Spring-hibernate mapping problem

Question

I have a spring-hibernate application which is failing to map an object properly: basically I have 2 domain objects, a Post and a User. The semantics are that every Post has 1 corresponding User.

The Post domain object looks roughly as follows:

class Post {

  private int pId;
  private String attribute;
  ...
  private User user;

  //getters and setters here

}

As you can see, Post contains a reference to User. When I load a Post object, I want to corresponding User object to be loaded (lazily - only when its needed).

My mapping looks as follows:

<class name="com...Post" table="post">
    <id name="pId" column="PostId" />
    <property name="attribute" column="Attribute" type="java.lang.String" />

    <one-to-one name="User" fetch="join"
        class="com...User"></one-to-one>
</class>

And of course I have a basic mapping for User set up.

As far as my table schema is concerned, I have a table called post with a foreign UserId which links to the user table.

I thought this setup should work, BUT when I load a page that forces the lazy loading of the User object, I notice the following Hiberate query being generated:

Select ... from post this_ left outer join user user2_ on this.PostId=user2_.UserId ...

Obviously this is wrong: it should be joining UserId from post with UserId from user, but instead its incorrectly joining PostId from post (its primary key) with UserId from user.

Any ideas? Thanks!

Update: Thanks to a couple of the posts below I now realize that I should have been using a many-to-one mapping instead of a one-to-one. I changed the mapping under post to the following:

<many-to-one name="User" class="com...User" column="uId"/>

But now I get a run-time error telling me that there is no attribute called uId. This makes sense since I do not have a uId column in my post domain object (I simply have a reference to a user object). Now I am really confused as to how I can get Hibernate to realize that it needs to map the foreign key from the post table to the user table. Should explicitly add a uId attribute to my post domain object to be a placeholder for the foreign key?

I hope I am making sense...

Answer 1

Since a user has many posts, your association is in fact a "many-to-one", not a "one-to-one". It should work if you map it accordingly.

Edit: Yes, you can map the property Post.user on the Post with a "many-to-one", or the set User.posts in User with a "one-to-many", or both. Have you specified the name of your foreign key column?

Edit2: In Hibernate speak, a "column" in the database is mapped to a "property" in your Java-Class. That is, the column attribute contains the name of your foreign key column in the database, not the name of any property in your Java class. If I read your question right, you should use "UserId", not "uId".

Oh, and a fetch="join" can not be lazy, as it mandates that the user is fetched in the same query as the post.

Answer 2

That is the behaviour of a one-to-one mapping. They usually share a primary key. Hibernate is assuming that the primary key of post is teh same as the primary key of user. This page summarizes this behaviour.

I suspect that one user can actually have more than one posts though. That makes your mapping a one-to-many.