等于grails中的域类

Question

The following equals code returns wrong result for domain

boolean equals(o) {
  if (o == null) return false
  if (this.is(o)) return true
  getClass() == o.class && id == o.id
}

For two loaded entities with same id return false. Id are equals (one record in DB). But classes not same.

The entities -- fields in another domains. And it looks like GORM used some wrapper classes.

How to avoid this kind of problem?

Answer 1

As you are seeing, requiring that the classes be the same is overly strict. Using instanceof is usually safer, eg

class Foo {
    boolean equals(o) {
        if (!o) return false
        if (is(o)) return true
        o instanceof Foo && id == o.id
    }
}

Using id in equals or hashCode is generally a bad idea in domain classes since you can't compare persistent and non-persistent classes. For example

class Foo {
    String name
    boolean equals(o) {
        if (!o) return false
        if (is(o)) return true
        o instanceof Foo && id == o.id
    }
}

With this class, this would fail:

new Foo(name: 'foo').save()
assert Foo.findByName('foo') == new Foo(name: 'foo')

but all of the important class data (in this case just the name property) is identical in both cases.

Even worse, assuming you create a hashCode method that's similarly broken, if you add a non-persistent instance to a hash-based collection (eg HashSet ) and then save it, its id will change from null to some long value and so will its hashcode value. This will cause the instance to become "lost" in the collection.

Answer 2

Where is special method in GormInstanceApi

/**
 * Proxy aware instanceOf implementation.
 */
boolean instanceOf(D o, Class cls) {
    if (o instanceof EntityProxy) {
        o = (D)((EntityProxy)o).getTarget()
    }
    return o in cls
}

Using of this method solved problem