如何根据父列有条件地预加载子级?

问题描述

我可以使用以下语法预加载与 Order 相关的所有 Guests :

I'm able to preload all Guests related to Order using this syntax:

Table(order).
Preload("Guests").
Where("order.code = ?", orderCode).
First(&order).
Error

是否可以根据 Order 表中一列的条件来预加载 Guests ?这是我要实现的SQL:

Is it possible to to preload Guests based on a condition on a column in Order table? here is SQL for what I want to achieve:

SELECT * FROM orders WHERE code = "xyz"
SELECT * FROM guests WHERE (order_id IN (1)) AND (some_column_in_guest_tbl = some_column_in_order_tbl)

注意:

我知道这种预加载语法(这不会从订单表列中获取价值，仅当我自己提供价值时才有效):

I'm aware of this Preload syntax (this doesn't take value from order table column, it only works if I provide value myself):

Preload("Guests", "some_column_in_guest_tbl = ?", some_column_in_order_tbl)

推荐答案

您可以在Gorm中使用自定义预加载SQL

You can use Custom Preloading SQL in Gorm

示例:按名称desc预加载来宾订单.

Example: Preload guests order by name desc.

    db.Where("code = ?", orderCode).Preload("Guests", func(db *gorm.DB) *gorm.DB {
        return db.Order("guests.name DESC")
    }).Find(&orders)
    //// SELECT * FROM orders WHERE orders.code = "ABC";
    //// SELECT * FROM guests WHERE order_id IN (1,2,3,4) order by guests.name DESC;

要处理父子条件，可以使用 JOIN

For working with parent-child condition you can use JOIN

db.Joins("JOIN orders ON orders.id = guests.order_id ").Find(&guests)

将两者组合可能会起作用.喜欢

Combining both may work. Like

    db.Where("code = ?", orderCode).Preload("Guests", func(db *gorm.DB) *gorm.DB {
        return db.Joins("JOIN orders ON orders.some_column_in_order_tbl = guests.some_column_in_guest_tbl")
    }).Find(&orders)

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