django 数据库|Django多数据库使用

时间:2021-08-15  来源:php与数据库  阅读:

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")

django 数据库|Django多数据库使用

http://m.bbyears.com/jiaocheng/136199.html

推荐访问:django数据库操作 django数据库迁移
相关阅读 猜你喜欢
本类排行 本类最新