[jboss-user] [Beginners Corner] - How to correctly work with Many to One relationships.
Hi...
In the scope of a project we're working on, we're sometimes running into trouble
with One to Many relationships. We've found two working scenarios but can't realy
figure out whether they are "principaly correct" or not.
Scenario 1
----------
Lets assume a table parent and a table child, knowing that many child records may
reference a single parent. (BTW we're using Postgres)
CREATE TABLE parent
(
id SERIAL PRIMARY KEY,
name VARCHAR(128)
);
CREATE TABLE child
(
id SERIAL PRIMARY KEY,
parent_id INTEGER,
name VARCHAR(128)
);
ALTER TABLE child ADD CONSTRAINT fk_parent_id FOREIGN KEY (parent_id) REFERENCES
parent(id) ON DELETE CASCADE;
In the parent Entity Bean, we have a property children, which is a collection, and which
is mapped as follows:
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
@JoinColumn(name = "parent_id")
public Collection getChildren ()
{
return children;
}
In the Child Entity Bean, we have a parentId property of type Integer, which is mapped as
follows:
@Column(name = "parent_id", updatable=false, insertable=false)
public Integer getParentId()
{
return parentId;
}
This scenario works fine as long as no NOT NULL constraints are being specified for the
foreign key parent_id. It also fails as soon as the updatable and insertable attributes
are removed from the child mapping.
The advantage of this scheme is:
We only have to add childs to the children collection an to perist the parent in order to
store both the parent and the children. No need to previously set the parentId of the
children.
Scenario 2
----------
Another scenario we have tried is as follows: Again the two tables (please note the
addition NOT NULL constraint.
CREATE TABLE parent
(
id SERIAL PRIMARY KEY,
name VARCHAR(128)
);
CREATE TABLE child
(
id SERIAL PRIMARY KEY,
parent_id INTEGER NOT NULL,
name VARCHAR(128)
);
ALTER TABLE child ADD CONSTRAINT fk_parent_id FOREIGN KEY (parent_id) REFERENCES
parent(id) ON DELETE CASCADE;
In the parent Entity Bean, we have a property children, which is a collection, and which
is mapped as follows:
@OneToMany(cascade = CascadeType.ALL, fetch = FetchType.EAGER)
@JoinColumn(name = "parent_id")
public Collection getChildren ()
{
return children;
}
In the Child Entity Bean, we have a parent property of type Parent, which is mapped as
follows:
@ManyToOne
@JoinColumn(name="parent_id")
public Parent getParent()
{
return parent;
}
Disadvantage of this solution is that we have to explicitly tell each child who his parent
is.
Question
--------
1.) Both solution appear to fail when it comes to deleting children, though. In both cases
we have to explicitely determine which children are no longer require and we have to
remove them explicitely. Are there other ways of using One to Many relationships ?
2.) My guts tell me that the second scenario appears to be more "as intended by the
inventor" or am I wrong ?
Many thanks in advance for your help...
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