feat(repository): add interface for hasManyThrough repository

The hasManyThrough repository interface defines the actions that can be performed on a many-to-many model relation.

This PR is a continuation of loopbackio#2359 and implements a step from loopbackio#2359 (comment).

packages/repository/src/relations/has-many/has-many-through.repository.ts

@@ -0,0 +1,108 @@
+// Copyright IBM Corp. 2020. All Rights Reserved.
+// Node module: @loopback/repository
+// This file is licensed under the MIT License.
+// License text available at https://opensource.org/licenses/MIT
+
+import {Count, DataObject, Entity, Filter, Options, Where} from '../..';
+
+/**
+ * CRUD operations for a target repository of a HasManyThrough relation
+ */
+export interface HasManyThroughRepository<
+  Target extends Entity,
+  TargetID,
+  Through extends Entity
+> {
+  /**
+   * Create a target model instance
+   * @param targetModelData - The target model data
+   * @param options - Options for the operation
+   * @returns A promise which resolves to the newly created target model instance
+   */
+  create(
+    targetModelData: DataObject<Target>,
+    options: Options & {
+      throughData?: DataObject<Through>;
+      throughOptions?: Options;
+    },
+  ): Promise<Target>;
+  /**
+   * Find target model instance(s)
+   * @param filter - A filter object for where, order, limit, etc.
+   * @param options - Options for the operation
+   * @returns A promise which resolves with the found target instance(s)
+   */
+  find(
+    filter?: Filter<Target>,
+    options?: Options & {
+      throughOptions?: Options;
+    },
+  ): Promise<Target[]>;
+  /**
+   * Delete multiple target model instances
+   * @param where - Instances within the where scope are deleted
+   * @param options
+   * @returns A promise which resolves the deleted target model instances
+   */
+  delete(
+    where?: Where<Target>,
+    options?: Options & {
+      throughOptions?: Options;
+    },
+  ): Promise<Count>;
+  /**
+   * Patch multiple target model instances
+   * @param dataObject - The fields and their new values to patch
+   * @param where - Instances within the where scope are patched
+   * @param options
+   * @returns A promise which resolves the patched target model instances
+   */
+  patch(
+    dataObject: DataObject<Target>,
+    where?: Where<Target>,
+    options?: Options & {
+      throughOptions?: Options;
+    },
+  ): Promise<Count>;
+
+  /**
+   * Creates a new many-to-many association to an existing target model instance
+   * @param targetModelId - The target model ID to link
+   * @param options
+   * @returns A promise which resolves to the linked target model instance
+   */
+  link(
+    targetModelId: TargetID,
+    options?: Options & {
+      throughData?: DataObject<Through>;
+      throughOptions?: Options;
+    },
+  ): Promise<Target>;
+
+  /**
+   * Removes an association to an existing target model instance
+   * @param targetModelId - The target model to unlink
+   * @param options
+   * @returns A promise which resolves to null
+   */
+  unlink(
+    targetModelId: TargetID,
+    options?: Options & {
+      throughOptions?: Options;
+    },
+  ): Promise<void>;
+
+  /**
+   * Checks if the given target model instance is associated with the source model.
+   * @param targetModelId - The target model ID to check
+   * @param options
+   * @returns A promise which resolves to true when the association exists
+   * and false otherwise.
+   */
+  exists(
+    targetModelId: TargetID,
+    options?: Options & {
+      throughOptions?: Options;
+    },
+  ): Promise<boolean>;
+}