A role grants privileges to perform sets of actions on defined resources. 角色授予对定义的资源执行一组操作的权限。A given role applies to the database on which it is defined and can grant access down to a collection level of granularity.给定的角色适用于定义它的数据库，并且可以授予访问权限，访问权限可以达到集合级别的粒度。
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The database this user's role should effect.此用户角色应影响的数据库。