如果我已经在数据库中有成千上万个相关实体，并且在集合中添加了新实体，那么在JPA中如何保持一对多关系

Question

We have two entities Entity1 and Entity2, where Entity1 contains set of Entity2, we already have thousands of entities stored in database of entity type Entity2 which all are referenced from an instance of Entity1, say myEntity.

Now if i add more Entity2 entities to the collection and try to persist myEntity, where newly added entities of Entity2 are already persisted.

My question is how will be the behavior on persist of myEntity , whether existing members of relation will travel to memory and new members will be added or new members are added to database without bringing existing members to memory

Answer 1

If you want to add new instances to a relation between two already existent entities (a one to many in this case) then you must first fetch from the database the entity that contains the collection; in your case myEntity .

So, when you load that entity you are bringing it to memory. If you had defined the relation between those two as EAGER then all the related entities (the ones in the collection) will be fetched as well at the same time than the parent one. If you, otherwise, had defined the relation as LAZY then the related entities will be loaded when you access the collection (in other, words, when you invoke the getter getXXX method for that collection).

This happens that way because JPA implementations (now I'm thinking on Hibernate) return proxies of the entities instead of actual instances so they can intercept the getter/setter method calls and perform any tracking on the state of the entities.

Right, so now you want to add more instances to the relation. It doesn't matter whether the relation is EAGER or LAZY in this case as you will eventually invoking the getter method of the collection in order to be able to perform add(myNewEntity); on it. So, the already existent entities are in the collection and you are just adding a (probably) untracked entity under the collection implementation semantics.

When persisting myEntity back to the database the JPA implementation will know which instances of the actual collection need either an update , a delete or an insert . If you just added new instances then just insert statements will be issued but you could also remove an entity from the collection or change the state (invoke the setter) of an already existent instance. JPA implementations are able to recognise those operations and issue the appropriate SQL statements to keep the database up to date.

Answer 2

If you have thousands of referenced entities, it might be better not to map the relationship and instead only query for it when needed - allowing you to use paging or other mechanisms to reduce the amount of entities read in at a time. It depends on what type of mapping it is, but only the owning relationship needs to be mapped (the one that doesn't have the mapped by) to set the foreign key in the database. Set the Entity2 side to be the owning side if it isn't already.

If this is a M:M with a relation table and doesn't make sense to map from the Entity2 side instead - you could add an entity for the relation table that you would read in the same way. The new entity would have a reference to Entity1, but Entity1 wouldn't reference it, and the app would query for the new entity when it needs to get Entity2s associated to a specific Entity1.