How to Efficiently Query Associated Entities in GORM?

Associations in GORM Golang

In Go, utilizing the GORM ORM, a common challenge arises when querying multiple entities with associated information. Let's delve into an example to illustrate this issue.

Consider structures representing a town and its associated places:

type Place struct {
  ID          int
  Name        string
  Town        Town
}

type Town struct {
  ID   int
  Name string
}

Assuming a database with the following sample data:

places table		towns Table
id	name	town_id		id	name
1	Place1	1		1	Town1
2	Place2	1		2	Town2

To retrieve all places along with their corresponding towns, one might attempt the following query:

db := gorm.Open("sqlite3", "./data.db")
defer db.Close()
places := []Place{}
db.Find(&places)

However, the result would yield:

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

Notice that the corresponding town information is missing. To rectify this, we need to specify the foreign key in the Place struct as TownID:

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

To obtain the expected result, one could opt for the following approach:

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

This would indeed produce the desired result, but it suffers from an n 1 problem, as it triggers an additional database query for each place.

A more efficient solution involves utilizing Preloads:

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

This concise approach results in the following optimized database queries:

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

By utilizing Preloads, we ensure that associated town information is retrieved in a single database query, thereby optimizing performance and eliminating the n 1 problem.

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