自定义查询来获取表的所有条目，并且仅包含基于特定列的多个重复项中的第一个

Question

我有一个位置模型，表格看起来像

id	名字	文	ip_地址	创建于	更新于
0	默认	0	0.0.0.0/0	2021-11-08 11:54:26.822623	2021-11-08 11:54:26.822623
1	管理员	1	10.108.150.143	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885
2	V122	122	10.108.150.122	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885
3	V123	123	10.108.150.123	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885
4	V124	124	10.108.150.124	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885
5	V122	122	10.108.150.122	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885
6	V125	122	10.108.150.125	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885

我在位置模型中的方法

   def self.find_all_non_duplicate
     return self.find(:all, :conditions => "id <> 1")
   end

我想获取位置表的所有条目，除了 id = 1 的条目，并且该条目仅包含基于列 ip_address 的许多重复项中的第一个条目。

因为 id = 2 和 id = 5 的 ip_address 是重复的。我想保留许多重复项中的第一个条目，即 id = 2。 预期结果是

id名字0默认V122V123V124V125id为1和5的条目将被忽略

文	ip_地址	创建于	更新于
0	0.0.0.0/0	2021-11-08 11:54:26.822623	2021-11-08 11:54:26.822623		2
122	10.108.150.122	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885		3
123	10.108.150.123	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885		4
124	10.108.150.124	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885		6
122	10.108.150.125	2021-11-08 11:54:26.82885	2021-11-08 11:54:26.82885

您需要的是最近

这里

Answer 1

distinct on

，但

尚未合并，正如@engineersmnky所指出的那样。在原始 SQL 形式中，它看起来像这样：

select distinct on (ip_address) * 
from test 
where id<>1 
order by ip_address,created_at;

这将转化为 RoR

self.where("id <> 1").distinct_on(:ip_address)

或者，直到新功能被接受：

self.where("id <> 1").select("distinct on (ip_address) *")

完整的数据库端测试：

drop table if exists test cascade;
create table test (
    id serial primary key,
    name text,
    vin integer,
    ip_address inet,
    created_at timestamp,
    updated_at timestamp);
insert into test 
(id,name,vin,ip_address,created_at,updated_at)
values
(0,'default', 0,'0.0.0.0/0'::inet,'2021-11-08 11:54:26.822623'::timestamp,'2021-11-08 11:54:26.822623'::timestamp),
(1,'admin',   1,'10.108.150.143'::inet,'2021-11-08 11:54:26.82885'::timestamp,'2021-11-08 11:54:26.82885'::timestamp),
(2,'V122',    122,'10.108.150.122'::inet,'2021-11-08 11:54:26.82885'::timestamp,'2021-11-08 11:54:26.82885'::timestamp),
(3,'V123',    123,'10.108.150.123'::inet,'2021-11-08 11:54:26.82885'::timestamp,'2021-11-08 11:54:26.82885'::timestamp),
(4,'V124',    124,'10.108.150.124'::inet,'2021-11-08 11:54:26.82885'::timestamp,'2021-11-08 11:54:26.82885'::timestamp),
(5,'V122',    122,'10.108.150.122'::inet,'2021-11-08 11:54:26.82885'::timestamp,'2021-11-08 11:54:26.82885'::timestamp),
(6,'V125',    122,'10.108.150.125'::inet,'2021-11-08 11:54:26.82885'::timestamp,'2021-11-08 11:54:26.82885'::timestamp);

select distinct on (ip_address) * 
from test where id<>1 
order by ip_address,created_at;
-- id |  name   | vin |   ip_address   |         created_at         |         updated_at
------+---------+-----+----------------+----------------------------+----------------------------
--  0 | default |   0 | 0.0.0.0/0      | 2021-11-08 11:54:26.822623 | 2021-11-08 11:54:26.822623
--  2 | V122    | 122 | 10.108.150.122 | 2021-11-08 11:54:26.82885  | 2021-11-08 11:54:26.82885
--  3 | V123    | 123 | 10.108.150.123 | 2021-11-08 11:54:26.82885  | 2021-11-08 11:54:26.82885
--  4 | V124    | 124 | 10.108.150.124 | 2021-11-08 11:54:26.82885  | 2021-11-08 11:54:26.82885
--  6 | V125    | 122 | 10.108.150.125 | 2021-11-08 11:54:26.82885  | 2021-11-08 11:54:26.82885
--(5 rows)