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One to many associations
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Database

One to Many Associations

In this chapter, you’ll learn how to write a one to many association in Pop.

Tags

One to many associations work using a pair of tags:

  • belongs_to for the model with the foreign key.
  • has_many for the model without the foreign key (the one with the slice).

Example

// Models

type Fruit struct {
    ID     int   `json:"id,omitempty" db:"id"`
    TreeID int   `json:"-" db:"tree_id"`
    Tree   *Tree `json:"tree,omitempty" belongs_to:"tree"`
}
    
type Tree struct {
    ID     int     `json:"id" db:"id"`
    Name   string  `json:"name" db:"name"`
    Fruits []Fruit `json:"fruits,omitempty" has_many:"fruits"`
}

// Eager creation:
// Create an apple tree with 2 fruits.
t := &models.Tree{
    Name: "Apple tree",
    Fruits: []models.Fruit{
        {},
        {},
    },
}

if err := tx.Eager().Create(t); err != nil {
    return err
}

// Eager fetch all the trees with their fruits.
trees := &models.Trees{}

if err := c.Eager().All(trees); err != nil {
    log.Printf("err: %v", err)
    return
}

log.Printf("eager fetch: %v", trees)

Custom Association Order

Since has_many is mapped to a slice, you’ll probably want to customize the order of this slice. order_by tag allows you to indicate the order for the association when loading it:

type Tree struct {
    ID     int     `json:"id" db:"id"`
    Name   string  `json:"name" db:"name"`
    Fruits []Fruit `json:"fruits,omitempty" has_many:"fruits" order_by:"id desc"`
}

The format to use is order_by:"<column_name> <asc | desc>".

Customize Foreign Keys Lookup

By default, has_many will fetch related records using a convention for the foreign key column. In our previous example, the fruits table (mapped to the Fruit struct) contains a tree_id foreign key column which references the ID of the tree the fruit is attached to.

You can use the fk_id tag to customize this foreign key column:

type Tree struct {
    ID     int     `json:"id" db:"id"`
    Name   string  `json:"name" db:"name"`
    Fruits []Fruit `json:"fruits,omitempty" has_many:"fruits" fk_id:"custom_tree_id"`
}

Here, the relation will be looked up using the column custom_tree_id in the fruits table, instead of the default tree_id one.

This can be really useful when you have structs with multiple fields pointing to the same model:

type Player struct {
    ID            int     `json:"id" db:"id"`
    Name          string  `json:"name" db:"name"`
    CurrentBandID int     `json:"current_band_id" db:"current_band_id"`
    FormerBandID  int     `json:"former_band_id" db:"former_band_id"`
}

type Band struct {
    ID             int      `json:"id" db:"id"`
    Name           string   `json:"name" db:"name"`
    CurrentPlayers []Player `json:"current_players,omitempty" has_many:"players" fk_id:"current_band_id"`
    FormerPlayers  []Player `json:"former_players,omitempty" has_many:"players" fk_id:"former_band_id"`
}

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