When the IDE generate a entity its generates somethign like this
@Override
    public int hashCode() {
        int hash = 0;
        hash += (productId != null ? productId.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set
        if (!(object instanceof Product)) {
            return false;
        }
        Product other = (Product) object;
        if ((this.productId == null && other.productId != null) || (this.productId != null &&
!this.productId.equals(other.productId))) {
            return false;
        }
        return true;
    }

in this code create a TODO that warning that this code doesnt work with null primary key, I think that will  be very
useful if we change this default code for the next

The idea is that is one of the primaries keys is null them you should compare the object hashcode so:
// Is necessary to change the equals method
@Override
    public int hashCode() {
        int hash = (productId != null ? productId.hashCode() : super.hashCode());
        return hash;
    }

and then the equals code
@Override
    public boolean equals(Object object) {
        if (!(object instanceof Product)) {
            return false;
        }
        Product other = (Product) object;
        if (this.productId == null || other.productId == null) {
            return this.hashCode() == other.hashCode();
        } else {
            return this.productId.equals(other.productId);
        }
    }

I hope for the next release this can be changed and so i dont have to make this change for each generated entities.

Thanks.