Efficiently mapping one-to-many many-to-many database to struct in Golang

I can suggest another approach which I have used before.

You make a json of the tags in this case in the query and return it.

Pros: You have 1 call to the db, which aggregates the data, and all you have to do is parse the json into an array.

Cons: It's a bit ugly. Feel free to bash me for it.

type jointItem struct {
  Item 
  ParsedTags string
  Tags []Tag `gorm:"-"`
}

var jointItems []*jointItem
db.Raw(`SELECT 
  items.*, 
  (SELECT CONCAT(
            '[', 
             GROUP_CONCAT(
                  JSON_OBJECT('id', id,
                             'name', name 
                  )
             ), 
            ']'
         )) as parsed_tags 
   FROM items`).Scan(&jointItems)

for _, o := range jointItems {
var tempTags []Tag
   if err := json.Unmarshall(o.ParsedTags, &tempTags) ; err != nil {
      // do something
   }
  o.Tags = tempTags
}

---

Edit: code might behave weirdly so I find it better to use a temporary tags array when moving instead of using the same struct.

the sql in postgres :

create schema temp;
set search_path = temp;
create table item
(
  id INT generated by default as identity primary key
);

create table tag
(
  id      INT generated by default as identity primary key,
  name    VARCHAR(160),
  item_id INT references item (id)
);

create view item_tags as
select id,
  (
          select
            array_to_json(array_agg(row_to_json(taglist.*))) as array_to_json
          from (
                select tag.name, tag.id
                 from tag
                         where item_id = item.id
               ) taglist ) as tags
from item ;


-- golang query this maybe 
select  row_to_json(row)
from (
    select * from item_tags
) row;

then golang query this sql:

select  row_to_json(row)
from (
    select * from item_tags
) row;

and unmarshall to go struct:

pro:

1. postgres manage the relation of data. add / update data with sql functions.

2. golang manage business model and logic.

it's easy way.

.

You can use carta.Map() from https://github.com/jackskj/carta It tracks has-many relationships automatically.