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From "Heath Thomann (JIRA)" <j...@apache.org>
Subject [jira] Created: (OPENJPA-1936) A database 'FOREIGN KEY' constraint is not detected by default
Date Thu, 03 Feb 2011 00:24:28 GMT
A database 'FOREIGN KEY' constraint is not detected by default
--------------------------------------------------------------

                 Key: OPENJPA-1936
                 URL: https://issues.apache.org/jira/browse/OPENJPA-1936
             Project: OpenJPA
          Issue Type: Improvement
    Affects Versions: 2.0.1
            Reporter: Heath Thomann
            Assignee: Heath Thomann
            Priority: Minor


Take the following SQL to create two tables in a database:

CREATE TABLE PARENT (
    "ID" INT NOT NULL,
    PRIMARY KEY ("ID")
  )
  
CREATE TABLE CHILD (
    "ID" INT NOT NULL,
    "PARENT_ID" INT,
    PRIMARY KEY ("ID"),
    FOREIGN KEY ("PARENT_ID") REFERENCES "PARENT" ("ID")
  )


Take the following two entities:

public class Parent implements Serializable {
  @Id
  private int id;

  @OneToMany(mappedBy = "parent", cascade = CascadeType.ALL, fetch = FetchType.EAGER, orphanRemoval
= true)
  private Collection<Child> childs;
.........


public class Child implements Serializable {
  @Id
  private int id;

  @ManyToOne
  private Parent parent;
.........


If a scenario is executed in which an existing Parent is removed, an existing Child(s) associated
with the Parent will also be removed given the definition of the @OneToMany relationship.
 However, when OpenJPA executes the SQL to remove the Parent and Child, the SQL to remove
the Parent will be executed first.  Given the 'FOREIGN KEY' constraint on the Child table,
a database will throw some kind of 'constraint violation' exception when a Parent is removed
before its Child (if it were not for the 'FOREIGN KEY' constraint on the Child table, the
SQL order would be fine).  In this case, OpenJPA should execute the SQL to remove the Child
first, then the Parent.  However, by default, OpenJPA knows nothing about the 'FOREIGN KEY'
constraint on the Child table and OpenJPA never assumes that there are database constraints
for relationships.  As a result, OpenJPA does not  take them into account when executing SQL.
 To tell OpenJPA that there are database level constraints, and thus to effect the order of
the SQL in this case, a user can perform one of the following options:

1) Use the @ForeignKey annotation (org.apache.openjpa.persistence.jdbc.ForeignKey) in entities
(on the ToOne fields).

2) Have OpenJPA read in the table definitions from the database by adding the following property:
  <property name="openjpa.jdbc.SchemaFactory" value="native(ForeignKeys=true)"/>


While either of these two options will properly handle the above scenario, it can be argued
that OpenJPA should detect the 'FOREIGN KEY' constraint, and not require a user to add an
annotation to their code or set a property.  This JIRA will be used to investigate possible
solutions to change the way the constraints are detected.

Thanks,

Heath

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