很多网站有多数据库联用的文章，如自强学堂http://code.ziqiangxuetang.com/django/django-multi-database.html
大都只讲解如何让不同的app对应不同的数据库，而没有谈到如何让同一个app里的不同class对应不同的数据库。
经过N多次试验，历经好几天时间，终于找出如下的简便易行的途径，而不需要自己造*，现总结如下：

方式一：通过数据库路由自动分发Model，无需手动using

settings配置如下：

1、增加了DATABASE_ROUTERS，用于指定路由路径
2、增加了DATABASE_APPS_MAPPING，指定app_label对应的数据库

DBNAME = ‘multi_test‘
DATABASES = {
    ‘default‘: {
        ‘ENGINE‘: ‘django.db.backends.sqlite3‘,
        ‘NAME‘: os.path.join(BASE_DIR, ‘db.sqlite3‘),
    },
    ‘db1‘: {
        ‘ENGINE‘: ‘django.db.backends.mysql‘,
        ‘NAME‘: DBNAME,
        ‘USER‘: ‘root‘,
        ‘PASSWORD‘: ‘xxx‘,
        ‘HOST‘: ‘127.0.0.1‘,
        ‘PORT‘: 3306,
        ‘OPTIONS‘: {
            ‘init_command‘: "SET sql_mode=‘STRICT_TRANS_TABLES‘",
        },
    },
    ‘db2‘: {
        ‘ENGINE‘: ‘django.db.backends.postgresql_psycopg2‘,
        ‘NAME‘: DBNAME,
        ‘USER‘: ‘root‘,
        ‘PASSWORD‘: ‘xxx‘,
        ‘HOST‘: ‘127.0.0.1‘,
        ‘PORT‘: 5432,
    },
    ‘db3‘: {
        ‘ENGINE‘: ‘django.db.backends.dummy‘,
    },
}
connect(DBNAME) #connect to mongodb
SUB_DIR = os.path.basename(os.path.dirname(__file__))
DATABASE_ROUTERS = [
    ‘{}.database_router.DatabaseAppsRouter‘.format(SUB_DIR)
]
DATABASE_APPS_MAPPING = {
        # example:
        #‘app_label‘:‘database_name‘,
        ‘mysql‘: ‘db1‘,
        ‘postgres‘: ‘db2‘,
}
# ‘mysql‘, ‘postgres‘需加到INSTALLED_APPS中，它们是通过startapp创建的两个空app

在settings同目录下增加database_router.py 文件，内容如下：
（直接从自强学堂拷贝，只注释掉print语句，无需做其他更改）

# -*- coding: utf-8 -*-
from django.conf import settings
 
DATABASE_MAPPING = settings.DATABASE_APPS_MAPPING
 
 
class DatabaseAppsRouter(object):
    """
    A router to control all database operations on models for different
    databases.
 
    In case an app is not set in settings.DATABASE_APPS_MAPPING, the router
    will fallback to the `default` database.
 
    Settings example:
 
    DATABASE_APPS_MAPPING = {‘app1‘: ‘db1‘, ‘app2‘: ‘db2‘}
    """
 
    def db_for_read(self, model, **hints):
        """"Point all read operations to the specific database."""
        if model._meta.app_label in DATABASE_MAPPING:
            return DATABASE_MAPPING[model._meta.app_label]
        return None
 
    def db_for_write(self, model, **hints):
        """Point all write operations to the specific database."""
        if model._meta.app_label in DATABASE_MAPPING:
            return DATABASE_MAPPING[model._meta.app_label]
        return None
 
    def allow_relation(self, obj1, obj2, **hints):
        """Allow any relation between apps that use the same database."""
        db_obj1 = DATABASE_MAPPING.get(obj1._meta.app_label)
        db_obj2 = DATABASE_MAPPING.get(obj2._meta.app_label)
        if db_obj1 and db_obj2:
            if db_obj1 == db_obj2:
                return True
            else:
                return False
        return None
 
    # for Django 1.4 - Django 1.6
    def allow_syncdb(self, db, model):
        """Make sure that apps only appear in the related database.""" 
        if db in DATABASE_MAPPING.values():
            return DATABASE_MAPPING.get(model._meta.app_label) == db
        elif model._meta.app_label in DATABASE_MAPPING:
            return False
        return None
 
    # Django 1.7 - Django 1.11
    def allow_migrate(self, db, app_label, model_name=None, **hints):
        """
        Make sure that apps only appear in the related database.
        根据app_label的值只在相应的数据库中创建一个表，如果删除该def或
        不指定过滤条件，则一个Model会在每个数据库里都创建一个表。
        """
        if db in DATABASE_MAPPING.values():
            return DATABASE_MAPPING.get(app_label) == db
        elif app_label in DATABASE_MAPPING:
            return False
        return None

models里在class Meta中指定app_label就可以指定它要使用的数据库

例如：settings的DATABASE_APPS_MAPPING中指定app1对应db1、app2对应db2，
那么app1中的Model默认使用db1，而当指定Meta中的app_label为app2时，则改用db2。
注：app_label指定的值须包含在settings的INSTALLED_APPS中，否则makemigrations不会自动创建表。
app1/models.py内容如下：

#coding=utf-8
from django.db import models
from django.utils.encoding import python_2_unicode_compatible

@python_2_unicode_compatible
class Animal(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

    class Meta:
        # 通过app_label来指定要使用的数据库
        # 需指定db_table，否则该class的表名会是mysql_animal
        # 如果不指定Meta的app_label，会使用默认数据库
        app_label = ‘mysql‘ 
        db_table = ‘app1_animal‘

@python_2_unicode_compatible
class Plant(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

    class Meta:
        app_label = ‘postgres‘
        db_table = ‘app1_plant‘

app2/models.py

#coding=utf-8
from django.db import models
from django.utils.encoding import python_2_unicode_compatible

@python_2_unicode_compatible
class Fruit(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

    class Meta:
        app_label = ‘mysql‘  #使用mysql数据库
        db_table = ‘app2_fruit‘  #指定表名为app2_fruit

@python_2_unicode_compatible
class Nut(models.Model):
    name = models.CharField(max_length=20)

    def __str__(self):
        return self.name

    class Meta:
        app_label = ‘postgres‘  #使用postgres数据库
        db_table = ‘app2_nut‘  #指定表名为app2_fruit

app1/test.py

from django.test import TestCase

from app1.models import Animal, Plant
from app2.models import Fruit, Nut

class ModelsTestCase(TestCase):
    # db1 -- mysql
    # db2 -- postgres
    def setUp(self):
        Animal.objects.create(name=‘db1‘)
        Plant.objects.create(name=‘db2‘)
        Fruit.objects.create(name=‘mysql_fruit‘)
        Nut.objects.create(name=‘post_nut‘)

    def test_methods_auto_choose_db(self):
        self.assertEqual(Animal.objects.all()[0],
                Animal.objects.using(‘db1‘).all()[0])
        self.assertEqual(Plant.objects.all()[0],
                Plant.objects.using(‘db2‘).all()[0])
        self.assertEqual(Fruit.objects.all()[0],
                Fruit.objects.using(‘db1‘).all()[0])
        self.assertEqual(Nut.objects.all()[0],
                Nut.objects.using(‘db2‘).all()[0])

        self.assertNotEqual(Animal.objects.all()[0],
                Fruit.objects.using(‘db1‘).all()[0])
        self.assertNotEqual(Plant.objects.all()[0],
                Nut.objects.using(‘db2‘).all()[0])
        self.assertNotEqual(Fruit.objects.all()[0],
                Plant.objects.using(‘db2‘).all()[0])

然后在命令行中运行

python manage.py makemigrations
python manage.py migrate --database=db1
python manage.py migrate --database=db2
python manage.py migrate
python manage.py test

完整代码放在coding上：

git clone https://git.coding.net/zhengwenjie/multi_dbs.git
cd multi_dbs
virtualenv venv
source venv/bin/activate
pip install -r requirements.txt
vi multi_dbs/settings.py #修改数据库名和密码等
alias mg=‘python manage.py‘
mg makemigrations
mg migrate --database=db1
mg migrate --database=db2
mg migrate
mg test

方式二：通过using手动指定数据库

例如settings中的数据库配置如下：

# Database
# https://docs.djangoproject.com/en/1.11/ref/settings/#databases

import pymysql
pymysql.install_as_MySQLdb()
from mongoengine import connect
DBNAME = ‘multi_test‘
DATABASES = {
    ‘default‘: {
        ‘ENGINE‘: ‘django.db.backends.sqlite3‘,
        ‘NAME‘: os.path.join(BASE_DIR, ‘db.sqlite3‘),
    },
    ‘mysql‘: {
        ‘ENGINE‘: ‘django.db.backends.mysql‘,
        ‘NAME‘: DBNAME,
        ‘USER‘: ‘root‘,
        ‘PASSWORD‘: ‘xxx‘,
        ‘HOST‘: ‘127.0.0.1‘,
        ‘PORT‘: 3306,
        ‘OPTIONS‘: {
            ‘init_command‘: "SET sql_mode=‘STRICT_TRANS_TABLES‘",
        },
    },
    ‘postgres‘: {
        ‘ENGINE‘: ‘django.db.backends.postgresql_psycopg2‘,
        ‘NAME‘: DBNAME,
        ‘USER‘: ‘root‘,
        ‘PASSWORD‘: ‘xxx‘,
        ‘HOST‘: ‘127.0.0.1‘,
        ‘PORT‘: 5432,
    },
    ‘mongo‘: {
        ‘ENGINE‘: ‘django.db.backends.dummy‘,
    },
}

则查询和创建数据语句如下（不加using为采用default数据库）

from app1.models import House
House.objects.using(‘mysql‘).create(name=‘Tom‘)
House.objects.using(‘mysql‘).get(name=‘Tom‘)
House.objects.using(‘postgres‘).get_or_create(name=‘Jim‘)