使用中间表（中间实体）与两个ManyToOne进行JPA映射

Question

I am trying to set a JPA mapping with JoinTable, and it seems to be ignored when Hibernate (my JPA implementation) is doing a query.

To explain the use case

Each time a user gets a page of my app, I insert a line in the USAGE_LOG table (with the id of the user and the id of the page).

Each page is related to a category (for instance: settings, orders, items, news...) and a type (for instance create, update, display, delete).

So, I have some kind of middle entity table, that links a page to: a category + a type. Like a triplet: (page, category, type)

My table structure

table USAGE_LOG (for information only, this one works well)
   ID PrimaryKey
   USER_ID Foreign key to column ID of table USER
   USAGE_LOG_PAGE_ID Foreign key to column ID of table USER_LOG_PAGE

table USAGE_LOG_PAGE
   ID PrimaryKey
   URL VARCHAR
   USER_ACTION_ID Foreign key to column ID of table USER_ACTION

table USER_ACTION
   ID PrimaryKey
   ACTION_CATEGORY_ID Foreign key to column ID of table ACTION_CATEGORY
   ACTION_TYPE_ID Foreign key to column ID of table ACTION_CATEGORY

table ACTION_CATEGORY
   ID PrimaryKey
   NAME VARCHAR

table ACTION_TYPE
   ID PrimaryKey
   NAME VARCHAR

So the USER_ACTION table is a join table with the particularity that it links a USAGE_LOG_PAGE to a ACTION_CATEGORY and a ACTION_TYPE at the same time.

Also, I can have several USAGE_LOG_PAGE that are linked to the same ACTION_CATEGORY and ACTION_TYPE.

Unfortunately, I cannot change the database structure (it is legacy code).

I have tried the following Mappping on the Entity "UsageLogPage"

@ManyToOne
@JoinTable(name="action",
        joinColumns=@JoinColumn(name="ID", referencedColumnName="USER_ACTION_ID"),
        inverseJoinColumns=@JoinColumn(name="ACTION_CATEGORY_ID", referencedColumnName="ID"))
@Getter @Setter
private ActionCategory actionCategory;

@ManyToOne
@JoinTable(name="action",
    joinColumns=@JoinColumn(name="ID", referencedColumnName="USER_ACTION_ID"),
    inverseJoinColumns=@JoinColumn(name="ACTION_TYPE_ID", referencedColumnName="ID"))
@Getter @Setter
private ActionType actionType;

(I use Lombok for @Getter and @Setter)

This mapping compiles, but when I try to get data, I have the following exception: com.mysql.jdbc.exceptions.jdbc4.MySQLSyntaxErrorException: Unknown column 'usagelogpa0_.actionCategory' in 'field list'

Indeed, the Hibernate query is:

select usagelogpa0_.ID as ID1_80_0_,
   usagelogpa0_.actionCategory as actionCa2_80_0_,
   usagelogpa0_.actionType as actionTy3_80_0_,
   usagelogpa0_.URL as URL5_80_0_
from usage_log_page usagelogpa0_
where usagelogpa0_.ID=?

(the key part is the "actionCategory" and "actionType" in the select)

This is not what I expect, Hibernate should do a join.

Have you any idea of what I did wrong?

Thanks !

Answer 1

After lots of investigations, I have found that:

• it wasn't working as expected because I put the @ManyToOne and the @JoinTable annotations at the attribute level. I created a getter by hand and put the annotations on it, and they were taken into account
• it still wasn't working correctly, because Hibernate didn't find the column "USER_ACTION_ID" on the USAGE_LOG_PAGE table, at run time. This column wasn't in the available fields, for a reason (that I coudn't find). When adding a field "usage_action_id" in the entity "UsageLogPage", it found the attribute, but refused to create the mapping because USAGE_ACTION_ID isn't a primary key.

At the end, even if I couldn't change the database, I could change the object model.

So I created the middle entity "UserAction", binded it with ManyToOne on the UsageLogPage entity, removed the attribute "actionCategory" and "actionType" from the UsageLogPage and added them as ManyToOne in the new UserAction entity.

If you have a table that acts as a middle entity for 2 different ManyToOne relationships, perhaps the best solution is to create the middle entity in your object model.