Added basic tests for collections caching

Added also comments for Mongo dummy mocks

src/packages/emmett-mongodb/src/eventStore/storage/mongoDBEventStoreStorage.unit.spec.ts

@@ -51,6 +51,20 @@ void describe('mongoDBEventStoreStorage', () => {
     assertEqual(collection.dbName, defaultDBName);
   });
 
+  void it('resolves the same instance of collection for multiple calls for the same stream type resolution', async () => {
+    // Given
+    const storage = mongoDBEventStoreStorage({
+      getConnectedClient,
+    });
+
+    // When
+    const firstResolution = await storage.collectionFor(testStreamTypeName);
+    const nextResolution = await storage.collectionFor(testStreamTypeName);
+
+    // Then
+    assertEqual(firstResolution, nextResolution);
+  });
+
   void describe('Single Collection storage', () => {
     void it('handles SINGLE_COLLECTION passed as string with default collection name and no db', async () => {
       // Given

src/packages/emmett-mongodb/src/testing/mongoMocks.ts → src/packages/emmett-mongodb/src/testing/mongoDummies.ts

@@ -7,6 +7,13 @@ import type {
   MongoClient,
 } from 'mongodb';
 
+/**
+ * Creates a dummy MongoDB collection. It should not be used as in-memory version,
+ * but just a dummy replacement for the basic structure and calls test.
+ * @param name collection name
+ * @param options collection setup options
+ * @returns Dummmy collection that has name, dbName and createIndex method
+ */
 export const getDummyCollection = <TSchema extends Document = Document>(
   name: string,
   options?: CollectionOptions & { dbName?: string },
@@ -20,6 +27,13 @@ export const getDummyCollection = <TSchema extends Document = Document>(
   return dummyCollection;
 };
 
+/**
+ * Creates a dummy MongoDB database. It should not be used as in-memory version,
+ * but just a dummy replacement for the basic structure and calls test.
+ * @param dbName database name
+ * @param options database setup options
+ * @returns Dummmy database that has name and can setup dummy collection
+ */
 export const getDummyDb = (dbName?: string, options?: DbOptions): Db => {
   const dummyDB: Db = {
     databaseName: dbName!,
@@ -30,6 +44,12 @@ export const getDummyDb = (dbName?: string, options?: DbOptions): Db => {
   return dummyDB;
 };
 
+/**
+ * Creates a dummy MongoDB connection. It should not be used as in-memory version,
+ * but just a dummy replacement for the basic structure and calls test.
+ * @param options setup options allowing to pass the default database name
+ * @returns Dummmy connection that can setup a dummy database
+ */
 export const getDummyClient = (options?: {
   defaultDBName?: string;
 }): MongoClient => {