java JPA @Column 注释以创建注释/描述
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JPA @Column annotation to create comment/description
提问by masterdany88
I was wondering is it possible to create from jpa/hibernate annotation a database column description/comment like this:
我想知道是否可以从 jpa/hibernate 注释创建这样的数据库列描述/评论：
ALTER TABLE tablename CHANGE status status INT(11) NOT NULL COMMENT 'sample description/comment';
ALTER TABLE tablename CHANGE status status INT(11) NOT NULL COMMENT 'sample description/comment';
It will be great functionality, but I cant find anything about this in JPA specification.
这将是很棒的功能，但我在 JPA 规范中找不到任何关于此的信息。
Maybe I should use @Column(columnDefinition="")property, but I dont have any clue.
Please help
也许我应该使用@Column(columnDefinition="")财产，但我没有任何线索。请帮忙
采纳答案by masterdany88
I found answer for my own question.
我找到了我自己问题的答案。
I am not sure is it ok? Will it work for all Databases?
我不确定可以吗？它适用于所有数据库吗？
For sure it works for mysql.
肯定它适用于 mysql。
Here is the working code:
这是工作代码：
@Column(columnDefinition=" INT(11) NOT NULL COMMENT '0 for no action, 1 for executed, 2 for validated, 3 for aproved'")
private int status;
回答by Guillaume Husta
Comment description already exists for Table annotations. We must use the Hibernate @Table annotation for that, complementary with the JPA @Table annotation.
表注释的注释描述已存在。为此，我们必须使用 Hibernate @Table 注释，与 JPA @Table 注释互补。
For example :
例如 ：
@javax.persistence.Table( name = "Cat" )
@org.hibernate.annotations.Table( comment = "Table for cats" )
public class Cat {
...
Concerning the column's comment : It seems there is no equivalent, even in Hibernate 5.2 / JPA 2.1.
关于该专栏的评论：即使在 Hibernate 5.2 / JPA 2.1 中，似乎也没有等效项。
There has been an issue submitted a long time ago (2007) on this subject, but still unresolved : Support @Comment or column attribute on @Table and @Column. Abandoned since 2012 ?
很久以前（2007 年）就这个主题提交了一个问题，但仍未解决：支持 @Table 和 @Column 上的 @Comment 或 column 属性。自 2012 年以来被遗弃？
I also found comment use in org.hibernate.dialect.Dialect:
我还在org.hibernate.dialect.Dialect 中发现了评论使用：
/**
* Does this dialect/database support commenting on tables, columns, etc?
*
* @return {@code true} if commenting is supported
*/
public boolean supportsCommentOn() {
return false;
}
/**
* Get the comment into a form supported for table definition.
*
* @param comment The comment to apply
*
* @return The comment fragment
*/
public String getTableComment(String comment) {
return "";
}
/**
* Get the comment into a form supported for column definition.
*
* @param comment The comment to apply
*
* @return The comment fragment
*/
public String getColumnComment(String comment) {
return "";
}
For example PostgreSQL81Dialect supports it (supportsCommentOn() returns true).
例如 PostgreSQL81Dialect 支持它（supportsCommentOn() 返回 true）。
It enables the use of the SQL command "COMMENT ON ...",
like in PostgreSQL (https://www.postgresql.org/docs/current/static/sql-comment.html).
For example :
它允许使用 SQL 命令“COMMENT ON ...”，就像在 PostgreSQL ( https://www.postgresql.org/docs/current/static/sql-comment.html) 中一样。
例如 ：
COMMENT ON TABLE my_schema.my_table IS 'Employee Information';
COMMENT ON COLUMN my_table.my_column IS 'Employee ID number';
It seems to be used there : org.hibernate.tool.schema.internal.StandardTableExporter#getSqlCreateStrings
它似乎在那里使用：org.hibernate.tool.schema.internal.StandardTableExporter#getSqlCreateStrings
The column's comment is extracted from the Hibernate Mapping with org.hibernate.mapping.Column#getComment.
该列的注释是从带有org.hibernate.mapping.Column#getComment的 Hibernate Mapping 中提取的。
Finally, If using Hibernate Toolsand reverse engineering, the column's comment is extracted from JDBC DatabaseMetaData, with org.hibernate.cfg.reveng.BasicColumnProcessor#processBasicColumns.
最后，如果使用Hibernate Tools和逆向工程，则列的注释是从 JDBC DatabaseMetaData 中提取的，使用org.hibernate.cfg.reveng.BasicColumnProcessor#processBasicColumns。
-> String comment = (String) columnRs.get("REMARKS");
回答by Vlad Mihalcea
While Hibernate mappings are needed for mapping Domain Models to Relational Data, it's not a good idea to stretch this feature to generating the database schema too.
虽然将领域模型映射到关系数据需要 Hibernate 映射，但将此功能扩展到生成数据库模式也不是一个好主意。
This might work for a relatively small project, but when you have a medium to large enterprise application, that evolves with every Sprint iteration, you then need a database schema migration tool, like Flyway.
这可能适用于一个相对较小的项目，但是当您有一个大中型企业应用程序时，它会随着每次 Sprint 迭代而发展，那么您就需要一个数据库模式迁移工具，比如Flyway。
The comment doesn't require a Domain Model mapping, since it's a relational specific description. The Domain Model should make use JavaDocs to document each field meaning (according to a specific domain logic requirement).
注释不需要域模型映射，因为它是关系特定的描述。域模型应该使用 JavaDocs 来记录每个字段的含义（根据特定的域逻辑要求）。
