We know that the database configuration supports configuring a default database, so the DB object and Model object are bound to a specific database when initialized. There are several ways to switch databases at runtime (assuming our databases include a user user database and an order database):

1. Achieved through different configuration groups. This requires configuring different group settings in the configuration file, and then you can obtain a singleton object of a specific database in the program via g.DB("group name").

2. Switch the database of a singleton object at runtime using the DB.SetSchema method. Note that since the database configuration of the singleton object is modified, the impact is global:

   g.DB().SetSchema("user-schema")
   g.DB().SetSchema("order-schema")

3. Create a Schema database object using the chaining operation Schema method, and create a model object through this database object to perform subsequent chaining operations:

   g.DB().Schema("user-schema").Model("user").All()
   g.DB().Schema("order-schema").Model("order").All()

4. You can also set the corresponding database for the current chaining operation using the chaining operation Model.Schema method. If not set, the default connected database of the DB or TX is used:

   g.Model("user").Schema("user-schema").All()
   g.Model("order").Schema("order-schema").All()
   tip

   Note the difference between the two usage methods: the former creates a Model object after the Schema object and then executes operations; the latter achieves database switching by modifying the database name of the current Model object operation.

5. Additionally, if the current database operation's configured user has permission, cross-domain operations can be achieved directly by including the database name in the table name, even for cross-domain association queries:

   // SELECT * FROM `order`.`order` o LEFT JOIN `user`.`user` u ON (o.uid=u.id) WHERE u.id=1 LIMIT 1
   g.Model("order.order o").LeftJoin("user.user u", "o.uid=u.id").Where("u.id", 1).One()