fix: create nested data with composite key

src/model/Model.ts

@@ -15,6 +15,7 @@ import * as Payloads from '../modules/payloads/Actions'
 import Fields from './contracts/Fields'
 import ModelState from './contracts/State'
 import Serializer from './Serializer'
+import NoKey from '../schema/NoKey'
 
 type InstanceOf<T> = T extends new (...args: any[]) => infer R ? R : any
 
@@ -391,7 +392,7 @@ export default class Model {
       return record[key]
     }
 
-    return key.map(k => record[k]).join('_')
+    return key.map(k => (record[k] || NoKey.increment())).join('_')
   }
 
   /**

test/feature/relations/BelongsTo.spec.js

@@ -90,6 +90,49 @@ describe('Features – Relations – Belongs To', () => {
     expect(store.state.entities).toEqual(expected)
   })
 
+  it('can generate relation field with composite key', () => {
+    class User extends Model {
+      static entity = 'users'
+
+      static fields() {
+        return {
+          id: this.attr(null)
+        }
+      }
+    }
+
+    class Post extends Model {
+      static entity = 'posts'
+      static primaryKey = ['id', 'user_id']
+
+      static fields() {
+        return {
+          id: this.attr(null),
+          user_id: this.attr(null),
+          user: this.belongsTo(User, 'user_id')
+        }
+      }
+    }
+
+    const store = createStore([{ model: User }, { model: Post }])
+
+    store.dispatch('entities/posts/create', {
+      data: [
+        { id: 1, user: { id: 10 } },
+        { id: 1, user: { id: 20 } }
+      ]
+    })
+
+    expect(store.state.entities.users.data[10].id).toBe(10)
+    expect(store.state.entities.users.data[20].id).toBe(20)
+    expect(store.state.entities.posts.data['1_10'].$id).toBe('1_10')
+    expect(store.state.entities.posts.data['1_10'].id).toBe(1)
+    expect(store.state.entities.posts.data['1_10'].user_id).toBe(10)
+    expect(store.state.entities.posts.data['1_20'].$id).toBe('1_20')
+    expect(store.state.entities.posts.data['1_20'].id).toBe(1)
+    expect(store.state.entities.posts.data['1_20'].user_id).toBe(20)
+  })
+
   it('returns created record from `create` method', async () => {
     const store = createStore([{ model: User }, { model: Post }])