Gorm Golang orm associations

I'm using Go with the GORM ORM.
I have the following structs. The relation is simple. One Town has multiple Places and one Place belongs to one Town.

type Place struct {
  ID          int
  Name        string
  Town        Town
}

type Town struct {
  ID   int
  Name string
}

Now i want to query all places and get along with all their fields the info of the corresponding town.
This is my code:

db, _ := gorm.Open("sqlite3", "./data.db")
defer db.Close()

places := []Place{}
db.Find(&places)
fmt.Println(places)

My sample database has this data:

/* places table */
id  name    town_id
 1  Place1        1
 2  Place2        1

/* towns Table */
id name
 1 Town1
 2 Town2

i'm receiving this:

[{1 Place1 {0 }} {2 Mares Place2 {0 }}]

But i'm expecting to receive something like this (both places belongs to the same town):

[{1 Place1 {1 Town1}} {2 Mares Place2 {1 Town1}}]

How can i do such query ? I tried using Preloads and Related without success (probably the wrong way). I can't get working the expected result.

TownID must be specified as the foreign key. The Place struct gets like this:

type Place struct {
  ID          int
  Name        string
  Description string
  TownID      int
  Town        Town
}

Now there are different approach to handle this. For example:

places := []Place{}
db.Find(&places)
for i, _ := range places {
    db.Model(places[i]).Related(&places[i].Town)
}

This will certainly produce the expected result, but notice the log output and the queries triggered.

[4.76ms]  SELECT  * FROM "places"
[1.00ms]  SELECT  * FROM "towns"  WHERE ("id" = '1')
[0.73ms]  SELECT  * FROM "towns"  WHERE ("id" = '1')

[{1 Place1  {1 Town1} 1} {2 Place2  {1 Town1} 1}]

The output is the expected but this approach has a fundamental flaw, notice that for every place there is the need to do another db query which produce a n + 1 problem issue. This could solve the problem but will quickly gets out of control as the amount of places grow.

It turns out that the good approach is fairly simple using preloads.

db.Preload("Town").Find(&places)

That's it, the query log produced is:

[22.24ms]  SELECT  * FROM "places"
[0.92ms]  SELECT  * FROM "towns"  WHERE ("id" in ('1'))

[{1 Place1  {1 Town1} 1} {2 Place2  {1 Town1} 1}]

This approach will only trigger two queries, one for all places, and one for all towns that has places. This approach scales well regarding of the amount of places and towns (only two queries in all cases).

You do not specify the foreign key of towns in your Place struct. Simply add TownId to your Place struct and it should work.

package main

import (
	"fmt"

	"github.com/jinzhu/gorm"
	_ "github.com/mattn/go-sqlite3"
)

type Place struct {
	Id     int
	Name   string
	Town   Town
	TownId int //Foregin key
}

type Town struct {
	Id   int
	Name string
}

func main() {
	db, _ := gorm.Open("sqlite3", "./data.db")
	defer db.Close()

	db.CreateTable(&Place{})
	db.CreateTable(&Town{})
	t := Town{
		Name: "TestTown",
	}

	p1 := Place{
		Name:   "Test",
		TownId: 1,
	}

	p2 := Place{
		Name:   "Test2",
		TownId: 1,
	}

	err := db.Save(&t).Error
	err = db.Save(&p1).Error
	err = db.Save(&p2).Error
	if err != nil {
		panic(err)
	}

	places := []Place{}
	err = db.Find(&places).Error
	for i, _ := range places {
		db.Model(places[i]).Related(&places[i].Town)
	}
	if err != nil {
		panic(err)
	} else {
		fmt.Println(places)
	}
}

To optimize query I use "in condition" in the same situation

places := []Place{}

DB.Find(&places)

keys := []uint{}
for _, value := range places {
	keys = append(keys, value.TownID)
}

rows := []Town{}
DB.Where(keys).Find(&rows)

related := map[uint]Town{}
for _, value := range rows {
	related[value.ID] = value
}

for key, value := range places {
	if _, ok := related[value.TownID]; ok {
		res[key].Town = related[value.TownID]
	}
}

First change your model:

type Place struct {
  ID          int
  Name        string
  Description string
  TownID      int
  Town        Town
}

And second, make preloading:
https://gorm.io/docs/preload.html

Click For Full Docs

Summary: preloading one-to-one relation: has one, belongs to

eager preload:

db.Preload("Orders").Preload("Profile").Find(&users)

join preload using inner join:

db.Joins("Orders").Joins("Profile").Find(&users)

preload all associations:

db.Preload(clause.Associations).Find(&users)

No need to loop for ids, just pluck the ids

townIDs := []uint{}
DB.Model(&Place{}).Pluck("town_id", &placeIDs)

towns := []Town{}
DB.Where(townIDs).Find(&towns)