fix: multiple instances of the same repository class

Fix DefaultCrudRepository to re-use legacy juggler Models across
all instances of the same Repository class.

Before this change, each call of DefaultCrudRepository constructor
was redefining the backing persisted model.

This commit adds a caching mechanism to DefaultCrudRepository:
when setting up a backing model, we check datasource's modelBuilder
registry to find if the backing model was not already created by
an older instance of the repository.

docs/site/Repositories.md

@@ -9,7 +9,9 @@ summary:
 ---
 
 A Repository is a type of _Service_ that represents a collection of data within
-a DataSource.
+a DataSource. A repository class is a lightweight object, its instances can be
+created with low runtime overhead. Typically a new repository instance is
+created for each incoming request.
 
 ## Example Application
 

packages/repository/src/loopback-datasource-juggler.ts

@@ -179,6 +179,8 @@ export declare namespace juggler {
       properties?: AnyObject,
       options?: Options,
     ): T;
+
+    modelBuilder: ModelBuilder;
   }
 
   /**

packages/repository/src/repositories/legacy-juggler-bridge.ts

@@ -15,7 +15,7 @@ import {
   NamedParameters,
   PositionalParameters,
 } from '../common-types';
-import {Entity} from '../model';
+import {Entity, ModelDefinition} from '../model';
 import {Filter, Where} from '../query';
 import {EntityCrudRepository} from './repository';
 
@@ -91,7 +91,19 @@ export class DefaultCrudRepository<T extends Entity, ID>
       `Entity ${entityClass.name} must have at least one id/pk property.`,
     );
 
-    // Create an internal legacy Model attached to the datasource
+    this.setupPersistedModel(definition);
+  }
+
+  // Create an internal legacy Model attached to the datasource
+  private setupPersistedModel(definition: ModelDefinition) {
+    const dataSource = this.dataSource;
+
+    const model = dataSource.modelBuilder.models[definition.name];
+    if (model) {
+      // The backing persisted model has been already defined.
+      this.modelClass = model as typeof juggler.PersistedModel;
+      return;
+    }
 
     // We need to convert property definitions from PropertyDefinition
     // to plain data object because of a juggler limitation

packages/repository/test/unit/repositories/legacy-juggler-bridge.unit.ts

@@ -71,6 +71,13 @@ describe('DefaultCrudRepository', () => {
     await model.deleteAll();
   });
 
+  it('shares the backing PersistedModel across repo instances', () => {
+    const model1 = new DefaultCrudRepository(Note, ds).modelClass;
+    const model2 = new DefaultCrudRepository(Note, ds).modelClass;
+
+    expect(model1 === model2).to.be.true();
+  });
+
   it('implements Repository.create()', async () => {
     const repo = new DefaultCrudRepository(Note, ds);
     const note = await repo.create({title: 't3', content: 'c3'});