 'readPreference', {
   enumerable: true,
   get: function() {
-    return ReadPreference.primary;
+    return this.s.readPreference;
   }
 });
 

test/functional/mongo_client.test.js

@@ -3,6 +3,7 @@
 var f = require('util').format;
 var test = require('./shared').assert;
 var setupDatabase = require('./shared').setupDatabase;
+const ReadPreference = require('../../lib/core/topologies/read_preference');
 const Db = require('../../lib/db');
 const expect = require('chai').expect;
 
@@ -831,4 +832,10 @@ describe('MongoClient', function() {
         }
       });
   });
+
+  it('should cache a resolved readPreference from options', function() {
+    const client = this.configuration.newClient({}, { readPreference: ReadPreference.SECONDARY });
+    expect(client.readPreference).to.be.instanceOf(ReadPreference);
+    expect(client.readPreference).to.have.property('mode', ReadPreference.SECONDARY);
+  });
 });