Gorm Golang orm associations

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).

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]
    }
}

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)
    }
}