一对多关联

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原文：https://gobuffalo.io/documentation/database/relations-one-to-many/

One to Many Associations 一对多关联

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

在本章中，您将学习如何在 Pop 中编写一对多关联。

Tags 标签

One to many associations work using a pair of tags:

一对多关联使用一对标签：

belongs_to for the model with the foreign key. belongs_to 用于具有外键的模型。
has_many for the model without the foreign key (the one with the slice). has_many 用于没有外键的模型（带有切片的模型）。

Example 示例

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// 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 自定义关联顺序

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:

由于 has_many 映射到切片，因此您可能希望自定义此切片的顺序。 order_by 标签允许您在加载关联时指示关联的顺序：

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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>".

要使用的格式是 order_by:"<column_name> <asc | desc>" 。

Customize Foreign Keys Lookup 自定义外键查找

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.

默认情况下， has_many 将使用外键列的约定来获取相关记录。在我们的上一个示例中， fruits 表（映射到 Fruit 结构）包含一个 tree_id 外键列，该列引用水果所附树的 ID。

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

您可以使用 fk_id 标记来自定义此外键列：

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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.

在此，将使用 fruits 表中的列 custom_tree_id 来查找关系，而不是默认的 tree_id 列。

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

当您有多个字段指向同一模型的结构时，这非常有用：与 Pop 的关联：1 对 n - 一篇关于 Pop 中 1 对 n 关联的文章。

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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"`
}

Associations with Pop: 1 to n - An article about 1 to n associations in Pop.

最后修改 February 4, 2024: 更新 (c4156b3)