Laravel is a popular PHP framework that provides an elegant way to build web applications and APIs. In the process of building applications, it is often necessary to perform related queries between tables in order to obtain more data information. This article will focus on how to use Laravel to perform join table queries.

1. Base Model Class

In Laravel, every relationship is established through methods between related models. We need to define relationship methods in the model class. The following example shows how to define belongsTo and hasMany relationship methods in a model class.

class?User?extends?Model
{
????/**
?????*?Get?the?post?that?belongs?to?the?user.
?????*/
????public?function?post()
????{
????????return?$this->belongsTo(Post::class);
????}
}

class?Post?extends?Model
{
????/**
?????*?Get?the?comments?for?the?blog?post.
?????*/
????public?function?comments()
????{
????????return?$this->hasMany(Comment::class);
????}
}

In the User model, the belongsTo method indicates that the User model has one Post model, while in the Post model, the hasMany method indicates that the Post model has multiple Comment models.

2. Relationship Query

In Laravel, the query builder provides some methods to perform relational queries. For example, we can use the with method to get the data of the associated model.

$users?=?User::with('post')->get();

This code will get all User models and use the with method to preload the related Post models. This way, we can create a relationship between the user and the post.

Similarly, we can also query the relationship between post and comment.

$posts?=?Post::with('comments')->get();

This code will get all Post models and use the with method to preload the related Comment models.

If we need to further filter the query results, we can pass in the closure function in the method. The example below shows how to get all published comments.

$comments?=?Comment::with(['post'?=>?function?($query)?{
????$query->where('published',?true);
}])->get();

This code will get all Comment models and use the with method to preload the related Post models. In the with method, we can also pass an associative array. At this time, the keys of the array represent the relationship name, and the values ??of the array represent the query closure function of the current relationship.

3. Custom relationship query

In some cases, we may need to make some custom queries. For example, we need to query based on the user's role. At this point, we can define a relationship method in the model class.

class?User?extends?Model
{
????/**
?????*?Get?the?posts?for?the?user?by?role.
?????*/
????public?function?postsByRole($role)
????{
????????return?$this->hasManyThrough(
????????????'App\Post',?
????????????'App\Category',?
????????????'user_id',?
????????????'category_id'
????????)->where('role',?'=',?$role);
????}
}

In this example, we define a postsByRole method in the User model. This method uses the hasManyThrough method to establish the relationship between the Posts model and the Categories model. Among them, the first parameter represents the Posts model, the second parameter represents the Categories model, the third parameter represents the foreign key name of the User model from which the Posts model can be obtained, and the fourth parameter represents the Posts model from which the Categories model can be obtained. Foreign key name.

4. Many-to-many relationship

In Laravel, many-to-many relationships are established through intermediate tables. In the model class, we need to define the belongsToMany relationship method to create a many-to-many relationship. The following example shows how to establish a many-to-many relationship between the User model and the Role model.

class?User?extends?Model
{
????/**
?????*?The?roles?that?belong?to?the?user.
?????*/
????public?function?roles()
????{
????????return?$this->belongsToMany(Role::class);
????}
}

class?Role?extends?Model
{
????/**
?????*?The?users?that?belong?to?the?role.
?????*/
????public?function?users()
????{
????????return?$this->belongsToMany(User::class);
????}
}

In the User model, the belongsToMany method indicates that a many-to-many relationship has been established between the User model and the Role model. Similarly, in the Role model, the belongsToMany method indicates that a many-to-many relationship is established between the Role model and the User model.

Regarding the query of many-to-many relationships, Laravel provides some methods to implement, such as: withCount, has, whereHas, etc.

5. Conclusion

This article focuses on how to perform related queries between tables in Laravel, including basic model classes, relationship queries, custom relationship queries, and multiple pairs Multi-relationship query. I hope that through studying this article, readers can master the basic knowledge of Laravel's join table query and be able to flexibly apply it in actual projects.

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