The following post is for playing with the assiocations.
Uni-directional
public class SourceEntity {
@Id
private Long id;
private String name;
@OneToMany
private List<TargetEntity> targets;
}
public class TargetEntity {
@Id
private Long id;
private String description;
}
Both tables have two columns each. No FK column is added.
For the able table, the third table is "source_entity_targets" is created with two columns "source_entity_id" and "target_id".
Bi-directional
If I add convert it to bi-directional, still it will create a third table. Following changes are added for bi-directional and Target Entity looks like.
public class TargetEntity {
@Id
private Long id;
private String description;
@ManyToOne(fetch = FetchType.LAZY)
private SourceEntity sourceEntity;
}
This time the target entity table has "source_entity_id" column as well. It is "MUL" key.
Adding @JoinColumn annotation in Source table
public class SourceEntity {
@Id
private Long id;
private String name;
@OneToMany(cascade = {CascadeType.ALL})
@JoinColumn
private List<TargetEntity> targets;
}
public class TargetEntity {
@Id
private Long id;
private String description;
@ManyToOne(fetch = FetchType.LAZY)
private SourceEntity sourceEntity;
}
This creates two tables. The source table has 2 columns, Target tables have 4 columns out of which 2 columns are: source_entity_id and targets_id.
On saving the object, the source_entity id was saved in target_ids.
Join Column name
When I added the name to the Join column annotation as following in the source entity class.
Two tables were created.
@OneToMany(cascade = {CascadeType.ALL})
@JoinColumn(name="source_id")
private List<TargetEntity> targets;
Target table's column name "targets_id" is converted to "source_id".
When I added the @JoinColumn annotation in targetClass as
@ManyToOne(fetch = FetchType.LAZY)
@JoinColumn(name="source_id")
private SourceEntity sourceEntity;
Three tables were created.
source_entity has 2 columns.
The target entity has now 3 columns. The third column is "source_id".
And source_entity_targets table with two columns as discussed above.
When saved an object, the source_id was set to null and the source_entity_targets columns had values.
After adding the following to source
@OneToMany(cascade = {CascadeType.ALL}, mappedBy = "sourceEntity")
private List<TargetEntity> targets;
2 tables were created but the source id was not saved in the target table. the source_id column value was set to null.
To fix this, a new sourceEntity should be added to the target entity before saving the source entity. Example:
SourceEntity src = new SourceEntity();
src.setName("source");
List<TargetEntity> targetEntities = new ArrayList<>();
TargetEntity target = new TargetEntity();
target.setDescription("description");
target.setSourceEntity(src); // <<<<< this line adds the source to target.
targetEntities.add(target);
src.setTargets(targetEntities);
dataService.persistData(src);
