add field mixin

src/register.ts

-import { Context, Keys, Model } from 'koishi';
+import { Context, Dict, Flatten, Keys, Model, Tables } from 'koishi';
 import { ModelClassType } from './def';
 import { reflector } from './meta/meta';
+import _ from 'lodash';
 
 class ModelRegistrar<T = any> {
   constructor(private cls: ModelClassType<T>, private prefix = '') {}
@@ -170,3 +171,23 @@ export function registerModel(
   ctx.model.extend(tableName, ...registrar.getModelResult());
   Object.assign(ctx.model.config[tableName].internal, registrar.getInternal());
 }
+
+export function mixinModel<K extends Keys<Tables>>(
+  ctx: Context,
+  tableName: K,
+  classDict: {
+    [F in Keys<Tables[K]>]?: ModelClassType<Flatten<Tables[K][F]>>;
+  },
+) {
+  for (const _key in classDict) {
+    const key = _key as Keys<Tables[K]>;
+    const cls = classDict[key];
+    const registrar = new ModelRegistrar<any>(cls, key + '.');
+    const result = registrar.getModelResult();
+    ctx.model.extend(tableName, ...result);
+    Object.assign(
+      ctx.model.config[tableName].internal,
+      registrar.getInternal(),
+    );
+  }
+}

tests/user-mixin.spec.ts

 import { ChildModel, DefineModel, ModelField } from '../src/decorators';
 import { App } from 'koishi';
-import { registerModel } from '../src/register';
+import { mixinModel, registerModel } from '../src/register';
 import MemoryDatabase from '@koishijs/plugin-database-memory';
+import exp from 'constants';
 
 declare module 'koishi' {
   // eslint-disable-next-line @typescript-eslint/no-empty-interface
@@ -11,30 +12,38 @@ declare module 'koishi' {
   }
 }
 
-class Dress {
+class WearingPreference {
   @ModelField('string(8)')
   color: string;
-  @ModelField('integer(7)')
-  size: string;
+  @ModelField('string(12)')
+  shape: string;
+
+  format() {
+    return `${this.color} ${this.shape}`;
+  }
 }
 
-class Shirt {
-  @ModelField('string(8)')
-  color: string;
-  @ModelField('integer(7)')
+class Wearing {
+  @ModelField('string(3)')
   size: string;
-}
 
-@DefineModel('user')
-class UserMixin1 {
+  getSize() {
+    return this.size;
+  }
+
   @ChildModel()
-  dress: Dress;
+  preference: WearingPreference;
 }
 
-@DefineModel('user')
-class UserMixin2 {
-  @ChildModel()
-  shirt: Shirt;
+class Dress extends Wearing {
+  getDressSize() {
+    return this.getSize();
+  }
+}
+class Shirt extends Wearing {
+  getShirtSize() {
+    return this.getSize();
+  }
 }
 
 describe('Model test', () => {
@@ -43,22 +52,63 @@ describe('Model test', () => {
   beforeEach(async () => {
     app = new App();
     app.plugin(MemoryDatabase);
+    mixinModel(app, 'user', { dress: Dress, shirt: Shirt });
     await app.start();
   });
 
   it('should register model fields', () => {
-    registerModel(app, UserMixin1);
-    registerModel(app, UserMixin2);
     const { user } = app.model.config;
-    expect(user.fields['dress.color'].type).toBe('string');
-    expect(user.fields['dress.size'].type).toBe('integer');
-    expect(user.fields['dress.color'].length).toBe(8);
-    expect(user.fields['dress.size'].length).toBe(7);
-    expect(user.fields['shirt.color'].type).toBe('string');
-    expect(user.fields['shirt.size'].type).toBe('integer');
-    expect(user.fields['shirt.color'].length).toBe(8);
-    expect(user.fields['shirt.size'].length).toBe(7);
+    expect(user.fields['dress.size'].type).toBe('string');
+    expect(user.fields['dress.size'].length).toBe(3);
+    expect(user.fields['shirt.size'].type).toBe('string');
+    expect(user.fields['shirt.size'].length).toBe(3);
+    expect(user.fields['dress.preference.color'].type).toBe('string');
+    expect(user.fields['dress.preference.color'].length).toBe(8);
+    expect(user.fields['shirt.preference.color'].type).toBe('string');
+    expect(user.fields['shirt.preference.color'].length).toBe(8);
+    expect(user.fields['dress.preference.shape'].type).toBe('string');
+    expect(user.fields['dress.preference.shape'].length).toBe(12);
+    expect(user.fields['shirt.preference.shape'].type).toBe('string');
+    expect(user.fields['shirt.preference.shape'].length).toBe(12);
+
     expect(user.internal['dress.']).toEqual(Dress.prototype);
     expect(user.internal['shirt.']).toEqual(Shirt.prototype);
+    expect(user.internal['dress.preference.']).toEqual(
+      WearingPreference.prototype,
+    );
+    expect(user.internal['shirt.preference.']).toEqual(
+      WearingPreference.prototype,
+    );
+  });
+
+  it('should make class instance', async () => {
+    await app.database.upsert('user', [
+      {
+        id: '111111',
+        dress: {
+          size: 'S',
+          preference: {
+            color: 'red',
+            shape: 'round',
+          },
+        },
+        shirt: {
+          size: 'M',
+          preference: {
+            color: 'blue',
+            shape: 'square',
+          },
+        },
+      },
+    ]);
+    const [user] = await app.database.get('user', { id: '111111' });
+    expect(user.dress).toBeInstanceOf(Dress);
+    expect(user.dress.getDressSize()).toBe('S');
+    expect(user.dress.preference).toBeInstanceOf(WearingPreference);
+    expect(user.dress.preference.format()).toBe('red round');
+    expect(user.shirt).toBeInstanceOf(Shirt);
+    expect(user.shirt.getShirtSize()).toBe('M');
+    expect(user.shirt.preference).toBeInstanceOf(WearingPreference);
+    expect(user.shirt.preference.format()).toBe('blue square');
   });
 });