add getDMByUserId in data store

lib/data-store/data-store.js

@@ -158,6 +158,15 @@ SlackDataStore.prototype.getDMByName = function getDMByName(name) {
 };
 
 
+/**
+ * Returns the DM object between the registered user and the user with the supplied id.
+ * @param id
+ * @return {Object}
+ */
+SlackDataStore.prototype.getDMByUserId = function getDMByName(userId) {
+};
+
+
 /**
  * Returns the bot object matching the supplied id.
  * @param botId

lib/data-store/memory-data-store.js

@@ -135,6 +135,10 @@ SlackMemoryDataStore.prototype.getDMByName = function getDMByName(name) {
   return find(this.dms, ['user', user.id]);
 };
 
+/** @inheritdoc */
+SlackMemoryDataStore.prototype.getDMByUserId = function getDMByUserId(userId) {
+  return find(this.dms, ['user', userId]);
+};
 
 /** @inheritdoc */
 SlackMemoryDataStore.prototype.getBotById = function getBotById(botId) {

test/data-store/memory-data-store.js

@@ -27,6 +27,15 @@ describe('MemoryDataStore', function () {
     });
   });
 
+  describe('#getDMByUserId', function () {
+    it('should get a DM with another user when passed the id of that user', function () {
+      var dataStore = getMemoryDataStore();
+      var user = dataStore.getUserByName('bob');
+      var dm = dataStore.getDMByUserId(user.id);
+      expect(dm.id).to.equal('D0CHZQWNP');
+    });
+  });
+
   describe('#getChannelByName()', function () {
     it('should get a channel by name', function () {
       var dataStore = getMemoryDataStore();