许多数据库会自动对这些键施加约束，以便值真正匹配另一个表中的一条记录，这有助于摆脱这一困难。
Many databases will enforce constraints on these keys automatically so that value actually matches a record in the another table, which can help this out.
当定义一个主键约束时，联邦系统自动创建一个惟一的索引规范。
When a primary key constraint is defined, the federated system automatically creates a unique index specification.
一方面，它生成了特定模型的实例，并自动确认它们是否与已有的OCL约束兼容。
On one hand, it generates instances of a certain model, and it verifies automatically whether they are compatible with the existing OCL constraints.
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