feat(core): add Reference.getEntity() and Reference.getProperty()

Similar to `Collection.getItems()`, it will first check if the entity is initialized, and throw if not.

Closes #304

docs/docs/entity-references.md

@@ -85,6 +85,20 @@ console.log((await book.author.load()).name); // ok, author already loaded
 console.log(book.author.unwrap().name); // ok, author already loaded
 ```
 
+There are also `getEntity()` and `getProperty()` methods that are synchronous getters, 
+that will first check if the wrapped entity is initialized, and if not, it will throw 
+and error.
+
+```typescript
+const book = await orm.em.findOne(Book, 1);
+console.log(book.author instanceof Reference); // true
+console.log(book.author.isInitialized()); // false
+console.log(book.author.getEntity()); // Error: Reference<Author> 123 not initialized
+console.log(book.author.getProperty('name')); // Error: Reference<Author> 123 not initialized
+console.log((await book.author.get('name'))); // ok, loading the author first
+console.log(book.author.getProperty('name')); // ok, author already loaded
+```
+
 If you use different metadata provider than `TsMorphMetadataProvider` 
 (e.g. `ReflectMetadataProvider`), you will also need to explicitly set `wrappedReference` 
 parameter:

lib/entity/Reference.ts

@@ -58,7 +58,19 @@ export class Reference<T extends AnyEntity<T>> {
   }
 
   unwrap(): T {
-    return wrap(this.entity);
+    return this.entity;
+  }
+
+  getEntity(): T {
+    if (!this.isInitialized()) {
+      throw new Error(`Reference<${this.__meta.name}> ${this.__primaryKey} not initialized`);
+    }
+
+    return this.entity;
+  }
+
+  getProperty<K extends keyof T>(prop: K): T[K] {
+    return this.getEntity()[prop];
   }
 
   isInitialized(): boolean {

tests/EntityManager.mongo.test.ts

@@ -1611,6 +1611,8 @@ describe('EntityManagerMongo', () => {
     expect(ref._id).toBeInstanceOf(ObjectId);
     expect(ref.unwrap()).toBeInstanceOf(Author);
     expect(wrap(ref.unwrap()).isInitialized()).toBe(false);
+    expect(() => ref.getEntity()).toThrowError(`Reference<Author> ${ref.id} not initialized`);
+    expect(() => ref.getProperty('email')).toThrowError(`Reference<Author> ${ref.id} not initialized`);
 
     const ref2 = Reference.create(author);
     const ref3 = Reference.create(ref2);
@@ -1632,6 +1634,7 @@ describe('EntityManagerMongo', () => {
     expect(ref4.isInitialized()).toBe(false);
     await expect(ref4.get('name')).resolves.toBe('God');
     expect(ref4.isInitialized()).toBe(true);
+    expect(ref4.getProperty('name')).toBe('God');
     await expect(ref4.get('email')).resolves.toBe('hello@heaven.god');
   });