Hibernate self referencing relationship

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hibernate self referencing relationship

Following is the directory structure of the project for your reference - The best way to model a one-to-many relationship in hibernate. Post subject: Tree (self referencing table) with JPA. Post . Make the One-to- Many side of the relation as the owning side (to make Hibernate. [Hibernate] Self Join many-to-one relationship A Self-Join or Self-Reference is basically one which has some sort of Parent/Child sturcture.

Various supported techniques In hibernate there are 3 ways to create one-to-one relationships between two entities. Either way we have to use OneToOne annotation. First technique is most widely used and uses a foreign key column in one of the tables. Second technique uses a rather known solution of having a third table to store mapping between first two tables.

Third technique is something new which uses a common primary key value in both the tables. Hibernate one to one mapping with foreign key association In this kind of association, a foreign key column is created in owner entity. This column will store the foreign key for Account table.

Table structure will be like this: To make such association, refer the Account entity in EmployeeEntity class as follow: It has one more parameters named referencedColumnName. This parameter declares the column in the targeted entity that will be used to the join. If no JoinColumn is declared on the owner side, the defaults apply. In a bidirectional relationship, one of the sides and only one has to be the owner. The owner is responsible for the association column s update. To declare a side as not responsible for the relationship, the attribute mappedBy is used.

Lets test above mappings in running code: You can verify the data and mappings in both tables when you run above program. Hibernate one to one mapping with common join table This approach is not new to all of us.

Lets start with targeted DB structure in this technique. In this technique, main annotation to be used is JoinTable. This annotation is used to define the new table name mandatory and foreign keys from both of the tables. Lets see how it is used: Testing above entities generates following SQL queries in log files: Hibernate one to one mapping with shared primary key In this technique, hibernate will ensure that it will use a common primary key value in both the tables.

This way primary key of EmployeeEntity can safely be assumed the primary key of AccountEntity also. The shipment of the goods seems to be outside the scope of the application; the seller and the buyer agree on a method of shipment and payment after the auction ends.

They can do this offline, outside of CaveatEmptor. On the other hand, you could offer an extra escrow service in CaveatEmptor. Sellers would use this service to create a trackable shipment once the auction completed. But you want more service in CaveatEmp-tor.

This scenario calls for a Shipment entity with an optional one-to-one association to an Item. Look at the class diagram for this domain model in figure 7. A row in this table represents a Shipment made in the context of an auction. The tables are shown in figure 7. You now map two classes to three tables: But if a row needs to be inserted because you called aShipment. You could map this association bidirectional, with the same technique on the other side. However, optional one-to-one associations are unidirectional most of the time.

JPA also supports association join tables as secondary tables for an entity. Mapping secondary join tables with annotations You can map an optional one-to-one association to an intermediate join table with annotations: The auction property mapping is a OneToOne; and as before, the foreign key column referencing the ITEM table is moved to the intermediate secondary table: The table for the target JoinColumn is named explicitly.

Why would you use this approach instead of the simpler JoinTable strategy? Declaring a secondary table for an entity is useful if not only one property the many-to-one in this case but several properties must be moved into the secondary table. This completes our discussion of one-to-one association mappings.

To summarize, use a shared primary key association if one of the two entities seems more important and can act as the primary key source. Use a foreign key association in all other cases, and a hidden intermediate join table when your one-to-one association is optional.

We now focus on many-valued entity associations, including more options for one-to-many, and finally, many-to-many mappings. Many-valued entity associations A many-valued entity association is by definition a collection of entity references.

You mapped one of these in the previous topic, section 6. One-to-many associations are the most important kind of entity association that involves a collection. A many-to-many association may always be represented as two many-to-one associations to an intervening class. This model is usually more easily extensible, so we tend not to use many-to-many associations in applications.

If you decide to map collections of entity references, there are a few options and more complex situations that we discuss now, including a many-to-many relationship. The many end of this association was implemented in Java with a Set; you had a collection of bids in the Item class. Why would you do this? To map a bidirectional one-to-many association as a bag, you have to replace the type of the bids collection in the Item persistent class with a Collection and an ArrayList implementation.

The mapping for the association between Item and Bid is left essentially unchanged: Even the tables are the same: Unidirectional and bidirectional lists If you need a real list to hold the position of the elements in a collection, you have to store that position in an additional column.

Verify this with the table shown in figure 7. Also, because this mapping is unidirectional the collection is noninverseyou have to assume that there is no opposite side mapped to the same foreign key column where this constraint could be declared.

hibernate self referencing relationship

Whenever you have a noninverse collection of entity references most of the time a one-to-many with a list, map, or array and the foreign key join column in the target table is not nullable, you need to tell Hibernate about this. Remember that Hibernate ignores the state of an inverse collection! This time, however, the collection contains information that is needed to update the database correctly: If you map a bidirectional one-to-many entity association with an indexed collection this is also true for maps and arraysyou have to switch the inverse sides.

The collection becomes responsible for state synchronization, and the one side, the Bid, has to be made inverse.

Advanced entity association mappings (Hibernate)

Setting insert and update to false has the desired effect. As we discussed earlier, these two attributes used together make a property effectively read-only. This side of the association is therefore ignored for any write operations, and the state of the collection including the index of the elements is the relevant state when the in-memory state is synchronized with the database.

The equivalent in JPA, an indexed collection in a bidirectional one-to-many mapping, is as follows: This mapping is noninverse because no mappedBy attribute is present. We now discuss one more scenario with a one-to-many relationship: Optional one-to-many association with a join table A useful addition to the Item class is a buyer property. You can then call anItem. If made bidirectional, this association will also help to render a screen that shows all auctions a particular user has won: From the point of view of the User class, the association is one-to-many.

The classes and their relationship are shown in figure 7. Why is this association different than the one between Item and Bid? The column has to be nullable—a particular Item may not have been bought as long as the auction is still running. We always try to avoid nullable columns in a relational database schema. Information that is unknown degrades the quality of the data you store.

An optional entity association, be it one-to-one or one-to-many, is best represented in an SQL database with a join table. You added a join table earlier in this topic, for a one-to-one association. To guarantee the multiplicity of one-to-one, you applied unique constraints on both foreign key columns of the join table.

hibernate self referencing relationship

A particular item can be bought only once. You use a Set as the collection type. By forcing a unique constraint on the foreign key column that references the target entity table, you effectively force a one-to-many multiplicity. You can map this association bidirectional with the buyer property of Item. With the join table, you have to move this foreign key column into the join table. First, the association is optional, and you tell Hibernate not to insert a row into the join table if the grouped properties only one here, buyer are null.

Second, this is a bidirectional entity association. As always, one side has to be the inverse end. The Java code to create a link between a bought item and a user object is the same in both cases: To make this mapping bidirectional, add a collection on the User class and make it inverse with mappedBy: The JoinTable annotation is the equivalent in annotations. Many-to-many associations The association between Category and Item is a many-to-many association, as can be seen in figure 7.

In a real system, you may not have a many-to-many association. Our experience is that there is almost always other information that must be attached to each link between associated instances such as the date and time when an item was added You can map secondary tables in JPA to create a one-to-many association with a join table. First, map a ManyToOne to a join table: In Hibernate, you can map the association class as an entity and map two one-to-many associations for either side.

Perhaps more conveniently, you can also map a composite element class, a technique we show later. A simple unidirectional many-to-many association If you require only unidirectional navigation, the mapping is straightforward.

Hibernate One to Many Self Join using Annotations

Unidirectional many-to-many associations are essentially no more difficult than the collections of value-type instances we discussed earlier. For example, if the Category has a set of Items, you can create this mapping: The join table or link table, as some developers call it has two columns: The primary key is a composite of both columns. The full table structure is shown in figure 7.

Duplicate links are therefore allowed; the same Item can be added twice to a Category. With annotations, you can switch to an identifier bag with the Hibernate CollectionId: You may even switch to an indexed collection a map or list in a many-to-many association. The following example maps a list in Hibernate XML: As discussed earlier, JPA only supports ordered collections with an optional OrderBy annotation or ordered by primary keyso you again have to use a Hibernate extension for indexed collection support.

Creating a link between a Category and an Item is easy: Bidirectional many-to-many associations are slightly more difficult. A bidirectional many-to-many association You know that one side in a bidirectional association has to be mapped as inverse because you have named the foreign key column s twice.

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The same principle applies to bidirectional many-to-many associations: Each row of the link table is represented by two collection elements, one element at each end of the association.

An association between an Item and a Category is represented in memory by the Item instance in the items collection of the Category, but also by the Category instance in the categories collection of the Item. Before we discuss the mapping of this bidirectional case, you have to be aware that the code to create the object association also changes: As always, a bidirectional association no matter of what multiplicity requires that you set both ends of the association.

You can choose which side should be inverse. Recall this mapping of the items collection from the previous section: You may reuse this mapping for the Category end of the bidirectional association and map the other side as follows: Again, this setting tells Hibernate to ignore changes made to the categories collection and that the other end of the association, the items collection, is the representation that should be synchronized with the database if you link instances in Java code.

In JPA and with annotations, making a many-to-many association bidirectional is easy. First, the noninverse side: Now the opposite inverse side: In JPA, a bag is a collection without a persistent index: What types of collections may be used for bidirectional many-to-many associations? Do you need the same type of collection at each end? No other mappings can be used for the inverse end of a many-to-many association.

We already frowned at the use of many-to-many associations, because additional columns on the join table are almost always inevitable. Adding columns to join tables In this section, we discuss a question that is asked frequently by Hibernate users: What do I do if my join table has additional columns, not only two foreign key columns?

Imagine that you need to record some information each time you add an Item to a Category. For example, you may need to store the date and the name of the user who added the item to this category.

This requires additional columns on the join table, as you can see in figure 7. The first strategy requires an intermediate entity class for the join table and is mapped with one-to-many associations. The second strategy utilizes a collection of components, with a value-type class for the join table. Mapping the join table to an intermediate entity The first option we discuss now resolves the many-to-many relationship between Category and Item with an intermediate entity class, CategorizedItem.

Hence, the entity class also has a composite key, which you encapsulate in a static nested class for convenience. You can also see that constructing a CategorizedItem involves setting the values of the identifier—composite key values are assigned by the application.

Pay extra attention to the constructor and how it sets the field values and guarantees referential integrity by managing collections on either side of the association. This is necessary because the columns are mapped twice, once in the composite key which is responsible for insertion of the values and again for the many-to-one associations. The Category and Item entities can have a one-to-many association to the CategorizedItem entity, a collection.

For example, in Category: Add the same collection and mapping to Item to complete the association. This code creates and stores a link between a category and an item: The referential integrity of the Java objects is guaranteed by the constructor o CategorizedItem, which manages the collection in aCategory and in anItem Remove and delete the link between a category and an item: The primary advantage of this strategy is the possibility for bidirectional navigation: You can get all items in a category by calling aCategory.

A disadvantage is the more complex code needed to manage the CategorizedItem entity instances to create and remove associations—they have to be saved and deleted independently, and you need some infrastructure in the CategorizedItem class, such as the composite identifier.

However, you can enable transitive persistence with cascading options on the collections from Category and Item to CategorizedItem.

Mapping the join table to a collection of components First, simplify the CategorizedItem class, and make it a value type, without an identifier or any complex constructor: As for all value types, this class has to be owned by an entity. The owner is the Category, and it has a collection of these components: This is the complete mapping for a many-to-many association with extra columns on the join table. There is only one change to the database tables: Except for this change, the tables still look as shown in figure 7.

This is a ternary association mapping: This is a fairly exotic beast! The advantage of a collection of components is clearly the implicit lifecycle of the link objects. To create an association between a Category and an Item, add a new CategorizedItem instance to the collection.

To break the link, remove the element from the collection. No extra cascading settings are required, and the Java code is simplified: The downside of this approach is that there is no way to enable bidirectional navigation: