How Can GORM's Preload Function Optimize Queries with Associations?

GORM Golang ORM Associations

In Go, GORM is a popular ORM for interacting with databases. When working with associations in GORM, it's important to establish proper relations between structs. Consider the following structs representing a Town and Place:

type Place struct {
    ID          int
    Name        string
    Town        Town
}

type Town struct {
    ID   int
    Name string
}

Here, each Town can have multiple Places, while each Place belongs to only one Town.

Suppose you want to query all Places and their associated Town information. Below is a sample database and the expected result:

Places Table

id	name	town_id
1	Place1	1
2	Place2	1

Towns Table

id	name
1	Town1

Expected Result:

id	name	Town
1	Place1	Town1
2	Place2	Town1

Initial Implementation

The initial implementation attempts to query all Places but fails to include associated Town information:

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

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

Using Preload

The optimal solution involves using GORM's Preload method before the Find operation. This ensures that related data is included in the main query, eliminating the need for additional queries:

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

With this approach, only two queries will be triggered, significantly improving performance:

Query Log:

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

Conclusion

When working with associations in GORM, it's crucial to define foreign keys correctly. Utilizing Preload allows you to efficiently include associated information in your queries, preserving scalability and optimizing performance.

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