Gorm 关联多对多

Question

我知道 gorm 多对多关联会创建一个连接表，但是如果我想要/需要一个带有附加字段的自定义连接表怎么办

下面我的例子

所以我有两个 gorm 模型

type Exercise struct {
    gorm.Model
    Name        string `gorm:"not null" json:"name"`
    Description string `gorm:"not null json:"description"`
}
type Workout struct {
    gorm.Model
    Name string `gorm:"not null" json:"name"`
    CreatedAt time.Time `gorm:"not null" json:"created_at"`
}

如果我使用 gorm 多对多关联，它将创建一个锻炼锻炼连接表，但我将如何确保填写诸如 reps 和 set 之类的其他字段。这是需要在控制器中完成的事情，而不是真正的 gorm 处理。我用 Nodejs 和 sequelize 做过类似的事情，但对于 go 和 gorm 世界来说真的很新。

Answer 1

聚会有点晚了，但是……我的目标是创造类似的东西，花了一整天的时间，碰壁后，退后一步重新考虑……

经过深思熟虑，很多游戏，以及朋友的好建议，决定从稍微不同的角度接近。

如果您以不同的方式看待它，您可以将其“改写”为两个 has-one（或 belongs-to）和两个 one-to-many，而不是多对多：

• 运动有很多锻炼-运动
• 锻炼有很多锻炼-锻炼
• workout-exercise 有锻炼和一项运动

另外，在我的游戏中，我意识到多对多表的属性（列）在上述模型中的任何地方都没有引用，因此无论如何它都没有用。

如果您将锻炼-锻炼视为“一流”模型，它开始变得更有意义并变得更实用：

• 您可以创建练习 - 它是一种静态资源池
• 您可以创建锻炼方式
• 然后您可以使用锻炼-锻炼模型在锻炼中填充锻炼。

总而言之，我的建议是这样的：

type Exercise struct {
    gorm.Model
    Name        string `gorm:"not null" json:"name"`
    Description string `gorm:"not null" json:"description"`
    Workouts    []WorkoutExercise
}

type Workout struct {
    gorm.Model
    Name      string    `gorm:"not null" json:"name"`
    CreatedAt time.Time `gorm:"not null" json:"created_at"`
    Exercises []WorkoutExercise
}

type WorkoutExercise struct {
    gorm.Model
    WorkoutID  uint
    Workout    Workout `gorm:"foreignKey:WorkoutID;references:ID"`
    ExerciseID uint
    Exercise   Exercise `gorm:"foreignKey:ExerciseID;references:ID"`
    Sets       uint
    Reps       uint
    Weights    uint
}

它产生以下架构（我相信这是需要的）：

gym=# \dt
               List of relations
 Schema |       Name        | Type  |   Owner   
--------+-------------------+-------+-----------
 public | installations     | table | gym
 public | workout_exercises | table | gym
 public | workouts          | table | gym
(3 rows)

gym=# \d workouts
                                       Table "public.workouts"
   Column   |           Type           | Collation | Nullable |               Default                
------------+--------------------------+-----------+----------+--------------------------------------
 id         | bigint                   |           | not null | nextval('workouts_id_seq'::regclass)
 created_at | timestamp with time zone |           | not null | 
 updated_at | timestamp with time zone |           |          | 
 deleted_at | timestamp with time zone |           |          | 
 name       | text                     |           | not null | 
Indexes:
    "workouts_pkey" PRIMARY KEY, btree (id)
    "idx_workouts_deleted_at" btree (deleted_at)
Referenced by:
    TABLE "workout_exercises" CONSTRAINT "fk_workouts_exercises" FOREIGN KEY (workout_id) REFERENCES workouts(id)

gym=# \d exercises
                                       Table "public.exercises"
   Column    |           Type           | Collation | Nullable |                Default                
-------------+--------------------------+-----------+----------+---------------------------------------
 id          | bigint                   |           | not null | nextval('exercises_id_seq'::regclass)
 created_at  | timestamp with time zone |           |          | 
 updated_at  | timestamp with time zone |           |          | 
 deleted_at  | timestamp with time zone |           |          | 
 name        | text                     |           | not null | 
 description | text                     |           | not null | 
Indexes:
    "exercises_pkey" PRIMARY KEY, btree (id)
    "idx_exercises_deleted_at" btree (deleted_at)
Referenced by:
    TABLE "workout_exercises" CONSTRAINT "fk_exercises_workouts" FOREIGN KEY (exercise_id) REFERENCES exercises(id)

gym=# \d workout_exercises
                                       Table "public.workout_exercises"
   Column    |           Type           | Collation | Nullable |                    Default                    
-------------+--------------------------+-----------+----------+-----------------------------------------------
 id          | bigint                   |           | not null | nextval('workout_exercises_id_seq'::regclass)
 created_at  | timestamp with time zone |           |          | 
 updated_at  | timestamp with time zone |           |          | 
 deleted_at  | timestamp with time zone |           |          | 
 workout_id  | bigint                   |           |          | 
 exercise_id | bigint                   |           |          | 
 sets        | bigint                   |           |          | 
 reps        | bigint                   |           |          | 
 weights     | bigint                   |           |          | 
Indexes:
    "workout_exercises_pkey" PRIMARY KEY, btree (id)
    "idx_workout_exercises_deleted_at" btree (deleted_at)
Foreign-key constraints:
    "fk_exercises_workouts" FOREIGN KEY (exercise_id) REFERENCES exercises(id)
    "fk_workouts_exercises" FOREIGN KEY (workout_id) REFERENCES workouts(id)

如果您考虑一下，将练习拉到锻炼中，而没有其他属性是没有意义的。如果您只想获得锻炼的练习，可以从中间模型中提取它*：

workout := Workout
db.Preload("WorkoutExercise.Exercise").First(&workout, 10)

for _, we := range workout.Exercises {
    // access reps, etc directly from we
    // and access the exercise from the property we.Exercise
    log.Printf("do %d sets of %d repetitions of %s", we.Sets, we.Reps, 
               we.Exercise.Name)
}

*免责声明 - 没有实际运行此代码（其余的工作）。请查看Nested Preloading 以实现一些深度嵌套的急切加载。

Answer 2

type Exercise struct {
    gorm.Model
    Name        string `gorm:"not null" json:"name"`
    Description string `gorm:"not null json:"description"`
    Workouts []Workout `gorm:"many2many:exercise_workout;"`
}
type Workout struct {
    gorm.Model
    Name string `gorm:"not null" json:"name"`
    CreatedAt time.Time `gorm:"not null" json:"created_at"`
}

many2many 的标记值是您定义的关联表名的名称。 并使用以下代码加载带有锻炼数据的练习。

var exercises []Exercise
db.Preload("Workouts").Find(&exercises) 

如果你想用运动数据加载锻炼，只需在 Workout 结构中添加 many2many 关联，例如：

type Exercise struct {
    gorm.Model
    Name        string `gorm:"not null" json:"name"`
    Description string `gorm:"not null json:"description"`
    Workouts []Workout `gorm:"many2many:exercise_workout;"`
}
type Workout struct {
    gorm.Model
    Name string `gorm:"not null" json:"name"`
    CreatedAt time.Time `gorm:"not null" json:"created_at"`
    Exercises []Exercise `gorm:"many2many:exercise_workout;"`
}

var workouts []Workout
db.Preload("Exercises").Find(&workouts)