duplicate persistent field names in the embeddable

[Fay Wang <fy...@yahoo.com>]

Hi,
     If an embeddable contains persistent field names which clashes with other persistent field names (either in the owner class, other embeddables, or nested embeddables), openjpa currently maps these fields to the same column, resulting in lost data in some situation or SQL exception in others. JIRA-793 is open to fix this problem. The patch attached in this JIRA is to detect name clashes and throw an exception. In most cases, application developers can either change the field name or use @AttributeOverrides to change the column name as a workaround. However, in situations that involved inheritance or multiple embeddable instances of the same type, there is no workaround:

(1) in the case of inheritance: see
    org.apache.openjpa.persistence.embed.Address (Entity),
    org.apache.openjpa.persistence.embed.BaseEntity (MappedSuperclass),
    org.apache.openjpa.persistence.embed.Geocode (Embeddable)

    Both Address and Geocode inherits from BaseEntity. Address contains Geocode. This results in the same field name for the embeddable (Geocode) and entity class (Address). 

(2) in the case of multiple embeddables of the same type:
    Suppose EntityA contains:
       private Embeddable1 embedObj1;
       private Embeddable1 embedObj2;
    
Throwing an exception in these situations can prevent data loss or sql error from occurring, but leave the application no way to fix the problem. However, it is not appropriate for openjpa to provide unique column names automatically without applications knowing it. 

    To address this issue, one proposal is to make this an option which can be configured by adding an openjpa property, say,   openjpa.jdbc.createUniqueColumnNameIfNameClashes (any other idea?). 

    If this option is on, and there is name clash, openjpa will automatically create unique column names for the duplicate field names. If this option is off, and there is name clash, an exception will be thrown. 

    It is likely this proposal may break some existing applications, which may either need to change the field name, use @AttributeOverrides, or turn openjpa.jdbc.createUniqueColumnNameIfNameClashes on.  

    Any suggestions on the resolution of this issue is mostly appreciated. 

Regards,
Fay