safe to "reuse" table for mapping of separate entities/relations?

[Andy Schlaikjer]

Hi all,

I was curious if/how I might get into trouble by reusing a table to 
support mapping of an @Entity as well as a @JoinTable. Here's an example:

I have the following entities A, BRef, and B where a unidirectional 
many-to-many relation between A and B is defined via BRef (acting here 
as an explicit join table):

@Entity
class A {
  @Id
  protected long id;

  @OneToMany(mappedBy = "a")
  @OrderBy("idx")
  protected List<BRef> brefs;
}

@Entity
@Table(uniqueConstraints = @UniqueConstraint(columnNames = {"a_id", 
"b_id"}))
class BRef {
  @Id
  protected long id;

  @ManyToOne(optional = false)
  protected A a;

  @ManyToOne(optional = false)
  protected B b;

  @Basic
  protected int idx;
}

@Entity
class B {
  @Id
  protected long id;
}

With this mapping (using the mysql DBDictionary), the following CREATE 
TABLE statement reflects the BRef table:

CREATE TABLE `BRef` (
  `id` bigint(20) NOT NULL,
  `idx` int(11) default NULL,
  `a_id` bigint(20) default NULL,
  `b_id` bigint(20) default NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `a_id` (`a_id`,`b_id`),
  KEY `I_BREF_A` (`a_id`),
  KEY `I_BREF_B` (`b_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

Now I'd like to extend the mapping to include the reverse many-to-many 
relation from B to A:

@Entity
class B {
  @Id
  protected long id;

  @OneToMany
  @JoinTable(
    name = "BRef",
    joinColumns = @JoinColumn(name = "b_id"),
    inverseJoinColumns = @JoinColumn(name = "a_id"),
    uniqueConstraints = @UniqueConstraint(columnNames = {"a_id", "b_id"})
  )
  protected Collection<A> as;
}

When I do this, the CREATE TABLE statement for table "BRef" changes 
slightly (ordering of columns, names of indices):

CREATE TABLE `BRef` (
  `id` bigint(20) NOT NULL,
  `idx` int(11) default NULL,
  `b_id` bigint(20) default NULL,
  `a_id` bigint(20) default NULL,
  PRIMARY KEY  (`id`),
  UNIQUE KEY `a_id` (`a_id`,`b_id`),
  KEY `I_BREF_B_ID` (`b_id`),
  KEY `I_BREF_ELEMENT` (`a_id`)
) ENGINE=InnoDB DEFAULT CHARSET=utf8

So it is clear that the two separate mappings for table "BRef" are 
competing with one another. How else might the OpenJPA runtime be 
affected by this?

Thanks,
Andy