refactor(client): provide default namespace and read preference

Author: mbroadst

lib/mongo_client.js

@@ -8,6 +8,8 @@ const inherits = require('util').inherits;
 const MongoError = require('./core').MongoError;
 const deprecate = require('util').deprecate;
 const WriteConcern = require('./write_concern');
+const MongoDBNamespace = require('./utils').MongoDBNamespace;
+const ReadPreference = require('./core/topologies/read_preference');
 
 // Operations
 const ConnectOperation = require('./operations/connect');
@@ -150,7 +152,8 @@ function MongoClient(url, options) {
     promiseLibrary: null,
     dbCache: new Map(),
     sessions: [],
-    writeConcern: WriteConcern.fromOptions(options)
+    writeConcern: WriteConcern.fromOptions(options),
+    namespace: new MongoDBNamespace('admin')
   };
 
   // Get the promiseLibrary
@@ -172,6 +175,13 @@ Object.defineProperty(MongoClient.prototype, 'writeConcern', {
   }
 });
 
+Object.defineProperty(MongoClient.prototype, 'readPreference', {
+  enumerable: true,
+  get: function() {
+    return ReadPreference.primary;
+  }
+});
+
 /**
  * The callback format for results
  * @callback MongoClient~connectCallback