How Can I Efficiently Query Places and Their Associated Towns Using GORM in Go?

GORM Golang ORM Associations: Querying Places with Associated Towns

In GORM for Go, it's possible to define relationships between models, such as the relationship between towns and places. When querying places, it's often desirable to retrieve the associated town information as well.

The Issue

Consider the following structs representing towns and places:

type Place struct {
  ID          int
  Name        string
  Town        Town
}

type Town struct {
  ID   int
  Name string
}

With a simple database containing two towns and two places related to one of the towns, querying all places may not yield the expected results. Instead of retrieving places with their associated town information, you may receive an array of places with empty town fields.

The Solution

To correctly query for places and their associated town information, the TownID must be specified as the foreign key in the Place struct:

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

Several approaches can be taken to handle the querying:

Approach 1: Using Related() Method

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

This approach triggers an additional query for each place to retrieve the associated town, resulting in an n 1 problem.

Approach 2: Using Preloads

The preferred approach is to use preloads, which allows for efficient querying of associated models.

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

This approach triggers only two queries: one to retrieve all places and one to retrieve all associated towns.

By using this approach, the query will return the expected result, where each place object contains the associated town information.

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