为什么在 DBIx::Class 中 'has_many' 关系得到 'JOIN' 而不是 'LEFT JOIN'？

Question

下一段关系我有3个table：

package SafeVPN::DB::Result::Locality;
__PACKAGE__->has_many( servers        => 'SafeVPN::DB::Result::Server', 'locality_id', {cascade_delete => 0});
__PACKAGE__->has_many( addresses_view => 'SafeVPN::DB::Result::Pool::Address_view', 'locality_id', {cascade_delete => 0});

package SafeVPN::DB::Result::Server;
__PACKAGE__->has_many('addresses_view', 'SafeVPN::DB::Result::Pool::Address_view', 'server_id', {cascade_delete => 0});
__PACKAGE__->belongs_to('locality', 'SafeVPN::DB::Result::Locality', 'locality_id', {cascade_delete => 0});


package SafeVPN::DB::Result::Pool::Address_view;
__PACKAGE__->belongs_to( server   => 'SafeVPN::DB::Result::Server',       'server_id'  );
__PACKAGE__->belongs_to( locality => 'SafeVPN::DB::Result::Locality',     'locality_id');
__PACKAGE__->belongs_to( subnet   => 'SafeVPN::DB::Result::Pool::Subnet', 'subnet_id'  );

在 Locality.pm 运行
这个：$self->search_related('servers')->search_related('addresses_view')->as_query
或者这个：$self->servers->search_related('addresses_view')->as_query

生成下一个查询：

SELECT 
  "addresses_view"."id", "addresses_view"."subnet_id", 
  "addresses_view"."ip", "addresses_view"."usage",
  "addresses_view"."notes", "addresses_view"."locality_id", 
  "addresses_view"."server_id" 
FROM "servers" "me"  
JOIN "pool_addresses_view" "addresses_view" 
  ON "addresses_view"."server_id" = "me"."id" 
WHERE ( "me"."locality_id" = ? )

但依赖'has_many'关系（doc说："This relationship refers to zero or more records in the foreign table (e.g. a LEFT JOIN)"）

我希望 'LEFT JOIN' 但是，如您所见，我得到 'JOIN'

因此，如果 'locality_id' 中的 'server' 没有 IP 地址，我会从结果

中删除 'server'

更新
添加属性 join_type => 'left' 如：

__PACKAGE__->has_many( addresses_view => 'SafeVPN::DB::Result::Pool::Address_view', 'locality_id', {cascade_delete => 0,join_type => 'left'});
__PACKAGE__->has_many('addresses_view', 'SafeVPN::DB::Result::Pool::Address_view', 'server_id', {cascade_delete => 0,join_type => 'left'});

没有效果。可能是BUG？

Answer 1

好吧，servers->search_related('addresses_view') 的意思是“我想要这个服务器的地址”而不是“这个服务器的地址”，所以LEFT JOIN 没有意义，它会在结果中保存没有地址的服务器，但您不再关心服务器（search_related 的意思）。

search_related returns 新类型的结果集：SafeVPN::DB::ResultSet::Pool::Address_view 而不是 SafeVPN::DB::ResultSet::Pool::Server.

如果您想获取带地址的服务器（即保留结果集类型，但添加一些额外信息），那么您可能想使用 prefetch，而不是 search_related。

my @servers = $self->servers->search({}, prefetch => 'addresses_view')->all();
foreach my $server (@servers) {
    print $server->addresses_view(); # Doesn't make a SQL query, data is *prefetched*.
}

您不会获得原始连接关系，所有地址都存储在 server 结果对象中，这就是 ORM 的工作方式。

为什么在 DBIx::Class 中 'has_many' 关系得到 'JOIN' 而不是 'LEFT JOIN'？

Why I get 'JOIN' instead of 'LEFT JOIN' for 'has_many' relationship in DBIx::Class?

perl

join

dbix-class