【www.bbyears.com--php与数据库】
对于Django的使用中,大家有时候会遇到一些项目需要使用多个数据库,这下问题来了,怎么去使用呢?下面文章就给大家具体介绍下使用方法,感兴趣的一起来了解下。
1.在settings中设定DATABASE
比如要使用两个数据库:
DATABASES = { "default": { "NAME": "app_data", "ENGINE": "django.db.backends.postgresql", "USER": "postgres_user", "PASSWORD": "s3krit" }, "users": { "NAME": "user_data", "ENGINE": "django.db.backends.mysql", "USER": "mysql_user", "PASSWORD": "priv4te" } }
这样就确定了2个数据库,别名一个为default,一个为user。数据库的别名可以任意确定。
default的别名比较特殊,一个Model在路由中没有特别选择时,默认使用default数据库。
当然,default也可以设置为空:
DATABASES = { "default": {}, "users": { "NAME": "user_data", "ENGINE": "django.db.backends.mysql", "USER": "mysql_user", "PASSWORD": "superS3cret" }, "customers": { "NAME": "customer_data", "ENGINE": "django.db.backends.mysql", "USER": "mysql_cust", "PASSWORD": "veryPriv@ate" } }
这样,因为没有了默认的数据库,就需要为所有的Model,包括使用的第三方库中的Model做好数据库路由选择。
2.为需要做出数据库选择的Model规定app_label
class MyUser(models.Model): ... class Meta: app_label = "users"
3.写Database Routers
Database Router用来确定一个Model使用哪一个数据库,主要定义以下四个方法:
db_for_read(model, **hints)
规定model使用哪一个数据库读取。
db_for_write(model, **hints)
规定model使用哪一个数据库写入。
allow_relation(obj1, obj2, **hints)
确定obj1和obj2之间是否可以产生关联, 主要用于foreign key和 many to many操作。
allow_migrate(db, app_label, model_name=None, **hints)
确定migrate操作是否可以在别名为db的数据库上运行。
一个完整的例子:
数据库设定:
DATABASES = { "default": {}, "auth_db": { "NAME": "auth_db", "ENGINE": "django.db.backends.mysql", "USER": "mysql_user", "PASSWORD": "swordfish", }, "primary": { "NAME": "primary", "ENGINE": "django.db.backends.mysql", "USER": "mysql_user", "PASSWORD": "spam", }, "replica1": { "NAME": "replica1", "ENGINE": "django.db.backends.mysql", "USER": "mysql_user", "PASSWORD": "eggs", }, "replica2": { "NAME": "replica2", "ENGINE": "django.db.backends.mysql", "USER": "mysql_user", "PASSWORD": "bacon", }, }
如果想要达到如下效果:
app_label为auth的Model读写都在auth_db中完成,其余的Model写入在primary中完成,读取随机在replica1和replica2中完成。
auth:
class AuthRouter(object): """ A router to control all database operations on models in the auth application. """ def db_for_read(self, model, **hints): """ Attempts to read auth models go to auth_db. """ if model._meta.app_label == "auth": return "auth_db" return None def db_for_write(self, model, **hints): """ Attempts to write auth models go to auth_db. """ if model._meta.app_label == "auth": return "auth_db" return None def allow_relation(self, obj1, obj2, **hints): """ Allow relations if a model in the auth app is involved. """ if obj1._meta.app_label == "auth" or \ obj2._meta.app_label == "auth": return True return None def allow_migrate(self, db, app_label, model_name=None, **hints): """ Make sure the auth app only appears in the "auth_db" database. """ if app_label == "auth": return db == "auth_db" return None
这样app_label为auth的Model读写都在auth_db中完成,允许有关联,migrate只在auth_db数据库中可以运行。
其余的:
import random class PrimaryReplicaRouter(object): def db_for_read(self, model, **hints): """ Reads go to a randomly-chosen replica. """ return random.choice(["replica1", "replica2"]) def db_for_write(self, model, **hints): """ Writes always go to primary. """ return "primary" def allow_relation(self, obj1, obj2, **hints): """ Relations between objects are allowed if both objects are in the primary/replica pool. """ db_list = ("primary", "replica1", "replica2") if obj1._state.db in db_list and obj2._state.db in db_list: return True return None def allow_migrate(self, db, app_label, model_name=None, **hints): """ All non-auth models end up in this pool. """ return True
这样读取在随机在replica1和replica2中完成,写入使用primary。
最后在settings中设定:
DATABASE_ROUTERS = ["path.to.AuthRouter", "path.to.PrimaryReplicaRouter"]
就可以了。
进行migrate操作时:
$ ./manage.py migrate $ ./manage.py migrate --database=users
migrate操作默认对default数据库进行操作,要对其它数据库进行操作,可以使用--database选项,后面为数据库的别名。
与此相应的,dbshell,dumpdata,loaddata命令都有--database选项。
也可以手动的选择路由:
查询:
>>> # This will run on the "default" database. >>> Author.objects.all() >>> # So will this. >>> Author.objects.using("default").all() >>> # This will run on the "other" database. >>> Author.objects.using("other").all()
保存:
>>> my_object.save(using="legacy_users")
移动:
>>> p = Person(name="Fred") >>> p.save(using="first") # (statement 1) >>> p.save(using="second") # (statement 2)
以上的代码会产生问题,当p在first数据库中第一次保存时,会默认生成一个主键,这样使用second数据库保存时,p已经有了主键,这个主键如果未被使用不会产生问题,但如果先前被使用了,就会覆盖原先的数据。
有两个解决方法;
1.保存前清除主键:
>>> p = Person(name="Fred") >>> p.save(using="first") >>> p.pk = None # Clear the primary key. >>> p.save(using="second") # Write a completely new object.
2.使用force_insert
>>> p = Person(name="Fred") >>> p.save(using="first") >>> p.save(using="second", force_insert=True)
删除:
从哪个数据库取得的对象,从哪删除
>>> u = User.objects.using("legacy_users").get(username="fred") >>> u.delete() # will delete from the `legacy_users` database
如果想把一个对象从legacy_users数据库转移到new_users数据库:
>>> user_obj.save(using="new_users") >>> user_obj.delete(using="legacy_users")