fix(pinia-orm): hasManyThrough doesn't work correctly with mulitple model instances

packages/pinia-orm/src/model/attributes/relations/HasManyThrough.ts

@@ -95,7 +95,7 @@ export class HasManyThrough extends Relation {
       const key = model[this.localKey]
 
       dictionary[key]
-        ? model.$setRelation(relation, dictionary[key][0])
+        ? model.$setRelation(relation, dictionary[key].flat().filter(item => item !== undefined))
         : model.$setRelation(relation, [])
     })
   }

packages/pinia-orm/tests/feature/relations/has_many_through_retrieve.spec.ts

@@ -1,7 +1,7 @@
 import { describe, expect, it } from 'vitest'
 
 import { Model, useRepo } from '../../../src'
-import { Attr, HasManyThrough, Str } from '../../../src/decorators'
+import { Attr, HasMany, HasManyThrough, Str } from '../../../src/decorators'
 import { assertInstanceOf, assertModel, fillState } from '../../helpers'
 
 describe('feature/relations/has_many_through_retrieve', () => {
@@ -115,4 +115,72 @@ describe('feature/relations/has_many_through_retrieve', () => {
     expect(country.posts[0].id).toBe(2)
     expect(country.posts[1].id).toBe(1)
   })
+
+  it('can retrieve multi-level "has many through" relations with manual comparison', () => {
+    // Extend the Country model with a direct relationship to users to manifest the difference
+    class ExtendedCountry extends Country {
+      @HasMany(() => User, 'countryId')
+      declare users: User[]
+    }
+
+    // Extend the User model with a direct relationship to posts to manifest the difference
+    class ExtendedUser extends User {
+      @HasMany(() => Post, 'userId')
+      declare posts: Post[]
+    }
+
+    const countryRepo = useRepo(ExtendedCountry)
+
+    // Set up test data with:
+    // - 2 countries
+    // - 3 users (2 in country 1, 1 in country 2)
+    // - 5 posts (3 from user 1, 1 from user 2, 1 from user 3)
+    fillState({
+      users: {
+        1: { id: 1, name: 'John Doe', countryId: 1 },
+        2: { id: 2, name: 'Jane Doe', countryId: 1 },
+        3: { id: 3, name: 'Johnny Doe', countryId: 2 },
+      },
+      countries: {
+        1: { id: 1 },
+        2: { id: 2 },
+      },
+      posts: {
+        1: { id: 1, userId: 1, title: 'Title 01' },
+        2: { id: 2, userId: 1, title: 'Title 02' },
+        3: { id: 3, userId: 1, title: 'Title 03' },
+        4: { id: 4, userId: 2, title: 'Title 04' },
+        5: { id: 5, userId: 3, title: 'Title 05' },
+      },
+    })
+
+    // Register the extended user model to make posts accessible
+    useRepo(ExtendedUser)
+
+    // Retrieve country with all related data
+    const country = countryRepo.withAllRecursive().first()!
+
+    // Verify the country is retrieved correctly
+    expect(country.id).toBe(1)
+
+    // Verify users are retrieved correctly
+    expect(country.users.length).toBe(2)
+
+    // Verify posts are retrieved correctly through HasManyThrough
+    expect(country.posts.length).toBe(4) // Should include all posts from all users in country 1
+
+    // Manually retrieve all posts through users to compare
+    const manualPosts = country.users.flatMap((user) => {
+      // Cast user to ExtendedUser to access posts
+      return (user as unknown as ExtendedUser).posts
+    })
+
+    // Compare the manual retrieval with the HasManyThrough relationship
+    // This tests that HasManyThrough correctly retrieves all related models
+    expect(country.posts.length).toEqual(manualPosts.length)
+
+    // Verify all expected post IDs are present
+    const postIds = country.posts.map(post => post.id).sort()
+    expect(postIds).toEqual([1, 2, 3, 4])
+  })
 })