JPA not instantiating mapped collections.

I struggled a while now to figure out why I am getting a NullPointerException
when trying to access a mapped collection:

@OneToMany(cascade = CascadeType.ALL, mappedBy = "cmTypedef")

private List cmParamdefs;

I instantiated a object, then em.merge() it but still I got a NPE.
I really thought that merge would populate all the fields.


Then I tried to make it Eager:

@OneToMany(cascade = CascadeType.ALL, mappedBy = "cmTypedef",

   fetch = FetchType.EAGER) 

private List cmParamdefs = new ArrayList();

Still no luck :(


So merge didn't do the trick, but trying everything I saw that if I do

em.flush();

em.refresh(ret);

after the merge, then like magic the collection is populated..


Finally I remember I had this issue before and the simple solution I have so far is
to manually instantiate the collection:

@OneToMany(cascade = CascadeType.ALL, mappedBy = "cmTypedef")

private List cmParamdefs = new ArrayList();

I'm not sure if this is the best practice, but it works and is simple..