Use one single table "blank_fields" for both A and B. "blank_fields" has fields: 'ref_id', 'blank_field', 'type'. 'type' is used to identify which entity the record belongs to. Use 'type' + 'ref_id' to specify the collection of elements for one entity.
@Entity
@Table(name = "table_a")
public class A {
private Set<BlankField> blankFields = new HashSet<BlankField>();
@CollectionOfElements
@Fetch(FetchMode.SUBSELECT)
@Enumerated(EnumType.ORDINAL)
@JoinTable(name = "blank_fields", joinColumns = { @JoinColumn(name = "ref_id") })
@Cascade(value = org.hibernate.annotations.CascadeType.DELETE_ORPHAN)
@Column(name = "blank_field", nullable = false)
@SQLInsert(sql = "INSERT INTO blank_fields(ref_id, blank_field, type) VALUES(?,?,0)")
@Where(clause = "type=0")
public Set<BlankField> getBlankFields() { // BlankField is an enum
return blankFields;
}
@SuppressWarnings("unused")
private void setBlankFields(Set<BlankField> blankFields) {
this.blankFields = blankFields;
}
} // End B
@Entity
@Table(name = "table_b")
public class B {
private Set<BlankField> blankFields = new HashSet<BlankField>();
@CollectionOfElements
@Fetch(FetchMode.SUBSELECT)
@Enumerated(EnumType.ORDINAL)
@JoinTable(name = "blank_fields", joinColumns = { @JoinColumn(name = "ref_id") })
@Cascade(value = org.hibernate.annotations.CascadeType.DELETE_ORPHAN)
@Column(name = "blank_field", nullable = false)
@SQLInsert(sql = "INSERT INTO blank_fields(ref_id, blank_field, type) VALUES(?,?,1)") // used for insert
@Where(clause = "type=1") // used for query, if not @CollectionOfElements, such as @OneToMany, use @WhereJoinTable instead
public Set<BlankField> getBlankFields() {
return blankFields;
}
@SuppressWarnings("unused")
private void setBlankFields(Set<BlankField> blankFields) {
this.blankFields = blankFields;
}
}
當(dāng)然還有其他的方式來實現(xiàn)上面的需求，上面采用的單表來記錄不同實體的associations（這兒是CollectionOfElements，并且返回的是Set<Enum>，不是Set<Embeddable>)，然后用'type'來區(qū)分不同的實體，這樣做的好處是：數(shù)據(jù)庫冗余少，易于擴展，對于新的實體，只需加一個type值，而不需更改數(shù)據(jù)庫表結(jié)構(gòu)。另外一種采用單表的方式是為每個實體增加新的字段，如"blank_fields": 'a_id', 'b_id', 'blank_field', a_id reference table_a (id), b_id reference table_b (id). 這樣在映射的時候更簡單，
對于A，映射為
@JoinTable(name = "blank_fields", joinColumns = { @JoinColumn(name = "a_id") })
對于B，映射為
@JoinTable(name = "blank_fields", joinColumns = { @JoinColumn(name = "b_id") })
這樣作的缺點是：帶來了數(shù)據(jù)庫冗余，對于blank_fields來講，任一條記錄，a_id和b_id中只有一個不為null。當(dāng)多個實體共用這個表時，用上面的方法更合理，如果共用實體不多時，這種方法更方便。