JPA Persist vs Merge

Once again, Persist versus Merge

There are tons of posts on internet on this topic; I will not duplicate already existing info but just emphasize a simple sample showing that not understanding the basics of JPA notions can lead to very unpleasant side effects.

To began with some (very) simplified assertions on above mentioned operations:

• Persist will:
  • turn your newly created entity into a managed one and will issue an INSERT statement at transaction commit or flush operation;
  • For IDENTITY/SEQUENCE based PK generation the latest PK value is retrieved from the database and filled in the managed entity (which is the original one passed to the Persist operation!)

• Merge will:

  • Create a managed copy of your entity; this copy will be returned by the merge call while your original entity will not become managed(!)
  • SELECT SQL statements will be issued in order to decide if and INSERT or UPDATE will be used prior the end of transaction/flush operation
  • The appropriate SQL statement will be then issued (INSERT/UPDATE)
  • For IDENTITY/SEQUENCE based PK generation the latest PK value is retrieved from the database and filled in the managed entity (not in the original one!)

Now, in conjunction with CascadeType.Persist/All applied on a @OneToMany relationship the disaster is eminent. Let me explain on a simple sample.

Suppose we have a Car entity (owned by a company) having as a property the current driver and we want to manage also the history of allocated drivers. We will be tempted to have something like this:

    @OneToMany(cascade = CascadeType.ALL, mappedBy = "carId")
    private Collection<PersonHistory> personHistoryCollection;

Imagine the following scenario:

• We change the driver for a Car; the back end action automatically does the following operations:
  • add a new driver to the personHistoryCollection
  • changes the previous current driver in personHistoryCollection last date he owned the car
  • changes the current driver reference in the Car instance
• We call merge operation on the Car instance

At the moment of calling the merge we have the following situation:

• Car instance modified
• Previous PersonHistory ( member in personHistoryCollection) having last date modified
• A new PersonHistory entity added to the collection

The Persist operation will update in the database the Car and, due to the CascadeType.All, will be subsequently applied also to all members of personHistoryCollection updating the previous PersonHistory entity and inserting a new PersonHistory. The INSERT will not update the PK of the newly created PersonHistory (the PK will remain NULL) due to the way Merge operation works on copies of original entity and thus leaving the collection in an inconsistent state.

Suppose you have chosen the @SessionScoped model for your backing bean and during the same Web session you add a new driver: the next merge operation sees that the previous PersonHistory has a NULL PK and performs an INSERT instead on an UPDATE. Nice, isn’t it?

So, be sure you have a at least a basic understanding of persist/merge JPA operations and be cautious using CascadeTpe attributes. Apart from the inconsistencies between in session image and database image the merge operation applied in conjunction with CascadeType.* will lead to a lot of futile SQLECT statements.

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