Unidirectional @OneToOne W/OChanging the Existing Schema

Daythryl · **Joined:** Mon Dec 11, 2006 8:28 am **Posts:** 13

I have an existing table/class for which I occasionally have extra information. I'd like to put the extra information into a separate table/class and associate it with the original class, without making any changes to the schema of the original table. Something like this:

Code:

class Char {
   @Id
   private int ID;
   private String font;
   private String size;

   @OneToOne
   private Style style;
}

class Style{
   private boolean bold;
   private boolean italic;
   private boolean underlined'
}

What I'd like to see in the schema is:

Code:

Table                    Create Table                                 
-----------------------  ---------------------------------------------
Char         CREATE TABLE `Char` (     
            `id` bigint(20) NOT NULL auto_increment,   
            `font` varchar(255),         
            `size` varchar(255),     
            PRIMARY KEY  (`id`)                        
                         ) ENGINE=InnoDB   

Style         CREATE TABLE `Style` (                                                                     
            `id` bigint(20) NOT NULL,   
            `bold` tinyint(1),                                                        
            `italic` tinyint(1),                                                                 
            `underlined` tinyint(1),                                                               
            PRIMARY KEY  (`id`)                        
            CONSTRAINT `FK89B773C384085057` FOREIGN KEY (`id`) REFERENCES `Char` (`id`)  
            ) ENGINE=InnoDB                                                         

It seems clear to me that this can work in terms of the SQL, and it's advantageous to me to not have to change the schema of the existing table (there's also a savings if only a small percentage of Chars have Styles).
I've been looking through many, many examples of unidirectional on-to-one mappings and can't see how to do this, or even whether it can be done.

Thanks in advance for any replies.

Michael

Masterplan79th · **Joined:** Fri Jun 26, 2009 6:59 am **Posts:** 23

i ran into a similar problem with a unidirectional one-to-one, unfortunately. unlike the one-to-many relationship. you cannot simply used a mappedBy, or JoinColumn on the target table for a one-to-one relationship. It seems to be a severe limitation to me.

My solution was to add a FK to my source table to link it to my target with a single joincolumn annotation. However, if you truly CANNOT make this change. i would suggest changing your mapping from one-to-one to one-to-many and simply retrieve the Object at index 0 from the set, this ensures that the mapping is from target to source, and requires no changes to your source schema. It isn't an ideal situation, since you're not accurately representing the relationship, but it's the only one i've come across for now.

Hope this helps.