Persisting an instance of the many side of a in one to many relationship

It seems as if most books only tell you how to setup the relationship for a one to many relationship but seems to fail to tell you the appropriate way to persist in this relationship. So, I have a parent class with many child classes. Also, I am using services and so I am converting from Entities to Dtos and visa versa.
First, i want to insert a new child for the parent. Do I just convert the child Dto to a child entity and then persist the child entity with the reference to the parent? Then do I find the parent and add the child to the parents children so the entities are consistent?
Or do I do a find on the parent and add the child to the parents children list and the child. Will the child be persisted automatically?
With this said, what about updating a child? Do I remove the existing child and add a new one? or do I somehow find the child in the parents children set and update the info there and then somehow the child is persisted automatically? And since I am using Dto to entity, do I use a find to find the parent and then iterate through that parents children til I reach the child I modified by id somehow.
What about Many to Many relationships? With a middle join table? How does JPA know to remove a join if I change the child reference? Does it remove the old and insert the new? Or do I have to do it myself somehow. Which if the later then I really can not have a parent child Dto I really need a parent childlink table where the child is referenced by the linktable? The book I have been reading has not made this very clear.

Hello,
The relationships cascade settings determine what should happen. I would not recommend persisting a new entity that references a non-managed entity. Instead either persist the child then make the associations or find the parent, make the associations then persist the child. In the second case, you do not need to explicietly persist the child if the parent->child relationship is marked cascade persist, since it will be found on flush or commit.
Since you are using DTOs, updates require you merge the changes. You can directly merge the child, and if it has a link to its parent, modify the parent to add the child on the managed child that gets returned.
As for ManyToMany, you must modify the side that controls the relationship. So if the parent owns it, then you must modify or merge the changes into the parent for the changes to be persisted to the database. If it is bi-directional, you must modify both sides so that the cache remains consistent with the database. This should be transparent to your application, and you only need an entity for the relation table if you want to map more complex information that doesn't fit into the JPA ManyToMany options. Otherwise, JPA should handle inserting and deleting enteries to the relation table for you.
Best Regards,
Chris

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If this Answers your issue please mark as answered.

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