Hi Michael Simons

a problem I see in your mapping is the same name used for the @JoinColum

@JoinColumn(name="commentable_id", referencedColumnName="id")

in 2 classes

BulletinComment

and

PageComment 

belonging to SINGLE_TABLE inheritance and referring to 2 different Entities.

@JoinColumn(name="commentable_id", referencedColumnName="id")
	@ManyToOne(fetch=FetchType.EAGER, optional=false)
	private Bulletin commentable;

involves the creation of
foreign key (commentable_id)
references news

@JoinColumn(name="commentable_id", referencedColumnName="id")
	@ManyToOne(fetch=FetchType.EAGER, optional=false)
	private Page commentable;

while creates
foreign key (commentable_id)
references page

he same column is used for 2 differents FK and this is not possible.
So your problem can be solved assigning 2 different names for the @JoinColumn.

The issue instead remains when no name is assigned to the 2 @JoinComun.